Pilot’s Handbook of A

eronautical Know

ledge

FAA

-H-8083-25

PILOT’S HANDBOOK of

Aeronautical Knowledge

2003

U.S. DEPARTMENT OF TRANSPORTATIONFEDERAL AVIATION ADMINISTRATION

Flight Standards Service

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PREFACE

The Pilot’s Handbook of Aeronautical Knowledge provides basic knowledge that is essential for pilots. This hand-book introduces pilots to the broad spectrum of knowledge that will be needed as they progress in their pilot train-ing. Except for the Code of Federal Regulations pertinent to civil aviation, most of the knowledge areas applicableto pilot certification are presented. This handbook is useful to beginning pilots, as well as those pursuing moreadvanced pilot certificates.

Occasionally, the word “must” or similar language is used where the desired action is deemed critical. The use ofsuch language is not intended to add to, interpret, or relieve a duty imposed by Title 14 of the Code of FederalRegulations (14 CFR).

It is essential for persons using this handbook to also become familiar with and apply the pertinent parts of 14 CFRand the Aeronautical Information Manual (AIM). The AIM is available online at http://www.faa.gov/atpubs.

The current Flight Standards Service airman training and testing material and subject matter knowledge codes for allairman certificates and ratings can be obtained from the Flight Standards Service Web site at http://av-info.faa.gov.

This handbook supersedes Advisory Circular (AC) 61-23C, Pilot’s Handbook of Aeronautical Knowledge, dated1997.

This publication may be purchased from the Superintendent of Documents, U.S. Government Printing Office (GPO),Washington, DC 20402-9325, or from http://bookstore.gpo.gov. This handbook is also available for download fromthe Flight Standards Service Web site at http://av-info.faa.gov.

This handbook is published by the U.S. Department of Transportation, Federal Aviation Administration, AirmanTesting Standards Branch, AFS-630, P.O. Box 25082, Oklahoma City, OK 73125. Comments regarding this hand-book should be sent in e-mail form to AFS630comments@faa.gov.

AC 00-2, Advisory Circular Checklist, transmits the current status of FAA advisory circulars and other flight information and publications. This checklist is available via the Internet athttp://www.faa.gov/aba/html_policies/ac00_2.html.

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Chapter 1—Aircraft StructureMajor Components ........................................1-1

Fuselage ....................................................1-2Wings........................................................1-3Empennage ...............................................1-4Landing Gear ............................................1-4The Powerplant.........................................1-5

Chapter 2—Principles of FlightStructure of the Atmosphere..........................2-1

Atmospheric Pressure...............................2-2Effects of Pressure on Density .................2-2Effect of Temperature on Density ............2-2Effect of Humidity on Density .................2-2

Newton’s Laws of Motion and Force............2-2Magnus Effect ...............................................2-3Bernoulli’s Principle of Pressure...................2-3Airfoil Design................................................2-4Low Pressure Above......................................2-5High Pressure Below.....................................2-6Pressure Distribution .....................................2-6

Chapter 3—Aerodynamics of FlightForces Acting on the Airplane.......................3-1

Thrust........................................................3-2Drag ..........................................................3-3Weight.......................................................3-5Lift ............................................................3-6

Wingtip Vortices ............................................3-6Ground Effect ................................................3-7Axes of an Airplane.......................................3-8Moments and Moment Arm ..........................3-9Design Characteristics ...................................3-9

Basic Concepts of Stability ....................3-10Static Stability ........................................3-10Dynamic Stability ...................................3-11Longitudinal Stability (Pitching) ............3-11Lateral Stability (Rolling) ......................3-14Vertical Stability (Yawing) .....................3-15Free Directional Oscillations(Dutch Roll)...........................................3-16Spiral Instability .....................................3-16

Aerodynamic Forces in Flight Maneuvers ..3-17Forces in Turns .......................................3-17Forces in Climbs.....................................3-19Forces in Descents..................................3-19

Stalls ............................................................3-20Basic Propeller Principles ...........................3-21

Torque and P Factor................................3-23Torque Reaction......................................3-23Corkscrew Effect ....................................3-24Gyroscopic Action ..................................3-24

Asymmetric Loading (P Factor).............3-25Load Factors ................................................3-26

Load Factors in Airplane Design............3-26Load Factors in Steep Turns...................3-27Load Factors and Stalling Speeds ..........3-28Load Factors and Flight Maneuvers.......3-29VG Diagram ...........................................3-30

Weight and Balance.....................................3-31Effects of Weight onFlight Performance ................................3-32Effect of Weight on Airplane Structure..3-32Effects of Weight on Stability andControllability........................................3-33Effect of Load Distribution ....................3-33

High Speed Flight........................................3-35Supersonic vs. Subsonic Flow................3-35Speed Ranges..........................................3-35Mach Number vs. Airspeed....................3-36Boundary Layer ......................................3-36Shock Waves...........................................3-37Sweepback ..............................................3-38Mach Buffet Boundaries.........................3-39Flight Controls........................................3-40

Chapter 4—Flight ControlsPrimary Flight Controls.................................4-1

Ailerons ....................................................4-1Adverse Yaw.............................................4-2

Differential Ailerons .............................4-2Frise-Type Ailerons ..............................4-2Coupled Ailerons and Rudder ..............4-3

Elevator.....................................................4-3T-Tail.........................................................4-3Stabilator...................................................4-4Canard.......................................................4-5Rudder ......................................................4-5V-Tail ........................................................4-6

Secondary Flight Controls.............................4-6Flaps..........................................................4-6Leading Edge Devices..............................4-7Spoilers .....................................................4-7Trim Systems............................................4-8

Trim Tabs..............................................4-8Balance Tabs.........................................4-8Antiservo Tabs......................................4-8Ground Adjustable Tabs .......................4-9Adjustable Stabilizer ............................4-9

Chapter 5—Aircraft SystemsPowerplant .....................................................5-1

Reciprocating Engines..............................5-1Propeller....................................................5-2

CONTENTS

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Fixed-Pitch Propeller............................5-3Adjustable-Pitch Propeller....................5-4

Induction Systems ....................................5-5Carburetor Systems ..............................5-5

Mixture Control ................................5-5Carburetor Icing................................5-6Carburetor Heat ................................5-7Carburetor Air Temperature Gauge..5-8Outside Air Temperature Gauge.......5-8

Fuel Injection Systems .........................5-8Superchargers and Turbosuperchargers....5-9

Superchargers .......................................5-9Turbosuperchargers ............................5-10

System Operation ...........................5-10High Altitude Performance.............5-11

Ignition System.......................................5-11Combustion.............................................5-12Fuel Systems...........................................5-13

Fuel Pumps .........................................5-14Fuel Primer .........................................5-14Fuel Tanks...........................................5-14Fuel Gauges ........................................5-14Fuel Selectors .....................................5-14Fuel Strainers, Sumps, and Drains .....5-14Fuel Grades.........................................5-15

Fuel Contamination ........................5-15Refueling Procedures......................5-16

Starting System.......................................5-16Oil Systems.............................................5-16Engine Cooling Systems ........................5-18Exhaust Systems.....................................5-19Electrical System....................................5-19Hydraulic Systems..................................5-22Landing Gear ..........................................5-22

Tricycle Landing Gear Airplanes .......5-22Tailwheel Landing Gear Airplanes.....5-23Fixed and Retractable Landing Gear..5-23Brakes .................................................5-23

Autopilot.................................................5-23Pressurized Airplanes ..................................5-24

Oxygen Systems .....................................5-26Masks..................................................5-27Diluter Demand Oxygen Systems ......5-27Pressure Demand Oxygen Systems....5-27Continuous Flow Oxygen System......5-27Servicing of Oxygen Systems ............5-28

Ice Control Systems................................5-28Airfoil Ice Control ..............................5-28Windscreen Ice Control ......................5-29Propeller Ice Control ..........................5-29Other Ice Control Systems .................5-29

Turbine Engines...........................................5-29Types of Turbine Engines.......................5-30

Turbojet...............................................5-30Turboprop ...........................................5-30Turbofan .............................................5-30Turboshaft...........................................5-31Performance Comparison ...................5-31

Turbine Engine Instruments ...................5-31Engine Pressure Ratio ........................5-32Exhaust Gas Temperature...................5-32Torquemeter........................................5-32N1 Indicator........................................5-32N2 Indicator........................................5-32

Turbine Engine Operational Considerations .......................................5-32

Engine Temperature Limitations ........5-32Thrust Variations ................................5-32Foreign Object Damage......................5-32Turbine Engine Hot/Hung Start .........5-33Compressor Stalls...............................5-33Flameout .............................................5-33

Chapter 6—Flight InstrumentsPitot-Static Flight Instruments.......................6-1

Impact Pressure Chamber and Lines........6-1Static Pressure Chamber and Lines..........6-1Altimeter...................................................6-2

Principle of Operation ..........................6-2Effect of Nonstandard Pressure andTemperature .........................................6-2Setting the Altimeter.............................6-3Altimeter Operation..............................6-4Types of Altitude ..................................6-4

Indicated Altitude .............................6-4True Altitude.....................................6-4Absolute Altitude..............................6-4Pressure Altitude...............................6-4Density Altitude................................6-5

Vertical Speed Indicator ...........................6-5Principle of Operation ..........................6-5

Airspeed Indicator ....................................6-6Indicated Airspeed ................................6-6Calibrated Airspeed ..............................6-6True Airspeed .......................................6-6Groundspeed.........................................6-6Airspeed Indicator Markings................6-6Other Airspeed Limitations ..................6-7

Blockage of the Pitot-Static System.........6-8Blocked Pitot System ...........................6-8Blocked Static System..........................6-8

Gyroscopic Flight Instruments ......................6-9Gyroscopic Principles...............................6-9

Rigidity in Space ..................................6-9Precession .............................................6-9

Sources of Power....................................6-10Turn Indicators .......................................6-10

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Turn-and-Slip Indicator ......................6-11Turn Coordinator ................................6-11

Inclinometer ............................................6-11The Attitude Indicator ............................6-12Heading Indicator ...................................6-12

Magnetic Compass ......................................6-14Compass Errors ......................................6-15

Variation..............................................6-15Compass Deviation.............................6-16Magnetic Dip ......................................6-16Using the Magnetic Compass.............6-16

Acceleration/Deceleration Errors ...6-16Turning Errors ................................6-16

Vertical Card Compass ...........................6-17Outside Air Temperature Gauge..................6-17

Chapter 7—Flight Manuals and OtherDocumentsAirplane Flight Manuals................................7-1

Preliminary Pages.....................................7-1General (Section 1)...................................7-2Limitations (Section 2).............................7-2

Airspeed................................................7-2Powerplant ............................................7-2Weight and Loading Distribution .........7-2Flight Limits .........................................7-3Placards.................................................7-3

Emergency Procedures (Section 3) ..........7-3Normal Procedures (Section 4) ................7-3Performance (Section 5) ...........................7-3Weight and Balance/Equipment List(Section 6) ...............................................7-3Systems Description (Section 7) ..............7-4Handling, Service, and Maintenance(Section 8) ...............................................7-4Supplements (Section 9)...........................7-4Safety Tips (Section 10) ...........................7-5

Aircraft Documents .......................................7-5Certificate of Aircraft Registration...........7-5Airworthiness Certificate..........................7-6

Aircraft Maintenance.....................................7-7Aircraft Inspections ..................................7-7

Annual Inspection.................................7-7100-Hour Inspection.............................7-7Other Inspection Programs...................7-8Altimeter System Inspection ................7-8Transponder Inspection ........................7-8Preflight Inspections.............................7-8Minimum Equipment Lists(MEL) and Operationswith Inoperative Equipment ................7-8

Preventive Maintenance ...........................7-9Repairs and Alterations ............................7-9Special Flight Permits ..............................7-9

Airworthiness Directives ........................7-10Aircraft Owner/OperatorResponsibilities......................................7-11

Chapter 8—Weight and BalanceWeight Control ..............................................8-1

Effects of Weight ......................................8-1Weight Changes........................................8-2

Balance, Stability, and Center of Gravity......8-2Effects of Adverse Balance ......................8-2Management of Weight andBalance Control .......................................8-3Terms and Definitions ..............................8-3Basic Principles of Weight andBalance Computations.............................8-4Weight and Balance Restrictions..............8-6

Determining Loaded Weight and Centerof Gravity......................................................8-6

Computational Method.............................8-6Graph Method...........................................8-6Table Method............................................8-8Computations with a Negative Arm.........8-8Computations with Zero Fuel Weight ......8-9Shifting, Adding,and Removing Weight .............................8-9

Weight Shifting.....................................8-9Weight Addition or Removal..............8-10

Chapter 9—Aircraft PerformanceImportance of Performance Data ..................9-1Structure of the Atmosphere..........................9-1

Atmospheric Pressure...............................9-1Pressure Altitude.......................................9-2Density Altitude........................................9-3

Effects of Pressure on Density .............9-4Effects of Temperature on Density.......9-4Effect of Humidity (Moisture)on Density............................................9-4

Performance...................................................9-4Straight-and-Level Flight .........................9-5Climb Performance...................................9-6Range Performance ..................................9-8Ground Effect .........................................9-10Region of Reversed Command ..............9-12Runway Surface and Gradient................9-13Water on the Runway and DynamicHydroplaning .........................................9-14

Takeoff and Landing Performance ..............9-15Takeoff Performance ..............................9-15Landing Performance .............................9-17

Performance Speeds ....................................9-18Performance Charts .....................................9-19

Interpolation............................................9-20Density Altitude Charts ..........................9-20Takeoff Charts ........................................9-22Climb and Cruise Charts ........................9-23

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Crosswind and HeadwindComponent Chart...................................9-28Landing Charts .......................................9-29Stall Speed Performance Charts .............9-30

Transport Category AirplanePerformance................................................9-31

Major Differences in TransportCategory versus Non-TransportCategory Performance Requirements....9-31Performance Requirements ....................9-31Runway Requirements............................9-32Balanced Field Length............................9-32Climb Requirements...............................9-34

First Segment......................................9-35Second Segment .................................9-35Third or Acceleration Segment ..........9-35Forth or Final Segment.......................9-35Second Segment Climb Limitations...9-35

Air Carrier Obstacle ClearanceRequirements .........................................9-36Summary of Takeoff Requirements........9-36Landing Performance .............................9-37

Planning the Landing..........................9-37Landing Requirements........................9-37Approach Climb Requirements ..........9-37Landing Runway Required.................9-37Summary of LandingRequirements .....................................9-38

Examples of Performance Charts ................9-39

Chapter 10—Weather TheoryNature of the Atmosphere ...........................10-1

Oxygen and the Human Body ................10-2Significance of Atmospheric Pressure....10-3

Measurement of AtmosphericPressure..............................................10-3Effect of Altitude on AtmosphericPressure..............................................10-4Effect of Altitude on Flight ................10-4Effect of Differences in Air Density ..10-5Wind ...................................................10-5

The Cause of Atmosphere Circulation ........10-5Wind Patterns .........................................10-6Convective Currents ...............................10-7Effect of Obstructions on Wind..............10-8Low-Level Wind Shear ..........................10-9Wind and Pressure Representationon Surface Weather Maps....................10-11

Atmospheric Stability................................10-12Inversion ...............................................10-13Moisture and Temperature....................10-13Relative Humidity ................................10-13Temperature/Dewpoint Relationship....10-13Methods By Which Air Reachesthe Saturation Point .............................10-14

Dew and Frost ......................................10-14Fog........................................................10-14Clouds...................................................10-15Ceiling ..................................................10-17Visibility ...............................................10-18Precipitation..........................................10-18

Air Masses .................................................10-18Fronts .........................................................10-18

Warm Front...........................................10-19Flight Toward an ApproachingWarm Front......................................10-20

Cold Front.............................................10-20Fast-Moving Cold Front...................10-21Flight Toward an ApproachingCold Front........................................10-21Comparison of Cold andWarm Fronts ....................................10-21

Wind Shifts ...........................................10-21Stationary Front ....................................10-22Occluded Front .....................................10-22

Chapter 11—Weather Reports, Forecasts,and ChartsObservations ................................................11-1

Surface Aviation WeatherObservations .........................................11-1

Upper Air Observations ..........................11-1Radar Observations.................................11-2

Service Outlets.............................................11-2FAA Flight Service Station.....................11-2Transcribed Information BriefingService (TIBS).......................................11-2Direct User Access TerminalService (DUATS)...................................11-2En Route Flight Advisory Service..........11-2Hazardous In-Flight WeatherAdvisory (HIWAS) ................................11-3Transcribed Weather Broadcast(TWEB) .................................................11-3

Weather Briefings ........................................11-3Standard Briefing....................................11-3Abbreviated Briefing ..............................11-4Outlook Briefing.....................................11-4

Aviation Weather Reports............................11-4Aviation Routine Weather Report(METAR) ...............................................11-4Pilot Weather Reports (PIREPs).............11-7Radar Weather Reports (SD) ..................11-8

Aviation Forecasts .......................................11-9Terminal Aerodrome Forecasts...............11-9Area Forecasts ......................................11-10In-Flight Weather Advisories................11-12

Airman’s MeteorologicalInformation (AIRMET) ...................11-12

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Significant MeteorologicalInformation (SIGMET)....................11-12Convective SignificantMeteorological Information(WST) ..............................................11-12

Winds and Temperature AloftForecast (FD).......................................11-13

Weather Charts...........................................11-14Surface Analysis Chart .........................11-14Weather Depiction Chart ......................11-15Radar Summary Chart ..........................11-16Significant Weather PrognosticCharts ...................................................11-18

Chapter 12—Airport OperationsTypes of Airports .........................................12-1

Controlled Airport ..................................12-1Uncontrolled Airport ..............................12-1

Sources for Airport Data .............................12-1Aeronautical Charts ................................12-1Airport/Facility Directory.......................12-1Notices to Airmen...................................12-3

Airport Markings and Signs ........................12-3Runway Markings ..................................12-3Taxiway Markings ..................................12-3Other Markings.......................................12-3Airport Signs ..........................................12-3

Airport Lighting...........................................12-5Airport Beacon .......................................12-5Approach Light Systems ........................12-6Visual Glideslope Indicators ..................12-6

Visual Approach Slope Indicator........12-6Other Glidepath Systems....................12-6

Runway Lighting ....................................12-6Runway End Identifier Lights ............12-6Runway Edge Lights ..........................12-7In-Runway Lighting ...........................12-7

Control of Airport Lighting ....................12-7Taxiway Lights .......................................12-8Obstruction Lights ..................................12-8

Wind Direction Indicators ...........................12-8Radio Communications ...............................12-8

Radio License .........................................12-8Radio Equipment ....................................12-8Lost Communication Procedures ...........12-9

Air Traffic Control Services ......................12-10Primary Radar.......................................12-10Air Traffic Control RadarBeacon System ....................................12-11Transponder ..........................................12-11Radar Traffic Information Service........12-11

Wake Turbulence .......................................12-12Vortex Generation.................................12-13Vortex Strength .....................................12-13

Vortex Behavior....................................12-13Vortex Avoidance Procedures...............12-13

Collision Avoidance...................................12-14Clearing Procedures..............................12-14Runway Incursion Avoidance...............12-14

Chapter 13—AirspaceControlled Airspace .....................................13-1

Class A Airspace.....................................13-1Class B Airspace.....................................13-1Class C Airspace.....................................13-1Class D Airspace ....................................13-3Class E Airspace.....................................13-3

Uncontrolled Airspace .................................13-3Class G Airspace ....................................13-3

Special Use Airspace ...................................13-3Prohibited Areas .....................................13-3Restricted Areas......................................13-3Warning Areas ........................................13-4Military Operation Areas........................13-4Alert Areas..............................................13-4Controlled Firing Areas ..........................13-4

Other Airspace Areas...................................13-4Airport Advisory Areas ..........................13-4Military Training Routes ........................13-4Temporary Flight Restrictions................13-4Parachute Jump Areas ............................13-4Published VFR Routes ...........................13-4Terminal Radar Service Areas................13-5National Security Areas..........................13-5

Chapter 14—NavigationAeronautical Charts .....................................14-1

Sectional Charts......................................14-1Visual Flight Rule Terminal AreaCharts.....................................................14-1World Aeronautical Charts .....................14-1

Latitude and Longitude (Meridians andParallels) .....................................................14-2

Time Zones .............................................14-2Measurement of Direction......................14-3Variation..................................................14-4Deviation ................................................14-5

Effect of Wind .............................................14-6Basic Calculations .......................................14-8

Converting Minutes to EquivalentHours .....................................................14-8Converting Knots to Miles Per Hour .....14-8Fuel Consumption ..................................14-8Flight Computers ....................................14-8Plotter......................................................14-8

Pilotage ......................................................14-10

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Dead Reckoning ........................................14-10The Wind Triangle or VectorAnalysis ...............................................14-10

Flight Planning ..........................................14-13Assembling Necessary Material ...........14-13Weather Check......................................14-13Use of the Airport/Facility Directory ...14-13Airplane Flight Manual or Pilot’sOperating Handbook ..........................14-13

Charting the Course...................................14-14Steps in Charting the Course................14-14

Filing a VFR Flight Plan ...........................14-16Radio Navigation.......................................14-17

Very High Frequency (VHF)Omnidirectional Range (VOR) ...........14-18

Using the VOR .................................14-19Tracking with VOR ..........................14-20Tips On Using the VOR ...................14-21

Distance Measuring Equipment ..........14-21VOR/DME RNAV................................14-21Automatic Direction Finder ................14-22Loran-C Navigation..............................14-24Global Position System .......................14-26

Lost Procedures .........................................14-27Flight Diversion.........................................14-27

Chapter 15—Aeromedical FactorsObtaining a Medical Certificate ..................15-1Environmental and Health FactorsAffecting Pilot Performance.......................15-2

Hypoxia ..................................................15-2Hypoxic Hypoxia................................15-2Hypemic Hypoxia...............................15-2Stagnant Hypoxia ...............................15-2Histotoxic Hypoxia.............................15-2Symptoms of Hypoxia........................15-2

Hyperventilation .....................................15-3Middle Ear and Sinus Problems.............15-3

Spatial Disorientation and Illusions .......15-4Motion Sickness .....................................15-6Carbon Monoxide Poisoning..................15-6Stress.......................................................15-6Fatigue ....................................................15-7Dehydration and Heatstroke...................15-7Alcohol ...................................................15-8Drugs ......................................................15-8Scuba Diving ..........................................15-9

Vision in Flight ............................................15-9Empty-Field Myopia ............................15-10Night Vision..........................................15-10Night Vision Illusions...........................15-11

Autokinesis .......................................15-11False Horizon....................................15-11Night Landing Illusions....................15-12

Chapter 16—Aeronautical Decision MakingOrigins of ADM Training............................16-2The Decision-Making Process.....................16-2

Defining the Problem .............................16-2Choosing a Course of Action .................16-3Implementing the Decision andEvaluating the Outcome ........................16-4

Risk Management........................................16-4Assessing Risk........................................16-5

Factors Affecting Decision Making ............16-5Pilot Self-Assessment.............................16-5Recognizing Hazardous Attitudes ..........16-6Stress Management.................................16-6Use of Resources ....................................16-7

Internal Resources ..............................16-7External Resources .............................16-8

Workload Management...........................16-8Situational Awareness.............................16-8Obstacles to Maintaining SituationalAwareness ..............................................16-9

Operational Pitfalls ......................................16-9

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According to the current Title 14 of the Code of FederalRegulations (14 CFR) part 1, Definitions andAbbreviations, an aircraft is a device that is used, orintended to be used, for flight. Categories of aircraft forcertification of airmen include airplane, rotorcraft,lighter-than-air, powered-lift, and glider. Part 1 alsodefines airplane as an engine-driven, fixed-wing aircraft heavier than air that is supported in flight by thedynamic reaction of air against its wings. This chapter

provides a brief introduction to the airplane and itsmajor components.

MAJOR COMPONENTSAlthough airplanes are designed for a variety of pur-poses, most of them have the same major components.The overall characteristics are largely determined bythe original design objectives. Most airplane structuresinclude a fuselage, wings, an empennage, landing gear,and a powerplant. [Figure 1-1]

Figure 1-1. Airplane components.

Empennage

Wing

Fuselage

PowerplantLanding Gear

Aircraft—A device that is used for flight in the air.

Airplane—An engine-driven, fixed-wing aircraft heavier than air that issupported in flight by the dynamic reaction of air against its wings.

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FUSELAGEThe fuselage includes the cabin and/or cockpit, whichcontains seats for the occupants and the controls forthe airplane. In addition, the fuselage may alsoprovide room for cargo and attachment points for theother major airplane components. Some aircraft uti-lize an open truss structure. The truss-type fuselage isconstructed of steel or aluminum tubing. Strength andrigidity is achieved by welding the tubing togetherinto a series of triangular shapes, called trusses.[Figure 1-2]

Construction of the Warren truss features longerons,as well as diagonal and vertical web members. Toreduce weight, small airplanes generally utilize aluminum alloy tubing, which may be riveted orbolted into one piece with cross-bracing members.

As technology progressed, aircraft designers began toenclose the truss members to streamline the airplaneand improve performance. This was originally accom-plished with cloth fabric, which eventually gave way tolightweight metals such as aluminum. In some cases,the outside skin can support all or a major portion ofthe flight loads. Most modern aircraft use a form of thisstressed skin structure known as monocoque or semi-monocoque construction.

The monocoque design uses stressed skin to supportalmost all imposed loads. This structure can be verystrong but cannot tolerate dents or deformation of thesurface. This characteristic is easily demonstrated by athin aluminum beverage can. You can exert considerableforce to the ends of the can without causing any damage.

However, if the side of the can is dented only slightly,the can will collapse easily. The true monocoque con-struction mainly consists of the skin, formers, andbulkheads. The formers and bulkheads provide shapefor the fuselage. [Figure 1-3]

Since no bracing members are present, the skin must bestrong enough to keep the fuselage rigid. Thus, a significant problem involved in monocoque construc-tion is maintaining enough strength while keeping theweight within allowable limits. Due to the limitations ofthe monocoque design, a semi-monocoque structure isused on many of today’s aircraft.

The semi-monocoque system uses a substructure towhich the airplane’s skin is attached. The substructure,which consists of bulkheads and/or formers of varioussizes and stringers, reinforces the stressed skin by taking some of the bending stress from the fuselage.The main section of the fuselage also includes wingattachment points and a firewall. [Figure 1-4]

Longeron

Diagonal Web Members

VerticalWeb

Members

Figure 1-2.The Warren truss.

Truss—A fuselage design made up of supporting structural membersthat resist deformation by applied loads.

Monocoque—A shell-like fuselage design in which the stressed outerskin is used to support the majority of imposed stresses. Monocoquefuselage design may include bulkheads but not stringers.

Skin Former

Bulkhead

Figure 1-3. Monocoque fuselage design.

Bulkheadsand/or

Formers

Stressed Skin

Wing AttachmentPoints

Firewall

Stringers

Figure 1-4. Semi-monocoque construction.

Semi-Monocoque—A fuselage design that includes a substructure ofbulkheads and/or formers, along with stringers, to support flight loadsand stresses imposed on the fuselage.

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On single-engine airplanes, the engine is usuallyattached to the front of the fuselage. There is a fireproofpartition between the rear of the engine and the cockpitor cabin to protect the pilot and passengers from accidental engine fires. This partition is called a firewall and is usually made of heat-resistant materialsuch as stainless steel.

WINGSThe wings are airfoils attached to each side of the fuselage and are the main lifting surfaces that supportthe airplane in flight. There are numerous wingdesigns, sizes, and shapes used by the various manu-facturers. Each fulfills a certain need with respect tothe expected performance for the particular airplane.How the wing produces lift is explained in subsequentchapters.

Wings may be attached at the top, middle, or lower por-tion of the fuselage. These designs are referred to ashigh-, mid-, and low-wing, respectively. The number ofwings can also vary. Airplanes with a single set ofwings are referred to as monoplanes, while those withtwo sets are called biplanes. [Figure 1-5]

Many high-wing airplanes have external braces, orwing struts, which transmit the flight and landing loads

through the struts to the main fuselage structure. Sincethe wing struts are usually attached approximatelyhalfway out on the wing, this type of wing structure iscalled semi-cantilever. A few high-wing and most low-wing airplanes have a full cantilever wingdesigned to carry the loads without external struts.

The principal structural parts of the wing are spars,ribs, and stringers. [Figure 1-6] These are reinforced by

Airfoil—An airfoil is any surface, such as a wing, propeller, rudder, oreven a trim tab, which provides aerodynamic force when it interactswith a moving stream of air.

Monoplane—An airplane that has only one main lifting surface orwing, usually divided into two parts by the fuselage.

Biplane—An airplane that has two main airfoil surfaces or wings oneach side of the fuselage, one placed above the other.

Figure 1-5. Monoplane and biplane.

Spar

Skin

Wing Flap

Aileron

Stringers

WingTip

Ribs

Spar

Fuel Tank

Figure 1-6. Wing components.

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trusses, I-beams, tubing, or other devices, including theskin. The wing ribs determine the shape and thicknessof the wing (airfoil). In most modern airplanes, the fueltanks either are an integral part of the wing’s structure,or consist of flexible containers mounted inside of thewing.

Attached to the rear, or trailing, edges of the wings aretwo types of control surfaces referred to as ailerons andflaps. Ailerons extend from about the midpoint of eachwing outward toward the tip and move in oppositedirections to create aerodynamic forces that cause theairplane to roll. Flaps extend outward from the fuselage to near the midpoint of each wing. The flapsare normally flush with the wing’s surface during cruising flight. When extended, the flaps move simul-taneously downward to increase the lifting force of thewing for takeoffs and landings.

EMPENNAGEThe correct name for the tail section of an airplane isempennage. The empennage includes the entire tailgroup, consisting of fixed surfaces such as the verticalstabilizer and the horizontal stabilizer. The movable sur-faces include the rudder, the elevator, and one or moretrim tabs. [Figure 1-7]

A second type of empennage design does not requirean elevator. Instead, it incorporates a one-piece hori-zontal stabilizer that pivots from a central hinge point.This type of design is called a stabilator, and is movedusing the control wheel, just as you would the eleva-tor. For example, when you pull back on the controlwheel, the stabilator pivots so the trailing edge movesup. This increases the aerodynamic tail load andcauses the nose of the airplane to move up. Stabilatorshave an antiservo tab extending across their trailingedge. [Figure 1-8]

The antiservo tab moves in the same direction as thetrailing edge of the stabilator. The antiservo tab alsofunctions as a trim tab to relieve control pressures andhelps maintain the stabilator in the desired position.

The rudder is attached to the back of the vertical stabi-lizer. During flight, it is used to move the airplane’snose left and right. The rudder is used in combinationwith the ailerons for turns during flight. The elevator,which is attached to the back of the horizontal stabi-lizer, is used to move the nose of the airplane up anddown during flight.

Trim tabs are small, movable portions of the trailingedge of the control surface. These movable trim tabs,which are controlled from the cockpit, reduce controlpressures. Trim tabs may be installed on the ailerons,the rudder, and/or the elevator.

LANDING GEARThe landing gear is the principle support of the airplanewhen parked, taxiing, taking off, or when landing. The

VerticalStabilizer

HorizontalStabilizer

Rudder

Trim Tabs

Elevator

Figure 1-7. Empennage components.

Empennage—The section of the airplane that consists of the verticalstabilizer, the horizontal stabilizer, and the associated control surfaces.

Stabilator

AntiservoTab

Pivot Point

Figure 1-8. Stabilator components.

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most common type of landing gear consists of wheels,but airplanes can also be equipped with floats for wateroperations, or skis for landing on snow. [Figure 1-9]

The landing gear consists of three wheels—two mainwheels and a third wheel positioned either at the front orrear of the airplane. Landing gear employing a rear-mounted wheel is called conventional landing gear.Airplanes with conventional landing gear are sometimesreferred to as tailwheel airplanes. When the third wheel islocated on the nose, it is called a nosewheel, and thedesign is referred to as a tricycle gear. A steerable nose-wheel or tailwheel permits the airplane to be controlledthroughout all operations while on the ground.

THE POWERPLANTThe powerplant usually includes both the engine andthe propeller. The primary function of the engine is toprovide the power to turn the propeller. It also gener-ates electrical power, provides a vacuum source forsome flight instruments, and in most single-engine

airplanes, provides a source of heat for the pilot andpassengers. The engine is covered by a cowling, or inthe case of some airplanes, surrounded by a nacelle.The purpose of the cowling or nacelle is to stream-line the flow of air around the engine and to help coolthe engine by ducting air around the cylinders. Thepropeller, mounted on the front of the engine, trans-lates the rotating force of the engine into a forward-acting force called thrust that helps move the airplanethrough the air. [Figure 1-10]

Engine

Cowling

Propeller

Firewall

Figure 1-10. Engine compartment.

Figure 1-9. Landing gear.

Nacelle—A streamlined enclosure on an aircraft in which an engine ismounted. On multiengine propeller-driven airplanes, the nacelle is normally mounted on the leading edge of the wing.

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This chapter discusses the fundamental physical lawsgoverning the forces acting on an airplane in flight, andwhat effect these natural laws and forces have on theperformance characteristics of airplanes. Tocompetently control the airplane, the pilot mustunderstand the principles involved and learn to utilizeor counteract these natural forces.

Modern general aviation airplanes have what maybe considered high performance characteristics.Therefore, it is increasingly necessary that pilotsappreciate and understand the principles upon whichthe art of flying is based.

STRUCTURE OF THE ATMOSPHEREThe atmosphere in which flight is conducted is anenvelope of air that surrounds the earth and restsupon its surface. It is as much a part of the earth asthe seas or the land. However, air differs from landand water inasmuch as it is a mixture of gases. It hasmass, weight, and indefinite shape.

Air, like any other fluid, is able to flow and change itsshape when subjected to even minute pressures becauseof the lack of strong molecular cohesion. For example,gas will completely fill any container into which it isplaced, expanding or contracting to adjust its shape tothe limits of the container.

The atmosphere is composed of 78 percent nitrogen, 21percent oxygen, and 1 percent other gases, such asargon or helium. As some of these elements are heavierthan others, there is a natural tendency of these heavierelements, such as oxygen, to settle to the surface of theearth, while the lighter elements are lifted up to theregion of higher altitude. This explains why most of theoxygen is contained below 35,000 feet altitude.

Because air has mass and weight, it is a body, and as abody, it reacts to the scientific laws of bodies in thesame manner as other gaseous bodies. This body of airresting upon the surface of the earth has weight and atsea level develops an average pressure of 14.7 poundson each square inch of surface, or 29.92 inches of

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mercury—but as its thickness is limited, the higherthe altitude, the less air there is above. For thisreason, the weight of the atmosphere at 18,000 feetis only one-half what it is at sea level. [Figure 2-1]

ATMOSPHERIC PRESSUREThough there are various kinds of pressure, thisdiscussion is mainly concerned with atmosphericpressure. It is one of the basic factors in weatherchanges, helps to lift the airplane, and actuates someof the important flight instruments in the airplane.These instruments are the altimeter, the airspeedindicator, the rate-of-climb indicator, and themanifold pressure gauge.

Though air is very light, it has mass and is affectedby the attraction of gravity. Therefore, like any othersubstance, it has weight, and because of its weight, ithas force. Since it is a fluid substance, this force isexerted equally in all directions, and its effect onbodies within the air is called pressure. Understandard conditions at sea level, the average pressureexerted on the human body by the weight of theatmosphere around it is approximately 14.7 lb./in.The density of air has significant effects on theairplane’s capability. As air becomes less dense, itreduces (1) power because the engine takes in lessair, (2) thrust because the propeller is less efficient inthin air, and (3) lift because the thin air exerts lessforce on the airfoils.

EFFECTS OF PRESSURE ON DENSITYSince air is a gas, it can be compressed or expanded.When air is compressed, a greater amount of air can

occupy a given volume. Conversely, when pressureon a given volume of air is decreased, the airexpands and occupies a greater space. That is, theoriginal column of air at a lower pressure contains asmaller mass of air. In other words, the density isdecreased. In fact, density is directly proportional topressure. If the pressure is doubled, the density isdoubled, and if the pressure is lowered, so is thedensity. This statement is true, only at aconstant temperature.

EFFECT OF TEMPERATURE ON DENSITYThe effect of increasing the temperature of asubstance is to decrease its density. Conversely,decreasing the temperature has the effect ofincreasing the density. Thus, the density of air variesinversely as the absolute temperature varies. Thisstatement is true, only at a constant pressure.

In the atmosphere, both temperature and pressuredecrease with altitude, and have conflicting effectsupon density. However, the fairly rapid drop inpressure as altitude is increased usually has thedominating effect. Hence, density can be expected todecrease with altitude.

EFFECT OF HUMIDITY ON DENSITYThe preceding paragraphs have assumed that the airwas perfectly dry. In reality, it is never completelydry. The small amount of water vapor suspended inthe atmosphere may be almost negligible undercertain conditions, but in other conditions humiditymay become an important factor in the performanceof an airplane. Water vapor is lighter than air;consequently, moist air is lighter than dry air. It islightest or least dense when, in a given set ofconditions, it contains the maximum amount ofwater vapor. The higher the temperature, the greateramount of water vapor the air can hold. Whencomparing two separate air masses, the first warmand moist (both qualities tending to lighten the air)and the second cold and dry (both qualities making itheavier), the first necessarily must be less dense thanthe second. Pressure, temperature, and humidityhave a great influence on airplane performance,because of their effect upon density.

NEWTON’S LAWS OF MOTION ANDFORCEIn the 17th century, a philosopher andmathematician, Sir Isaac Newton, propounded threebasic laws of motion. It is certain that he did not havethe airplane in mind when he did so, but almosteverything known about motion goes back to histhree simple laws. These laws, named after Newton,are as follows:

Newton’s first law states, in part, that: A body at resttends to remain at rest, and a body in motion tends to

29.92 30

25

20

15

10

5

0

Inch

es o

f Mer

cury

AtmosphericPressure

StandardSea LevelPressure

Figure 2-1. Standard sea level pressure.

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remain moving at the same speed and in thesame direction.

This simply means that, in nature, nothing starts orstops moving until some outside force causes it to doso. An airplane at rest on the ramp will remain at restunless a force strong enough to overcome its inertia isapplied. Once it is moving, however, its inertia keeps itmoving, subject to the various other forces acting on it.These forces may add to its motion, slow it down, orchange its direction.

Newton’s second law implies that: When a body isacted upon by a constant force, its resultingacceleration is inversely proportional to the mass of thebody and is directly proportional to the applied force.

What is being dealt with here are the factors involvedin overcoming Newton’s First Law of Inertia. It coversboth changes in direction and speed, including startingup from rest (positive acceleration) and coming to astop (negative acceleration, or deceleration).

Newton’s third law states that: Whenever one bodyexerts a force on another, the second body alwaysexerts on the first, a force that is equal in magnitude butopposite in direction.

The recoil of a gun as it is fired is a graphic example ofNewton’s third law. The champion swimmer whopushes against the side of the pool during theturnaround, or the infant learning to walk—both wouldfail but for the phenomena expressed in this law. In anairplane, the propeller moves and pushes back the air;consequently, the air pushes the propeller (and thus theairplane) in the opposite direction—forward. In a jetairplane, the engine pushes a blast of hot gasesbackward; the force of equal and opposite reactionpushes against the engine and forces the airplaneforward. The movement of all vehicles is a graphicillustration of Newton’s third law.

MAGNUS EFFECTThe explanation of lift can best be explained by lookingat a cylinder rotating in an airstream. The local velocitynear the cylinder is composed of the airstream velocityand the cylinder’s rotational velocity, which decreaseswith distance from the cylinder. On a cylinder, which isrotating in such a way that the top surface area is rotatingin the same direction as the airflow, the local velocity atthe surface is high on top and low on the bottom.

As shown in figure 2-2, at point “A,” a stagnation pointexists where the airstream line that impinges on thesurface splits; some air goes over and some under.Another stagnation point exists at “B,” where the two

airstreams rejoin and resume at identical velocities. Wenow have upwash ahead of the rotating cylinder anddownwash at the rear.

The difference in surface velocity accounts for a differ-ence in pressure, with the pressure being lower on thetop than the bottom. This low pressure area produces anupward force known as the “Magnus Effect.” Thismechanically induced circulation illustrates therelationship between circulation and lift.

An airfoil with a positive angle of attack develops aircirculation as its sharp trailing edge forces the rearstagnation point to be aft of the trailing edge, while thefront stagnation point is below the leading edge.[Figure 2-3]

BERNOULLI’S PRINCIPLE OFPRESSUREA half century after Sir Newton presented his laws,Mr. Daniel Bernoulli, a Swiss mathematician,explained how the pressure of a moving fluid (liquidor gas) varies with its speed of motion. Specifically,

B A

Increased Local Velocity(Decreased pressure)

Decreased Local Velocity

Downwash Upwash

Figure 2-2. Magnus Effect is a lifting force produced when arotating cylinder produces a pressure differential. This is thesame effect that makes a baseball curve or a golf ball slice.

Leading EdgeStagnation Point

Trailing EdgeStagnation Point

B

A

Figure 2-3. Air circulation around an airfoil occurs when thefront stagnation point is below the leading edge and the aftstagnation point is beyond the trailing edge.

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he stated that an increase in the speed of movementor flow would cause a decrease in the fluid’spressure. This is exactly what happens to air passingover the curved top of the airplane wing.

An appropriate analogy can be made with waterflowing through a garden hose. Water moving througha hose of constant diameter exerts a uniform pressureon the hose; but if the diameter of a section of the hoseis increased or decreased, it is certain to change thepressure of the water at that point. Suppose the hosewas pinched, thereby constricting the area throughwhich the water flows. Assuming that the same volumeof water flows through the constricted portion of thehose in the same period of time as before the hose waspinched, it follows that the speed of flow must increaseat that point.

Therefore, if a portion of the hose is constricted, it notonly increases the speed of the flow, but also decreasesthe pressure at that point. Like results could beachieved if streamlined solids (airfoils) wereintroduced at the same point in the hose. This sameprinciple is the basis for the measurement of airspeed(fluid flow) and for analyzing the airfoil’s ability toproduce lift.

A practical application of Bernoulli’s theorem is theventuri tube. The venturi tube has an air inlet whichnarrows to a throat (constricted point) and an outletsection which increases in diameter toward the rear.The diameter of the outlet is the same as that of theinlet. At the throat, the airflow speeds up and thepressure decreases; at the outlet, the airflow slowsand the pressure increases. [Figure 2-4]

If air is recognized as a body and it is accepted that itmust follow the above laws, one can begin to seehow and why an airplane wing develops lift as itmoves through the air.

AIRFOIL DESIGNIn the sections devoted to Newton’s and Bernoulli’sdiscoveries, it has already been discussed in general

terms the question of how an airplane wing cansustain flight when the airplane is heavier than air.Perhaps the explanation can best be reduced to itsmost elementary concept by stating that lift (flight)is simply the result of fluid flow (air) about anairfoil—or in everyday language, the result ofmoving an airfoil (wing), by whatever means,through the air.

Since it is the airfoil which harnesses the forcedeveloped by its movement through the air, adiscussion and explanation of this structure, as well assome of the material presented in previous discussionson Newton’s and Bernoulli’s laws, will be presented.

An airfoil is a structure designed to obtain reactionupon its surface from the air through which it moves orthat moves past such a structure. Air acts in variousways when submitted to different pressures andvelocities; but this discussion will be confined to theparts of an airplane that a pilot is most concerned within flight—namely, the airfoils designed to produce lift.By looking at a typical airfoil profile, such as the crosssection of a wing, one can see several obviouscharacteristics of design. [Figure 2-5] Notice that thereis a difference in the curvatures of the upper and lowersurfaces of the airfoil (the curvature is called camber).The camber of the upper surface is more pronouncedthan that of the lower surface, which is somewhat flatin most instances.

In figure 2-5, note that the two extremities of theairfoil profile also differ in appearance. The endwhich faces forward in flight is called the leadingedge, and is rounded; while the other end, thetrailing edge, is quite narrow and tapered.

LeadingEdge

Trailing Edge

Camber of Upper Surface

Camber of Lower Surface

Chord Line

Figure 2-5.Typical airfoil section.

Velocity Pressure

LOW HIGH LOW HIGH

Velocity Pressure

LOW HIGH LOW HIGH

Velocity Pressure

LOW HIGH LOW HIGH

Figure 2-4. Air pressure decreases in a venturi.

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A reference line often used in discussing the airfoil isthe chord line, a straight line drawn through the profileconnecting the extremities of the leading and trailingedges. The distance from this chord line to the upperand lower surfaces of the wing denotes the magnitudeof the upper and lower camber at any point. Anotherreference line, drawn from the leading edge to thetrailing edge, is the “mean camber line.” This mean lineis equidistant at all points from the upper andlower contours.

The construction of the wing, so as to provide actionsgreater than its weight, is done by shaping the wing sothat advantage can be taken of the air’s response tocertain physical laws, and thus develop two actionsfrom the air mass; a positive pressure lifting actionfrom the air mass below the wing, and a negativepressure lifting action from lowered pressure above thewing.

As the airstream strikes the relatively flat lower surfaceof the wing when inclined at a small angle to itsdirection of motion, the air is forced to rebounddownward and therefore causes an upward reactionin positive lift, while at the same time airstreamstriking the upper curved section of the “leadingedge” of the wing is deflected upward. In otherwords, a wing shaped to cause an action on the air,and forcing it downward, will provide an equalreaction from the air, forcing the wing upward. If awing is constructed in such form that it will cause alift force greater than the weight of the airplane, theairplane will fly.

However, if all the lift required were obtained merelyfrom the deflection of air by the lower surface of thewing, an airplane would need only a flat wing like akite. This, of course, is not the case at all; under certainconditions disturbed air currents circulating at thetrailing edge of the wing could be so excessive as tomake the airplane lose speed and lift. The balance ofthe lift needed to support the airplane comes from theflow of air above the wing. Herein lies the key to flight.The fact that most lift is the result of the airflow’sdownwash from above the wing, must be thoroughlyunderstood in order to continue further in the study offlight. It is neither accurate nor does it serve a usefulpurpose, however, to assign specific values to thepercentage of lift generated by the upper surface of anairfoil versus that generated by the lower surface.These are not constant values and will vary, not onlywith flight conditions, but with different wing designs.

It should be understood that different airfoils havedifferent flight characteristics. Many thousands ofairfoils have been tested in wind tunnels and in actualflight, but no one airfoil has been found that satisfiesevery flight requirement. The weight, speed, and

purpose of each airplane dictate the shape of itsairfoil. It was learned many years ago that the mostefficient airfoil for producing the greatest lift wasone that had a concave, or “scooped out” lowersurface. Later it was also learned that as a fixeddesign, this type of airfoil sacrificed too much speedwhile producing lift and, therefore, was not suitablefor high-speed flight. It is interesting to note,however, that through advanced progress inengineering, today’s high-speed jets can again takeadvantage of the concave airfoil’s high liftcharacteristics. Leading edge (Kreuger) flaps andtrailing edge (Fowler) flaps, when extended from thebasic wing structure, literally change theairfoil shape into the classic concave form,thereby generating much greater lift during slowflight conditions.

On the other hand, an airfoil that is perfectlystreamlined and offers little wind resistancesometimes does not have enough lifting power totake the airplane off the ground. Thus, modernairplanes have airfoils which strike a mediumbetween extremes in design, the shape varyingaccording to the needs of the airplane for which it isdesigned. Figure 2-6 shows some of the morecommon airfoil sections.

LOW PRESSURE ABOVEIn a wind tunnel or in flight, an airfoil is simply astreamlined object inserted into a moving stream ofair. If the airfoil profile were in the shape of ateardrop, the speed and the pressure changes of theair passing over the top and bottom would be thesame on both sides. But if the teardrop shaped airfoilwere cut in half lengthwise, a form resembling thebasic airfoil (wing) section would result. If theairfoil were then inclined so the airflow strikes it atan angle (angle of attack), the air molecules movingover the upper surface would be forced to movefaster than would the molecules moving along thebottom of the airfoil, since the upper molecules musttravel a greater distance due to the curvature of theupper surface. This increased velocity reduces thepressure above the airfoil.

Early Airfoil Laminar Flow Airfoil(Subsonic)

Later AirfoilCircular Arc Airfoil

(Supersonic)

Double Wedge Airfoil(Supersonic)

Clark 'Y' Airfoil(Subsonic)

Figure 2-6. Airfoil designs.

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Bernoulli’s principle of pressure by itself does notexplain the distribution of pressure over the uppersurface of the airfoil. A discussion of the influence ofmomentum of the air as it flows in various curvedpaths near the airfoil will be presented. [Figure 2-7]Momentum is the resistance a moving body offers tohaving its direction or amount of motion changed.When a body is forced to move in a circular path, itoffers resistance in the direction away from thecenter of the curved path. This is “centrifugal force.”While the particles of air move in the curved pathAB, centrifugal force tends to throw them in thedirection of the arrows between A and B and hence,causes the air to exert more than normal pressure onthe leading edge of the airfoil. But after the airparticles pass B (the point of reversal of thecurvature of the path) the centrifugal force tends tothrow them in the direction of the arrows between Band C (causing reduced pressure on the airfoil). Thiseffect is held until the particles reach C, the secondpoint of reversal of curvature of the airflow. Againthe centrifugal force is reversed and the particlesmay even tend to give slightly more than normalpressure on the trailing edge of the airfoil, asindicated by the short arrows between C and D.

Therefore, the air pressure on the upper surface ofthe airfoil is distributed so that the pressure is muchgreater on the leading edge than the surroundingatmospheric pressure, causing strong resistance toforward motion; but the air pressure is less thansurrounding atmospheric pressure over a largeportion of the top surface (B to C).

As seen in the application of Bernoulli’s theorem to aventuri, the speedup of air on the top of an airfoilproduces a drop in pressure. This lowered pressure is acomponent of total lift. It is a mistake, however, toassume that the pressure difference between the upperand lower surface of a wing alone accounts for the totallift force produced.

One must also bear in mind that associated with thelowered pressure is downwash; a downward backwardflow from the top surface of the wing. As already seenfrom previous discussions relative to the dynamicaction of the air as it strikes the lower surface of thewing, the reaction of this downward backward flow

results in an upward forward force on the wing. Thissame reaction applies to the flow of air over the topof the airfoil as well as to the bottom, and Newton’sthird law is again in the picture.

HIGH PRESSURE BELOWIn the section dealing with Newton’s laws as theyapply to lift, it has already been discussed how acertain amount of lift is generated by pressureconditions underneath the wing. Because of themanner in which air flows underneath the wing, apositive pressure results, particularly at higherangles of attack. But there is another aspect to thisairflow that must be considered. At a point close tothe leading edge, the airflow is virtually stopped(stagnation point) and then gradually increasesspeed. At some point near the trailing edge, it hasagain reached a velocity equal to that on the uppersurface. In conformance with Bernoulli’s principles,where the airflow was slowed beneath the wing, apositive upward pressure was created against thewing; i.e., as the fluid speed decreases, the pressuremust increase. In essence, this simply “accentuatesthe positive” since it increases the pressuredifferential between the upper and lower surface ofthe airfoil, and therefore increases total lift over thatwhich would have resulted had there been noincrease of pressure at the lower surface. BothBernoulli’s principle and Newton’s laws are inoperation whenever lift is being generated byan airfoil.

Fluid flow or airflow then, is the basis for flight inairplanes, and is a product of the velocity of theairplane. The velocity of the airplane is veryimportant to the pilot since it affects the lift and dragforces of the airplane. The pilot uses the velocity(airspeed) to fly at a minimum glide angle, atmaximum endurance, and for a number of otherflight maneuvers. Airspeed is the velocity of theairplane relative to the air mass through which itis flying.

PRESSURE DISTRIBUTIONFrom experiments conducted on wind tunnel modelsand on full size airplanes, it has been determined thatas air flows along the surface of a wing at differentangles of attack, there are regions along the surfacewhere the pressure is negative, or less thanatmospheric, and regions where the pressure ispositive, or greater than atmospheric. This negativepressure on the upper surface creates a relativelylarger force on the wing than is caused by thepositive pressure resulting from the air striking thelower wing surface. Figure 2-8 shows the pressuredistribution along an airfoil at three different anglesof attack. In general, at high angles of attack the

IncreasedPressure

IncreasedPressure

ReducedPressure

C

DA

B

Figure 2-7. Momentum influences airflow over an airfoil.

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center of pressure moves forward, while at lowangles of attack the center of pressure moves aft. Inthe design of wing structures, this center of pressuretravel is very important, since it affects the positionof the airloads imposed on the wing structure in lowangle-of-attack conditions and high angle-of-attackconditions. The airplane’s aerodynamic balance andcontrollability are governed by changes in the centerof pressure.

The center of pressure is determined throughcalculation and wind tunnel tests by varying theairfoil’s angle of attack through normal operatingextremes. As the angle of attack is changed, so arethe various pressure distribution characteristics.[Figure 2-8] Positive (+) and negative (–) pressureforces are totaled for each angle of attack and theresultant force is obtained. The total resultantpressure is represented by the resultant force vectorshown in figure 2-9.

The point of application of this force vector istermed the “center of pressure” (CP). For any given

angle of attack, the center of pressure is the pointwhere the resultant force crosses the chord line. Thispoint is expressed as a percentage of the chord of theairfoil. A center of pressure at 30 percent of a 60-inch chord would be 18 inches aft of the wing’sleading edge. It would appear then that if thedesigner would place the wing so that its center ofpressure was at the airplane’s center of gravity, theairplane would always balance. The difficulty arises,however, that the location of the center of pressurechanges with change in the airfoil’s angle of attack.[Figure 2-10]

In the airplane’s normal range of flight attitudes, ifthe angle of attack is increased, the center ofpressure moves forward; and if decreased, it movesrearward. Since the center of gravity is fixed at onepoint, it is evident that as the angle of attackincreases, the center of lift (CL) moves ahead of thecenter of gravity, creating a force which tends toraise the nose of the airplane or tends to increase theangle of attack still more. On the other hand, if theangle of attack is decreased, the center of lift (CL)moves aft and tends to decrease the angle a greateramount. It is seen then, that the ordinary airfoil isinherently unstable, and that an auxiliary device,such as the horizontal tail surface, must be added tomake the airplane balance longitudinally.

The balance of an airplane in flight depends, therefore,on the relative position of the center of gravity (CG)and the center of pressure (CP) of the airfoil.Experience has shown that an airplane with the center

+4°

+10°Angle ofAttack

Angle ofAttack

-8°Angle ofAttack

Figure 2-8. Pressure distribution on an airfoil.

Chord LineAngle ofAttack

Relative Wind

Lift

Drag

Res

ulta

ntFo

rce

Center ofPressure

Figure 2-9. Force vectors on an airfoil.

Angle of Attack

Angle of Attack

Angle of Attack

CP

CP

CP

CG

CG

CG

Figure 2-10. CP changes with an angle of attack.

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of gravity in the vicinity of 20 percent of the wingchord can be made to balance and fly satisfactorily.

The tapered wing presents a variety of wing chordsthroughout the span of the wing. It becomesnecessary then, to specify some chord about whichthe point of balance can be expressed. This chord,known as the mean aerodynamic chord (MAC),

usually is defined as the chord of an imaginaryuntapered wing, which would have the same centerof pressure characteristics as the wing in question.

Airplane loading and weight distribution also affectcenter of gravity and cause additional forces, whichin turn affect airplane balance.

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FORCES ACTING ON THE AIRPLANEIn some respects at least, how well a pilot performs inflight depends upon the ability to plan and coordinatethe use of the power and flight controls for changingthe forces of thrust, drag, lift, and weight. It is the bal-ance between these forces that the pilot must alwayscontrol. The better the understanding of the forces andmeans of controlling them, the greater will be thepilot’s skill at doing so.

The following defines these forces in relation tostraight-and-level, unaccelerated flight.

Thrust is the forward force produced by the power-plant/propeller. It opposes or overcomes the force ofdrag. As a general rule, it is said to act parallel to thelongitudinal axis. However, this is not always the caseas will be explained later.

Drag is a rearward, retarding force, and is caused bydisruption of airflow by the wing, fuselage, and otherprotruding objects. Drag opposes thrust, and acts rear-ward parallel to the relative wind.

Weight is the combined load of the airplane itself, thecrew, the fuel, and the cargo or baggage. Weight pullsthe airplane downward because of the force of gravity.It opposes lift, and acts vertically downward throughthe airplane’s center of gravity.

Lift opposes the downward force of weight, is pro-duced by the dynamic effect of the air acting on thewing, and acts perpendicular to the flightpath throughthe wing’s center of lift.

In steady flight, the sum of these opposing forces isequal to zero. There can be no unbalanced forces insteady, straight flight (Newton’s Third Law). This istrue whether flying level or when climbing ordescending. This is not the same thing as saying thatthe four forces are all equal. It simply means thatthe opposing forces are equal to, and thereby cancelthe effects of, each other. Often the relationshipbetween the four forces has been erroneouslyexplained or illustrated in such a way that this pointis obscured. Consider figure 3-1 on the next page,for example. In the upper illustration the force vectorsof thrust, drag, lift, and weight appear to be equal invalue. The usual explanation states (without stipulat-ing that thrust and drag do not equal weight and lift)that thrust equals drag and lift equals weight as shownin the lower illustration. This basically true statementmust be understood or it can be misleading. It shouldbe understood that in straight, level, unacceleratedflight, it is true that the opposing lift/weight forcesare equal, but they are also greater than the oppos-ing forces of thrust/drag that are equal only to eachother; not to lift/weight. To be correct about it, itmust be said that in steady flight:

• The sum of all upward forces (not just lift) equalsthe sum of all downward forces (not just weight).

• The sum of all forward forces (not just thrust)equals the sum of all backward forces (not justdrag).

This refinement of the old “thrust equals drag; liftequals weight” formula takes into account the fact that

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in climbs a portion of thrust, since it is directed upward,acts as if it were lift; and a portion of weight, since it isdirected backward, acts as if it were drag. In glides, aportion of the weight vector is directed forward, andtherefore acts as thrust. In other words, any time theflightpath of the airplane is not horizontal, lift, weight,thrust, and drag vectors must each be broken down intotwo components. [Figure 3-2]

Figure 3-2. Force vectors during a stabilized climb.

Discussions of the preceding concepts are frequentlyomitted in aeronautical texts/handbooks/manuals. Thereason is not that they are of no consequence, butbecause by omitting such discussions, the main ideaswith respect to the aerodynamic forces acting upon

an airplane in flight can be presented in their mostessential elements without being involved in thetechnicalities of the aerodynamicist. In point of fact,considering only level flight, and normal climbs andglides in a steady state, it is still true that wing lift isthe really important upward force, and weight is thereally important downward force.

Frequently, much of the difficulty encountered inexplaining the forces that act upon an airplane is largelya matter of language and its meaning. For example,pilots have long believed that an airplane climbsbecause of excess lift. This is not true if one is thinkingin terms of wing lift alone. It is true, however, if by liftit is meant the sum total of all “upward forces.” Butwhen referring to the “lift of thrust” or the “thrust ofweight,” the definitions previously established forthese forces are no longer valid and complicate mat-ters. It is this impreciseness in language that affords theexcuse to engage in arguments, largely academic, overrefinements to basic principles.

Though the forces acting on an airplane have alreadybeen defined, a discussion in more detail to establishhow the pilot uses them to produce controlled flightis appropriate.

THRUSTBefore the airplane begins to move, thrust must beexerted. It continues to move and gain speed untilthrust and drag are equal. In order to maintain a con-stant airspeed, thrust and drag must remain equal,just as lift and weight must be equal to maintain aconstant altitude. If in level flight, the engine poweris reduced, the thrust is lessened, and the airplaneslows down. As long as the thrust is less than thedrag, the airplane continues to decelerate until itsairspeed is insufficient to support it in the air.

Likewise, if the engine power is increased, thrustbecomes greater than drag and the airspeedincreases. As long as the thrust continues to begreater than the drag, the airplane continues to accel-erate. When drag equals thrust, the airplane flies at aconstant airspeed.

Straight-and-level flight may be sustained at speedsfrom very slow to very fast. The pilot must coordi-nate angle of attack and thrust in all speed regimes ifthe airplane is to be held in level flight. Roughly,these regimes can be grouped in three categories:low-speed flight, cruising flight, and high-speedflight.

When the airspeed is low, the angle of attack must berelatively high to increase lift if the balance betweenlift and weight is to be maintained. [Figure 3-3] Ifthrust decreases and airspeed decreases, lift becomes

Figure 3-1. Relationship of forces acting on an airplane.

Incorrect Relationship

Correct Relationship

Weight

Weight

Lift

Lift

Thrust

Thrust

Drag

Drag

Drag

Thrust

Flight Path

Relative Wind

Component of Weight Opposed

to Lift

Rearward Componentof Weight

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less than weight and the airplane will start todescend. To maintain level flight, the pilot canincrease the angle of attack an amount which willgenerate a lift force again equal to the weight of theairplane and while the airplane will be flying moreslowly, it will still maintain level flight if the pilothas properly coordinated thrust and angle of attack.

Straight-and-level flight in the slow speed regimeprovides some interesting conditions relative to theequilibrium of forces, because with the airplane in anose-high attitude, there is a vertical component ofthrust that helps support the airplane. For one thing,wing loading tends to be less than would beexpected. Most pilots are aware that an airplane willstall, other conditions being equal, at a slower speedwith the power on than with the power off. (Inducedairflow over the wings from the propeller also con-tributes to this.) However, if analysis is restricted tothe four forces as they are usually defined, one cansay that in straight-and-level slow speed flight thethrust is equal to drag, and lift is equal to weight.

During straight-and level-flight when thrust isincreased and the airspeed increases, the angle ofattack must be decreased. That is, if changes havebeen coordinated, the airplane will still remain inlevel flight but at a higher speed when the properrelationship between thrust and angle of attack isestablished.

If the angle of attack were not coordinated(decreased) with this increase of thrust, the airplanewould climb. But decreasing the angle of attackmodifies the lift, keeping it equal to the weight, andif properly done, the airplane still remains in levelflight. Level flight at even slightly negative angles ofattack is possible at very high speed. It is evidentthen, that level flight can be performed with anyangle of attack between stalling angle and the rela-tively small negative angles found at high speed.

DRAGDrag in flight is of two basic types: parasite dragand induced drag. The first is called parasitebecause it in no way functions to aid flight, whilethe second is induced or created as a result of thewing developing lift.

Parasite drag is composed of two basic elements:form drag, resulting from the disruption of thestreamline flow; and the resistance of skin friction.

Of the two components of parasite drag, form drag isthe easier to reduce when designing an airplane. Ingeneral, a more streamlined object produces the bestform to reduce parasite drag.

Skin friction is the type of parasite drag that is mostdifficult to reduce. No surface is perfectly smooth.Even machined surfaces, when inspected throughmagnification, have a ragged, uneven appearance.This rough surface will deflect the streamlines of airon the surface, causing resistance to smooth airflow.Skin friction can be minimized by employing a glossy,flat finish to surfaces, and by eliminating protrudingrivet heads, roughness, and other irregularities.

Another element must be added to the considera-tion of parasite drag when designing an airplane.This drag combines the effects of form drag andskin friction and is called interference drag. If twoobjects are placed adjacent to one another, theresulting turbulence produced may be 50 to 200percent greater than the parts tested separately.

The three elements, form drag, skin friction, andinterference drag, are all computed to determineparasite drag on an airplane.

Shape of an object is a big factor in parasite drag.However, indicated airspeed is an equally importantfactor when speaking of parasite drag. The profiledrag of a streamlined object held in a fixed positionrelative to the airflow increases approximately as thesquare of the velocity; thus, doubling the airspeedincreases the drag four times, and tripling the airspeedincreases the drag nine times. This relationship, how-ever, holds good only at comparatively low subsonicspeeds. At some higher airspeeds, the rate at whichprofile drag has been increased with speed suddenlybegins to increase more rapidly.

The second basic type of drag is induced drag. It isan established physical fact that no system, whichdoes work in the mechanical sense, can be 100 per-cent efficient. This means that whatever the nature

Flight Path

Relative Wind

12°

Level (Low Speed)

Flight Path

Relative Wind

6°

Level (Cruise Speed)

Flight Path

Relative Wind

3°

Level (High Speed)

Figure 3-3. Angle of attack at various speeds.

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of the system, the required work is obtained at theexpense of certain additional work that is dissipatedor lost in the system. The more efficient the system,the smaller this loss.

In level flight the aerodynamic properties of the wingproduce a required lift, but this can be obtained onlyat the expense of a certain penalty. The name given tothis penalty is induced drag. Induced drag is inherentwhenever a wing is producing lift and, in fact, thistype of drag is inseparable from the production of lift.Consequently, it is always present if lift is produced.

The wing produces the lift force by making use ofthe energy of the free airstream. Whenever the wingis producing lift, the pressure on the lower surface ofthe wing is greater than that on the upper surface. Asa result, the air tends to flow from the high pressurearea below the wingtip upward to the low pressurearea above the wing. In the vicinity of the wingtips,there is a tendency for these pressures to equalize,resulting in a lateral flow outward from the under-side to the upper surface of the wing. This lateralflow imparts a rotational velocity to the air at thewingtips and trails behind the wing. Therefore, flowabout the wingtips will be in the form of two vorticestrailing behind as the wings move on.

When the airplane is viewed from the tail, thesevortices will circulate counterclockwise about theright wingtip and clockwise about the left wingtip.[Figure 3-4] Bearing in mind the direction of rota-tion of these vortices, it can be seen that they inducean upward flow of air beyond the wingtip, and adownwash flow behind the wing’s trailing edge. Thisinduced downwash has nothing in common with thedownwash that is necessary to produce lift. It is, infact, the source of induced drag. The greater the sizeand strength of the vortices and consequent down-wash component on the net airflow over the wing,the greater the induced drag effect becomes. Thisdownwash over the top of the wing at the tip has thesame effect as bending the lift vector rearward;therefore, the lift is slightly aft of perpendicular tothe relative wind, creating a rearward lift component.This is induced drag.

It should be remembered that in order to create agreater negative pressure on the top of the wing, thewing can be inclined to a higher angle of attack; also,that if the angle of attack of an asymmetrical wingwere zero, there would be no pressure differentialand consequently no downwash component; there-fore, no induced drag. In any case, as angle of attackincreases, induced drag increases proportionally.

To state this another way—the lower the airspeed thegreater the angle of attack required to produce lift

equal to the airplane’s weight and consequently, thegreater will be the induced drag. The amount ofinduced drag varies inversely as the square of theairspeed.

From the foregoing discussion, it can be noted thatparasite drag increases as the square of the airspeed,and induced drag varies inversely as the square ofthe airspeed. It can be seen that as airspeeddecreases to near the stalling speed, the total dragbecomes greater, due mainly to the sharp rise ininduced drag. Similarly, as the airspeed reaches theterminal velocity of the airplane, the total dragagain increases rapidly, due to the sharp increaseof parasite drag. As seen in figure 3-5, at somegiven airspeed, total drag is at its maximumamount. This is very important in figuring themaximum endurance and range of airplanes; forwhen drag is at a minimum, power required toovercome drag is also at a minimum.

To understand the effect of lift and drag on an air-plane in flight, both must be combined and thelift/drag ratio considered. With the lift and drag data

AirflowAbove Wing

AirflowBelow Wing

Air Spillage

Vorte

x

Figure 3-4. Wingtip vortices.

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available for various airspeeds of the airplane insteady, unaccelerated flight, the proportions of CL(Coefficient of Lift) and CD (Coefficient of Drag)can be calculated for each specific angle of attack.The resulting plot for lift/drag ratio with angle ofattack shows that L/D increases to some maximum,then decreases at the higher lift coefficients andangles of attack, as shown in figure 3-6. Note thatthe maximum lift/drag ratio, (L/D max) occurs at onespecific angle of attack and lift coefficient. If the air-plane is operated in steady flight at L/D max, thetotal drag is at a minimum. Any angle of attack loweror higher than that for L/D max reduces the lift/dragratio and consequently increases the total drag for agiven airplane’s lift.

The location of the center of gravity (CG) is determinedby the general design of each particular airplane. The

designers determine how far the center of pressure (CP)will travel. They then fix the center of gravity forwardof the center of pressure for the corresponding flightspeed in order to provide an adequate restoring momentto retain flight equilibrium.

The configuration of an airplane has a great effect onthe lift/drag ratio. The high performance sailplanemay have extremely high lift/drag ratios. The super-sonic fighter may have seemingly low lift/drag ratiosin subsonic flight, but the airplane configurationsrequired for supersonic flight (and high L/Ds at highMach numbers) cause this situation.

WEIGHTGravity is the pulling force that tends to draw allbodies to the center of the earth. The center of gravity(CG) may be considered as a point at which all theweight of the airplane is concentrated. If the airplanewere supported at its exact center of gravity, it wouldbalance in any attitude. It will be noted that center ofgravity is of major importance in an airplane, for itsposition has a great bearing upon stability.

The location of the center of gravity is determinedby the general design of each particular airplane. Thedesigners determine how far the center of pressure(CP) will travel. They then fix the center of gravityforward of the center of pressure for the correspon-ding flight speed in order to provide an adequaterestoring moment to retain flight equilibrium.

Weight has a definite relationship with lift, and thrustwith drag. This relationship is simple, but importantin understanding the aerodynamics of flying. Lift

is the upward force onthe wing acting perpen-dicular to the relativewind. Lift is required tocounteract the airplane’sweight (which is causedby the force of gravityacting on the mass of theairplane). This weight(gravity) force actsdownward through theairplane’s center ofgravity. In stabilizedlevel flight, when the liftforce is equal to theweight force, the air-plane is in a state ofequilibrium and neithergains nor loses altitude.If lift becomes less thanweight, the airplane loses

Figure 3-5. Drag versus speed.

Dra

g -

Pou

nds Parasite Drag

Induced Drag

Total Drag

Speed

Sta

ll

(L/D)MAX

L/D

CLMAX

CD

CD

CLCL

STALL

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

18

16

14

12

10

8

6

4

2

0

LD

00

2 4 6 8 10 12 14 16 18 20 22

.2000

.1800

.1600

.1400

.1200

.1000

.0800

.0600

.0400

.0200

Angle of Attack, Degrees

Figure 3-6. Lift coefficients at various angles of attack.

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altitude. When the lift is greater than weight, the air-plane gains altitude.

LIFTThe pilot can control the lift. Any time the controlwheel is more fore or aft, the angle of attack ischanged. As angle of attack increases, lift increases(all other factors being equal). When the airplanereaches the maximum angle of attack, lift begins todiminish rapidly. This is the stalling angle of attack,or burble point.

Before proceeding further with lift and how it can becontrolled, velocity must be interjected. The shapeof the wing cannot be effective unless it continuallykeeps “attacking” new air. If an airplane is to keepflying, it must keep moving. Lift is proportional tothe square of the airplane’s velocity. For example, anairplane traveling at 200 knots has four times the liftas the same airplane traveling at 100 knots, if theangle of attack and other factors remain constant.

Actually, the airplane could not continue to travel inlevel flight at a constant altitude and maintain thesame angle of attack if the velocity is increased. Thelift would increase and the airplane would climb asa result of the increased lift force. Therefore, tomaintain the lift and weight forces in balance, andto keep the airplane “straight and level” (not accel-erating upward) in a state of equilibrium, as velocityis increased, lift must be decreased. This is normallyaccomplished by reducing the angle of attack; i.e.,lowering the nose. Conversely, as the airplane isslowed, the decreasing velocity requires increasingthe angle of attack to maintain lift sufficient tomaintain flight. There is, of course, a limit to howfar the angle of attack can be increased, if a stall isto be avoided.

Therefore, it may be concluded that for every angleof attack there is a corresponding indicated airspeedrequired to maintain altitude in steady, unacceleratedflight—all other factors being constant. (Bear inmind this is only true if maintaining “level flight.”)Since an airfoil will always stall at the same angleof attack, if increasing weight, lift must also beincreased, and the only method for doing so is byincreased velocity if the angle of attack is heldconstant just short of the “critical” or stalling angleof attack.

Lift and drag also vary directly with the density ofthe air. Density is affected by several factors: pres-sure, temperature, and humidity. Remember, at analtitude of 18,000 feet, the density of the air hasone-half the density of air at sea level. Therefore,in order to maintain its lift at a higher altitude, an

airplane must fly at a greater true airspeed for anygiven angle of attack.

Furthermore, warm air is less dense than cool air,and moist air is less dense than dry air. Thus, on ahot humid day, an airplane must be flown at a greatertrue airspeed for any given angle of attack than on acool, dry day.

If the density factor is decreased and the total liftmust equal the total weight to remain in flight, itfollows that one of the other factors must beincreased. The factors usually increased are the air-speed or the angle of attack, because these factorscan be controlled directly by the pilot.

It should also be pointed out that lift varies directlywith the wing area, provided there is no change inthe wing’s planform. If the wings have the same pro-portion and airfoil sections, a wing with a planformarea of 200 square feet lifts twice as much at thesame angle of attack as a wing with an area of 100square feet.

As can be seen, two major factors from the pilot’sviewpoint are lift and velocity because these are thetwo that can be controlled most readily and accu-rately. Of course, the pilot can also control densityby adjusting the altitude and can control wing areaif the airplane happens to have flaps of the type thatenlarge wing area. However, for most situations, thepilot is controlling lift and velocity to maneuver theairplane. For instance, in straight-and-level flight,cruising along at a constant altitude, altitude ismaintained by adjusting lift to match the airplane’svelocity or cruise airspeed, while maintaining astate of equilibrium where lift equals weight. In anapproach to landing, when the pilot wishes to landas slowly as practical, it is necessary to increase liftto near maximum to maintain lift equal to the weightof the airplane.

WINGTIP VORTICESThe action of the airfoil that gives an airplane liftalso causes induced drag. It was determined thatwhen a wing is flown at a positive angle of attack, apressure differential exists between the upper andlower surfaces of the wing—that is, the pressureabove the wing is less than atmospheric pressure andthe pressure below the wing is equal to or greaterthan atmospheric pressure. Since air always movesfrom high pressure toward low pressure, and the pathof least resistance is toward the airplane’s wingtips,there is a spanwise movement of air from the bottomof the wing outward from the fuselage around thewingtips. This flow of air results in “spillage” overthe wingtips, thereby setting up a whirlpool of air

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called a “vortex.” [Figure 3-4] At the same time, theair on the upper surface of the wing has a tendencyto flow in toward the fuselage and off the trailingedge. This air current forms a similar vortex at theinboard portion of the trailing edge of the wing,but because the fuselage limits the inward flow, thevortex is insignificant. Consequently, the deviationin flow direction is greatest at the wingtips wherethe unrestricted lateral flow is the strongest. As theair curls upward around the wingtip, it combineswith the wing’s downwash to form a fast spinningtrailing vortex. These vortices increase dragbecause of energy spent in producing the turbu-lence. It can be seen, then, that whenever the wingis producing lift, induced drag occurs, and wingtipvortices are created.

Just as lift increases with an increase in angle ofattack, induced drag also increases. This occursbecause as the angle of attack is increased, there is agreater pressure difference between the top and bot-tom of the wing, and a greater lateral flow of air;consequently, this causes more violent vortices to beset up, resulting in more turbulence and moreinduced drag.

The intensity or strength of the wingtip vortices isdirectly proportional to the weight of the airplane andinversely proportional to the wingspan and speed ofthe airplane. The heavier and slower the airplane, thegreater the angle of attack and the stronger the wingtipvortices. Thus, an airplane will create wingtip vorticeswith maximum strength occurring during the takeoff,climb, and landing phases of flight.

GROUND EFFECTIt is possible to fly an airplane just clear of theground (or water) at a slightly slower airspeedthan that required to sustain level flight at higheraltitudes. This is the result of a phenomenon,which is better known than understood even bysome experienced pilots.

When an airplane in flightgets within several feetfrom the ground surface, achange occurs in the three-dimensional flow patternaround the airplane becausethe vertical component ofthe airflow around the wingis restricted by the groundsurface. This alters thewing’s upwash, downwash,and wingtip vortices.[Figure 3-7] These generaleffects due to the presence

of the ground are referred to as “ground effect.”Ground effect, then, is due to the interference of theground (or water) surface with the airflow patternsabout the airplane in flight.

While the aerodynamic characteristics of the tail sur-faces and the fuselage are altered by ground effects,the principal effects due to proximity of the groundare the changes in the aerodynamic characteristics ofthe wing. As the wing encounters ground effect andis maintained at a constant lift coefficient, there isconsequent reduction in the upwash, downwash, andthe wingtip vortices.

Induced drag is a result of the wing’s work of sus-taining the airplane and the wing lifts the airplanesimply by accelerating a mass of air downward. Itis true that reduced pressure on top of an airfoil isessential to lift, but that is but one of the thingsthat contributes to the overall effect of pushing anair mass downward. The more downwash there is,the harder the wing is pushing the mass of airdown. At high angles of attack, the amount ofinduced drag is high and since this corresponds tolower airspeeds in actual flight, it can be said thatinduced drag predominates at low speed.

However, the reduction of the wingtip vortices dueto ground effect alters the spanwise lift distributionand reduces the induced angle of attack and induceddrag. Therefore, the wing will require a lower angleof attack in ground effect to produce the same liftcoefficient or, if a constant angle of attack is main-tained, an increase in lift coefficient will result.[Figure 3-8]

Figure 3-7. Ground effect changes airflow.

InGround Effect

InGround Effect

Out ofGround Effect

Out ofGround Effect

Angle of Attack

Lift

Coe

ffici

ent C

L

Velocity

Thr

ust R

equi

red

Figure 3-8. Ground effect changes drag and lift.

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Ground effect also will alter the thrust required ver-sus velocity. Since induced drag predominates at lowspeeds, the reduction of induced drag due to groundeffect will cause the most significant reduction ofthrust required (parasite plus induced drag) at lowspeeds.

The reduction in induced flow due to ground effectcauses a significant reduction in induced drag butcauses no direct effect on parasite drag. As a resultof the reduction in induced drag, the thrust requiredat low speeds will be reduced.

Due to the change in upwash, downwash, andwingtip vortices, there may be a change in position(installation) error of the airspeed system, associatedwith ground effect. In the majority of cases, groundeffect will cause an increase in the local pressure atthe static source and produce a lower indication ofairspeed and altitude. Thus, the airplane may be air-borne at an indicated airspeed less than that normallyrequired.

In order for ground effect to be of significant magni-tude, the wing must be quite close to the ground. Oneof the direct results of ground effect is the variationof induced drag with wing height above the ground ata constant lift coefficient. When the wing is at aheight equal to its span, the reduction in induced dragis only 1.4 percent. However, when the wing is at aheight equal to one-fourth its span, the reduction ininduced drag is 23.5 percent and, when the wing is ata height equal to one-tenth its span, the reduction ininduced drag is 47.6 percent. Thus, a large reductionin induced drag will take place only when the wing isvery close to the ground. Because of this variation,ground effect is most usually recognized during theliftoff for takeoff or just prior to touchdown whenlanding.

During the takeoff phase of flight, ground effect pro-duces some important relationships. The airplaneleaving ground effect after takeoff encounters justthe reverse of the airplane entering ground effectduring landing; i.e., the airplane leaving groundeffect will:

• Require an increase in angle of attack to maintainthe same lift coefficient.

• Experience an increase in induced drag and thrustrequired.

• Experience a decrease in stability and a nose-upchange in moment.

• Produce a reduction in static source pressure andincrease in indicated airspeed.

These general effects should point out the possibledanger in attempting takeoff prior to achieving therecommended takeoff speed. Due to the reduced dragin ground effect, the airplane may seem capable oftakeoff well below the recommended speed.However, as the airplane rises out of ground effectwith a deficiency of speed, the greater induced dragmay result in very marginal initial climb perform-ance. In the extreme conditions such as high grossweight, high density altitude, and high temperature, adeficiency of airspeed during takeoff may permit theairplane to become airborne but be incapable of flyingout of ground effect. In this case, the airplane maybecome airborne initially with a deficiency of speed,and then settle back to the runway. It is important thatno attempt be made to force the airplane to becomeairborne with a deficiency of speed; the recommendedtakeoff speed is necessary to provide adequate initialclimb performance. For this reason, it is imperativethat a definite climb be established before retractingthe landing gear or flaps.

During the landing phase of flight, the effect of prox-imity to the ground also must be understood andappreciated. If the airplane is brought into groundeffect with a constant angle of attack, the airplanewill experience an increase in lift coefficient and areduction in the thrust required. Hence, a “floating”effect may occur. Because of the reduced drag andpower off deceleration in ground effect, any excessspeed at the point of flare may incur a considerable“float” distance. As the airplane nears the point oftouchdown, ground effect will be most realized ataltitudes less than the wingspan. During the finalphases of the approach as the airplane nears theground, a reduced power setting is necessary or thereduced thrust required would allow the airplane toclimb above the desired glidepath.

AXES OF AN AIRPLANEWhenever an airplane changes its flight attitude orposition in flight, it rotates about one or more ofthree axes, which are imaginary lines that passthrough the airplane’s center of gravity. The axes ofan airplane can be considered as imaginary axlesaround which the airplane turns, much like the axlearound which a wheel rotates. At the point where allthree axes intersect, each is at a 90° angle to the othertwo. The axis, which extends lengthwise through thefuselage from the nose to the tail, is the longitudinalaxis. The axis, which extends crosswise fromwingtip to wingtip, is the lateral axis. The axis,which passes vertically through the center of gravity,is the vertical axis. [Figure 3-9]

The airplane’s motion about its longitudinal axisresembles the roll of a ship from side to side. In fact,

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the names used in describing the motion about anairplane’s three axes were originally nautical terms.They have been adapted to aeronautical terminologybecause of the similarity of motion between an air-plane and the seagoing ship.

In light of the adoption of nautical terms, the motionabout the airplane’s longitudinal axis is called “roll”;motion about its lateral axis is referred to as “pitch.”Finally, an airplane moves about its vertical axis in amotion, which is termed “yaw”—that is, a horizontal(left and right) movement of the airplane’s nose.

The three motions of the airplane (roll, pitch, andyaw) are controlled by three control surfaces. Roll iscontrolled by the ailerons; pitch is controlled by theelevators; yaw is controlled by the rudder. The use ofthese controls is explained in Chapter 4—FlightControls.

MOMENTS AND MOMENT ARMA study of physics shows that a body that is free torotate will always turn about its center of gravity. Inaerodynamic terms, the mathematical measure of anairplane’s tendency to rotate about its center ofgravity is called a “moment.” A moment is said to beequal to the product of the force applied and the dis-tance at which the force is applied. (A moment arm isthe distance from a datum [reference point or line] tothe applied force.) For airplane weight and balancecomputations, “moments” are expressed in terms ofthe distance of the arm times the airplane’s weight, orsimply, inch pounds.

Airplane designers locate the fore and aft position ofthe airplane’s center of gravity as nearly as possibleto the 20 percent point of the mean aerodynamicchord (MAC). If the thrust line is designed to passhorizontally through the center of gravity, it will notcause the airplane to pitch when power is changed,and there will be no difference in moment due tothrust for a power-on or power-off condition offlight. Although designers have some control over

the location of the drag forces, they are not alwaysable to make the resultant drag forces pass throughthe center of gravity of the airplane. However, theone item over which they have the greatest controlis the size and location of the tail. The objective isto make the moments (due to thrust, drag, and lift)as small as possible; and, by proper location of thetail, to provide the means of balancing the airplanelongitudinally for any condition of flight.

The pilot has no direct control over the location offorces acting on the airplane in flight, except forcontrolling the center of lift by changing the angleof attack. Such a change, however, immediatelyinvolves changes in other forces. Therefore, thepilot cannot independently change the location ofone force without changing the effect of others. Forexample, a change in airspeed involves a change inlift, as well as a change in drag and a change in theup or down force on the tail. As forces such as tur-bulence and gusts act to displace the airplane, thepilot reacts by providing opposing control forces tocounteract this displacement.

Some airplanes are subject to changes in the locationof the center of gravity with variations of load.Trimming devices are used to counteract the forcesset up by fuel burnoff, and loading or off-loading ofpassengers or cargo. Elevator trim tabs andadjustable horizontal stabilizers comprise the mostcommon devices provided to the pilot for trimmingfor load variations. Over the wide ranges of balanceduring flight in large airplanes, the force which thepilot has to exert on the controls would becomeexcessive and fatiguing if means of trimming werenot provided.

DESIGN CHARACTERISTICSEvery pilot who has flown numerous types of air-planes has noted that each airplane handles somewhatdifferently—that is, each resists or responds tocontrol pressures in its own way. A training typeairplane is quick to respond to control applications,

Lateral Axis

PITCHING

Longitudinal Axis

ROLLING

Vertical Axis

YAWING

Figure 3-9. Axes of an airplane.

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while a transport airplane usually feels heavy on thecontrols and responds to control pressures moreslowly. These features can be designed into an air-plane to facilitate the particular purpose the airplaneis to fulfill by considering certain stability andmaneuvering requirements. In the following discus-sion, it is intended to summarize the more importantaspects of an airplane’s stability; its maneuveringand controllability qualities; how they are analyzed;and their relationship to various flight conditions. Inbrief, the basic differences between stability, maneu-verability, and controllability are as follows:

• Stability—The inherent quality of an airplaneto correct for conditions that may disturb itsequilibrium, and to return or to continue on theoriginal flightpath. It is primarily an airplanedesign characteristic.

• Maneuverability—The quality of an airplanethat permits it to be maneuvered easily and towithstand the stresses imposed by maneuvers. Itis governed by the airplane’s weight, inertia, sizeand location of flight controls, structuralstrength, and powerplant. It too is an airplanedesign characteristic.

• Controllability—The capability of an airplane torespond to the pilot’s control, especially withregard to flightpath and attitude. It is the qualityof the airplane’s response to the pilot’s controlapplication when maneuvering the airplane,regardless of its stability characteristics.

BASIC CONCEPTS OF STABILITYThe flightpaths and attitudes in which an airplanecan fly are limited only by the aerodynamic charac-teristics of the airplane, its propulsive system, and its

structural strength. These limitations indicate themaximum performance and maneuverability of theairplane. If the airplane is to provide maximum util-ity, it must be safely controllable to the full extent ofthese limits without exceeding the pilot’s strengthor requiring exceptional flying ability. If an airplaneis to fly straight and steady along any arbitraryflightpath, the forces acting on it must be in staticequilibrium. The reaction of any body when itsequilibrium is disturbed is referred to as stability.There are two types of stability; static and dynamic.Static will be discussed first, and in this discussionthe following definitions will apply:

• Equilibrium—All opposing forces acting on theairplane are balanced; (i.e., steady, unacceleratedflight conditions).

• Static Stability—The initial tendency that the air-plane displays after its equilibrium is disturbed.

• Positive Static Stability—The initial tendencyof the airplane to return to the original state ofequilibrium after being disturbed. [Figure 3-10]

• Negative Static Stability—The initial tendencyof the airplane to continue away from the originalstate of equilibrium after being disturbed. [Figure3-10]

• Neutral Static Stability—The initial tendencyof the airplane to remain in a new condition afterits equilibrium has been disturbed. [Figure 3-10]

STATIC STABILITYStability of an airplane in flight is slightly more com-plex than just explained, because the airplane is freeto move in any direction and must be controllable in

Figure 3-10.Types of stability.

CG

AppliedForce

POSITIVESTATIC STABILITY

CG

CG

AppliedForce

NEUTRALSTATIC STABILITY

CG

AppliedForce

NEGATIVESTATIC STABILITY

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pitch, roll, and direction. When designing the airplane,engineers must compromise between stability,maneuverability, and controllability; and the problemis compounded because of the airplane’s three-axisfreedom. Too much stability is detrimental tomaneuverability, and similarly, not enough stabil-ity is detrimental to controllability. In the designof airplanes, compromise between the two is thekeyword.

DYNAMIC STABILITYStatic stability has been defined as the initial tendencythat the airplane displays after being disturbed from itstrimmed condition. Occasionally, the initial tendencyis different or opposite from the overall tendency, sodistinction must be made between the two. Dynamicstability is the overall tendency that the airplane dis-plays after its equilibrium is disturbed. The curves offigure 3-11 represent the variation of controlledfunctions versus time. It is seen that the unit of timeis very significant. If the time unit for one cycle oroscillation is above 10 seconds’ duration, it is calleda “long-period” oscillation (phugoid) and is easilycontrolled. In a longitudinal phugoid oscillation,the angle of attack remains constant when the air-speed increases and decreases. To a certain degree,a convergent phugoid is desirable but is notrequired. The phugoid can be determined only on astatically stable airplane, and this has a great effecton the trimming qualities of the airplane. If thetime unit for one cycle or oscillation is less thanone or two seconds, it is called a “short-period”oscillation and is normally very difficult, if notimpossible, for the pilot to control. This is the typeof oscillation that the pilot can easily “get in phasewith” and reinforce.

A neutral or divergent, short-period oscillation isdangerous because structural failure usuallyresults if the oscillation is not damped immedi-ately. Short-period oscillations affect airplane andcontrol surfaces alike and reveal themselves as“porpoising” in the airplane, or as in “buzz” or“flutter” in the control surfaces. Basically, theshort-period oscillation is a change in angle ofattack with no change in airspeed. A short-periodoscillation of a control surface is usually of suchhigh frequency that the airplane does not havetime to react. Logically, the Code of FederalRegulations require that short-period oscillationsbe heavily damped (i.e., die out immediately).Flight tests during the airworthiness certificationof airplanes are conducted for this condition byinducing the oscillation in the controls for pitch,roll, or yaw at the most critical speed (i.e., at VNE,the never-exceed speed). The test pilot strikes thecontrol wheel or rudder pedal a sharp blow andobserves the results.

LONGITUDINAL STABILITY (PITCHING)In designing an airplane, a great deal of effort isspent in developing the desired degree of stabilityaround all three axes. But longitudinal stability aboutthe lateral axis is considered to be the most affectedby certain variables in various flight conditions.

A

Time

Damped Oscillation

(Positive Static)(Positive Dynamic)

Time

Undamped Oscillation

(Positive Static)(Neutral Dynamic)

Time

Divergent Oscillation

(Positive Static)(Negative Dynamic)

Dis

plac

emen

tD

ispl

acem

ent

Dis

plac

emen

t

B

C

Figure 3-11. Damped versus undamped stability.

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Longitudinal stability is the quality that makes anairplane stable about its lateral axis. It involves thepitching motion as the airplane’s nose moves up anddown in flight. A longitudinally unstable airplane hasa tendency to dive or climb progressively into a verysteep dive or climb, or even a stall. Thus, an airplanewith longitudinal instability becomes difficult andsometimes dangerous to fly.

Static longitudinal stability or instability in an air-plane, is dependent upon three factors:

1. Location of the wing with respect to the center ofgravity;

2. Location of the horizontal tail surfaces withrespect to the center of gravity; and

3. The area or size of the tail surfaces.

In analyzing stability, it should be recalled that abody that is free to rotate will always turn about itscenter of gravity.

To obtain static longitudinal stability, the relation ofthe wing and tail moments must be such that, if themoments are initially balanced and the airplane issuddenly nosed up, the wing moments and tailmoments will change so that the sum of their forceswill provide an unbalanced but restoring momentwhich, in turn, will bring the nose down again.Similarly, if the airplane is nosed down, the resultingchange in moments will bring the nose back up.

The center of lift, sometimes called the center ofpressure, in most unsymmetrical airfoils has a ten-dency to change its fore and aft position with achange in the angle of attack. The center of pressuretends to move forward with an increase in angle ofattack and to move aft with a decrease in angle ofattack. This means that when the angle of attack ofan airfoil is increased, the center of pressure (lift) bymoving forward, tends to lift the leading edge of thewing still more. This tendency gives the wing aninherent quality of instability.

Figure 3-12 shows an airplane in straight-and-levelflight. The line CG-CL-T represents the airplane’slongitudinal axis from the center of gravity (CG) toa point T on the horizontal stabilizer. The center oflift (or center of pressure) is represented by thepoint CL.

Most airplanes are designed so that the wing’s centerof lift (CL) is to the rear of the center of gravity. Thismakes the airplane “nose heavy” and requires thatthere be a slight downward force on the horizontalstabilizer in order to balance the airplane and keep

the nose from continually pitching downward.Compensation for this nose heaviness is providedby setting the horizontal stabilizer at a slight neg-ative angle of attack. The downward force thusproduced, holds the tail down, counterbalancingthe “heavy” nose. It is as if the line CG-CL-T wasa lever with an upward force at CL and two down-ward forces balancing each other, one a strongforce at the CG point and the other, a much lesserforce, at point T (downward air pressure on thestabilizer). Applying simple physics principles, itcan be seen that if an iron bar were suspended atpoint CL with a heavy weight hanging on it at theCG, it would take some downward pressure atpoint T to keep the “lever” in balance.

Even though the horizontal stabilizer may be levelwhen the airplane is in level flight, there is adownwash of air from the wings. This downwashstrikes the top of the stabilizer and produces adownward pressure, which at a certain speed willbe just enough to balance the “lever.” The fasterthe airplane is flying, the greater this downwashand the greater the downward force on the horizontalstabilizer (except “T” tails). [Figure 3-13] In air-planes with fixed position horizontal stabilizers, theairplane manufacturer sets the stabilizer at an anglethat will provide the best stability (or balance)during flight at the design cruising speed andpower setting. [Figure 3-14]

If the airplane’s speed decreases, the speed of the air-flow over the wing is decreased. As a result of thisdecreased flow of air over the wing, the downwashis reduced, causing a lesser downward force on thehorizontal stabilizer. In turn, the characteristic noseheaviness is accentuated, causing the airplane’s noseto pitch down more. This places the airplane in anose-low attitude, lessening the wing’s angle ofattack and drag and allowing the airspeed toincrease. As the airplane continues in the nose-lowattitude and its speed increases, the downward forceon the horizontal stabilizer is once again increased.

CL

CG

CLCG

T

T

Figure 3-12. Longitudinal stability.

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Consequently, the tail is again pushed downward andthe nose rises into a climbing attitude.

As this climb continues, the airspeed again decreases,causing the downward force on the tail to decreaseuntil the nose lowers once more. However, becausethe airplane is dynamically stable, the nose does notlower as far this time as it did before. The airplanewill acquire enough speed in this more gradual diveto start it into another climb, but the climb is not sosteep as the preceding one.

After several of these diminishing oscillations, inwhich the nose alternately rises and lowers, the air-plane will finally settle down to a speed at whichthe downward force on the tail exactly counteractsthe tendency of the airplane to dive. When thiscondition is attained, the airplane will once again

be in balanced flight and will continue in stabi-lized flight as long as this attitude and airspeed arenot changed.

A similar effect will be noted upon closing thethrottle. The downwash of the wings is reducedand the force at T in figure 3-12 is not enough tohold the horizontal stabilizer down. It is as if theforce at T on the lever were allowing the force ofgravity to pull the nose down. This, of course, is adesirable characteristic because the airplane isinherently trying to regain airspeed and reestablishthe proper balance.

Power or thrust can also have a destabilizing effectin that an increase of power may tend to make thenose rise. The airplane designer can offset this byestablishing a “high thrustline” wherein the line ofthrust passes above the center of gravity. [Figures3-15 and 3-16] In this case, as power or thrust isincreased a moment is produced to counteract thedown load on the tail. On the other hand, a very“low thrust line” would tend to add to the nose-upeffect of the horizontal tail surface.

Figure 3-15.Thrust line affects longitudinal stability.

It can be concluded, then, that with the center of gravityforward of the center of lift, and with an aerodynamictail-down force, the result is that the airplane alwaystries to return to a safe flying attitude.

A simple demonstration of longitudinal stability maybe made as follows: Trim the airplane for “hands off”control in level flight. Then momentarily give thecontrols a slight push to nose the airplane down. If,

Cruise Speed

High Speed

BalancedTail Load

LesserDownwardTail Load

GreaterDownwardTail Load

Low Speed

CG

CG

CG

Figure 3-13. Effect of speed on downwash.

Figure 3-14. Reduced power allows pitch down.

L

T

W

W

T

L

Normal Downwash

Reduced Downwash

T

Below CG

CG

TCG

Through CG

CGT

Above CG

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within a brief period, the nose rises to the originalposition and then stops, the airplane is staticallystable. Ordinarily, the nose will pass the originalposition (that of level flight) and a series of slowpitching oscillations will follow. If the oscillationsgradually cease, the airplane has positive stability;if they continue unevenly, the airplane has neutralstability; if they increase, the airplane is unstable.

LATERAL STABILITY (ROLLING)Stability about the airplane’s longitudinal axis,which extends from nose to tail, is called lateralstability. This helps to stabilize the lateral orrolling effect when one wing gets lower than thewing on the opposite side of the airplane. There arefour main design factors that make an airplane sta-ble laterally: dihedral, keel effect, sweepback, andweight distribution.

The most common procedure for producing lateralstability is to build the wings with a dihedral anglevarying from one to three degrees. In other words, thewings on either side of the airplane join the fuselageto form a slight V or angle called “dihedral,” and thisis measured by the angle made by each wing above aline parallel to the lateral axis.

The basis of rolling stability is, of course, the lat-eral balance of forces produced by the airplane’swings. Any imbalance in lift results in a tendencyfor the airplane to roll about its longitudinal axis.Stated another way, dihedral involves a balance of

lift created by the wings’ angle of attack on eachside of the airplane’s longitudinal axis.

If a momentary gust of wind forces one wing of theairplane to rise and the other to lower, the airplanewill bank. When the airplane is banked without turn-ing, it tends to sideslip or slide downward toward thelowered wing. [Figure 3-17] Since the wings havedihedral, the air strikes the low wing at much greaterangle of attack than the high wing. This increases thelift on the low wing and decreases lift on the highwing, and tends to restore the airplane to its originallateral attitude (wings level)—that is, the angle ofattack and lift on the two wings are again equal.

Figure 3-17. Dihedral for lateral stability.

The effect of dihedral, then, is to produce a rollingmoment tending to return the airplane to a laterallybalanced flight condition when a sideslip occurs.

The restoring force may move the low wing up toofar, so that the opposite wing now goes down. If so,the process will be repeated, decreasing with eachlateral oscillation until a balance for wings-levelflight is finally reached.

Conversely, excessive dihedral has an adverse effecton lateral maneuvering qualities. The airplane may beso stable laterally that it resists any intentional rollingmotion. For this reason, airplanes that require fast rollor banking characteristics usually have less dihedralthan those designed for less maneuverability.

The contribution of sweepback to dihedral effect isimportant because of the nature of the contribution.In a sideslip, the wing into the wind is operating withan effective decrease in sweepback, while the wingout of the wind is operating with an effective increasein sweepback. The swept wing is responsive only tothe wind component that is perpendicular to thewing’s leading edge. Consequently, if the wing is

T

L

CG

Cruise Power

L

CGT

Idle Power

L

CGT

Full Power

Figure 3-16. Power changes affect longitudinal stability.

Normal Angleof Attack

Normal Angleof Attack

Lesser Angleof Attack

Greater Angleof Attack

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operating at a positive lift coefficient, the wing intothe wind has an increase in lift, and the wing out ofthe wind has a decrease in lift. In this manner, theswept back wing would contribute a positive dihedraleffect and the swept forward wing would contribute anegative dihedral effect.

During flight, the side area of the airplane’s fuselageand vertical fin react to the airflow in much the samemanner as the keel of a ship. That is, it exerts asteadying influence on the airplane laterally aboutthe longitudinal axis.

Such laterally stable airplanes are constructed so thatthe greater portion of the keel area is above andbehind the center of gravity. [Figure 3-18] Thus,when the airplane slips to one side, the combinationof the airplane’s weight and the pressure of the air-flow against the upper portion of the keel area (bothacting about the CG) tends to roll the airplane backto wings-level flight.

Figure 3-18. Keel area for lateral stability.

VERTICAL STABILITY (YAWING)Stability about the airplane’s vertical axis (the side-ways moment) is called yawing or directional stability.

Yawing or directional stability is the more easilyachieved stability in airplane design. The area of thevertical fin and the sides of the fuselage aft of thecenter of gravity are the prime contributors whichmake the airplane act like the well known weather-vane or arrow, pointing its nose into the relativewind.

In examining a weathervane, it can be seen that ifexactly the same amount of surface were exposedto the wind in front of the pivot point as behind it,the forces fore and aft would be in balance andlittle or no directional movement would result.Consequently, it is necessary to have a greatersurface aft of the pivot point that forward of it.

Similarly in an airplane, the designer must ensurepositive directional stability by making the sidesurface greater aft than ahead of the center ofgravity. [Figure 3-19] To provide more positivestability aside from that provided by the fuselage,a vertical fin is added. The fin acts similar to thefeather on an arrow in maintaining straight flight.Like the weathervane and the arrow, the farther aftthis fin is placed and the larger its size, the greaterthe airplane’s directional stability.

Figure 3-19. Fuselage and fin for vertical stability.

If an airplane is flying in a straight line, and a side-ward gust of air gives the airplane a slight rotationabout its vertical axis (i.e., the right), the motion isretarded and stopped by the fin because while theairplane is rotating to the right, the air is striking theleft side of the fin at an angle. This causes pressureon the left side of the fin, which resists the turningmotion and slows down the airplane’s yaw. In doingso, it acts somewhat like the weathervane by turningthe airplane into the relative wind. The initial changein direction of the airplane’s flightpath is generallyslightly behind its change of heading. Therefore,after a slight yawing of the airplane to the right, thereis a brief moment when the airplane is still movingalong its original path, but its longitudinal axis ispointed slightly to the right.

The airplane is then momentarily skidding sideways,and during that moment (since it is assumed thatalthough the yawing motion has stopped, the excesspressure on the left side of the fin still persists) there

CG

CG

Centerline

Centerline

CG

AreaForwardof CG

Area AFTof CG

CG

Relative Wind Yaw

Yaw

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is necessarily a tendency for the airplane to be turnedpartially back to the left. That is, there is a momen-tary restoring tendency caused by the fin.

This restoring tendency is relatively slow in develop-ing and ceases when the airplane stops skidding. Whenit ceases, the airplane will be flying in a directionslightly different from the original direction. Inother words, it will not of its own accord return tothe original heading; the pilot must reestablish theinitial heading.

A minor improvement of directional stability maybe obtained through sweepback. Sweepback is incor-porated in the design of the wing primarily to delaythe onset of compressibility during high-speed flight.In lighter and slower airplanes, sweepback aidsin locating the center of pressure in the correctrelationship with the center of gravity. A longitudinallystable airplane is built with the center of pressure aft ofthe center of gravity.

Because of structural reasons, airplane designerssometimes cannot attach the wings to the fuselage atthe exact desired point. If they had to mount thewings too far forward, and at right angles to thefuselage, the center of pressure would not be farenough to the rear to result in the desired amount oflongitudinal stability. By building sweepback intothe wings, however, the designers can move the cen-ter of pressure toward the rear. The amount ofsweepback and the position of the wings then placethe center of pressure in the correct location.

The contribution of the wing to static directional sta-bility is usually small. The swept wing provides astable contribution depending on the amount ofsweepback, but the contribution is relatively smallwhen compared with other components.

FREE DIRECTIONAL OSCILLATIONS (DUTCH ROLL)Dutch Roll is a coupled lateral/directional oscillationthat is usually dynamically stable but is objectionablein an airplane because of the oscillatory nature. Thedamping of the oscillatory mode may be weak orstrong depending on the properties of the particularairplane.

Unfortunately all air is not smooth. There are bumpsand depressions created by gusty updrafts and down-drafts, and by gusts from ahead, behind, or the sideof the airplane.

The response of the airplane to a disturbance fromequilibrium is a combined rolling/yawing oscillationin which the rolling motion is phased to precedethe yawing motion. The yawing motion is not too

significant, but the roll is much more noticeable.When the airplane rolls back toward level flight inresponse to dihedral effect, it rolls back too farand sideslips the other way. Thus, the airplaneovershoots each time because of the strong dihe-dral effect. When the dihedral effect is large incomparison with static directional stability, theDutch Roll motion has weak damping and isobjectionable. When the static directional stabilityis strong in comparison with the dihedral effect,the Dutch Roll motion has such heavy dampingthat it is not objectionable. However, these qualitiestend toward spiral instability.

The choice is then the least of two evils—Dutch Rollis objectionable and spiral instability is tolerable ifthe rate of divergence is low. Since the moreimportant handling qualities are a result of highstatic directional stability and minimum necessarydihedral effect, most airplanes demonstrate a mildspiral tendency. This tendency would be indicatedto the pilot by the fact that the airplane cannot beflown “hands off” indefinitely.

In most modern airplanes, except high-speedswept wing designs, these free directional oscilla-tions usually die out automatically in a very fewcycles unless the air continues to be gusty orturbulent. Those airplanes with continuing DutchRoll tendencies usually are equipped with gyrostabilized yaw dampers. An airplane that hasDutch Roll tendencies is disconcerting, to say theleast. Therefore, the manufacturer tries to reach amedium between too much and too little direc-tional stability. Because it is more desirable forthe airplane to have “spiral instability” than DutchRoll tendencies, most airplanes are designed withthat characteristic.

SPIRAL INSTABILITYSpiral instability exists when the static directionalstability of the airplane is very strong as comparedto the effect of its dihedral in maintaining lateralequilibrium. When the lateral equilibrium of theairplane is disturbed by a gust of air and a sideslipis introduced, the strong directional stability tendsto yaw the nose into the resultant relative windwhile the comparatively weak dihedral lags inrestoring the lateral balance. Due to this yaw, thewing on the outside of the turning moment travelsforward faster than the inside wing and as a conse-quence, its lift becomes greater. This produces anoverbanking tendency which, if not corrected bythe pilot, will result in the bank angle becomingsteeper and steeper. At the same time, the strongdirectional stability that yaws the airplane into therelative wind is actually forcing the nose to a lowerpitch attitude. Then, the start of a slow downward

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spiral which has begun, if not counteracted by thepilot, will gradually increase into a steep spiral dive.Usually the rate of divergence in the spiral motion isso gradual that the pilot can control the tendencywithout any difficulty.

All airplanes are affected to some degree by thischaracteristic although they may be inherently stablein all other normal parameters. This tendency wouldbe indicated to the pilot by the fact that the airplanecannot be flown “hands off” indefinitely.

Much study and effort has gone into developmentof control devices (wing leveler) to eliminate or atleast correct this instability. Advanced stages ofthis spiral condition demand that the pilot be verycareful in application of recovery controls, orexcessive loads on the structure may be imposed.Of the in-flight structural failures that haveoccurred in general aviation airplanes, improperrecovery from this condition has probably beenthe underlying cause of more fatalities than anyother single factor. The reason is that the airspeedin the spiral condition builds up rapidly, and theapplication of back elevator force to reduce thisspeed and to pull the nose up only “tightens theturn,” increasing the load factor. The results of theprolonged uncontrolled spiral are always thesame; either in-flight structural failure, crashinginto the ground, or both. The most common causeson record for getting into this situation are: loss ofhorizon reference, inability of the pilot to controlthe airplane by reference to instruments, or a com-bination of both.

AERODYNAMIC FORCES IN FLIGHT MANEUVERS

FORCES INTURNSIf an airplanewere viewed ins t r a i g h t - a n d -level flight fromthe rear [figure3-20], and if theforces acting onthe airplane actu-ally could beseen, two forces(lift and weight)

would be apparent, and if the airplane were in a bank itwould be apparent that lift did not act directly oppositeto the weight—it now acts in the direction of the bank.The fact that when the airplane banks, lift acts inwardtoward the center of the turn, as well as upward, is oneof the basic truths to remember in the consideration ofturns.

An object at rest or moving in a straight line willremain at rest or continue to move in a straight lineuntil acted on by some other force. An airplane, likeany moving object, requires a sideward force tomake it turn. In a normal turn, this force is suppliedby banking the airplane so that lift is exerted inwardas well as upward. The force of lift during a turn isseparated into two components at right angles toeach other. One component, which acts verticallyand opposite to the weight (gravity), is called the“vertical component of lift.” The other, which actshorizontally toward the center of the turn, is calledthe “horizontal component of lift,” or centripetalforce. The horizontal component of lift is the forcethat pulls the airplane from a straight flightpath tomake it turn. Centrifugal force is the “equal andopposite reaction” of the airplane to the change indirection and acts equal and opposite to the hori-zontal component of lift. This explains why, in acorrectly executed turn, the force that turns theairplane is not supplied by the rudder.

An airplane is not steered like a boat or an automo-bile; in order for it to turn, it must be banked. If theairplane is not banked, there is no force available thatwill cause it to deviate from a straight flightpath.Conversely, when an airplane is banked, it will turn,provided it is not slipping to the inside of the turn.Good directional control is based on the fact that theairplane will attempt to turn whenever it is banked.

Centripetal Force – The force opposite centrifugal force and attracts abody towards its axis of rotation.

Centrifugal Force—An apparent force resulting from the effect of iner-tia during a turn.

Figure 3-20. Forces during normal coordinated turn.

Lift

Weight

Level Flight

TotalLift

HorizontalComponent

VerticalComponent

CentrifugalForce

Weight ResultantLoad

Medium Banked Turn

TotalLift

VerticalComponent

HorizontalComponent

Weight

CentrifugalForce

ResultantLoad

Steep Banked Turn

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This fact should be borne in mind at all times, partic-ularly while attempting to hold the airplane instraight-and-level flight.

Merely banking the airplane into a turn produces nochange in the total amount of lift developed.However, as was pointed out, the lift during the bankis divided into two components: one vertical and theother horizontal. This division reduces the amount oflift which is opposing gravity and actually support-ing the airplane’s weight; consequently, the airplaneloses altitude unless additional lift is created. This isdone by increasing the angle of attack until the verti-cal component of lift is again equal to the weight.Since the vertical component of lift decreases as thebank angle increases, the angle of attack must beprogressively increased to produce sufficient verti-cal lift to support the airplane’s weight. The fact thatthe vertical component of lift must be equal to theweight to maintain altitude is an important fact toremember when making constant altitude turns.

At a given airspeed, the rate at which an airplaneturns depends upon the magnitude of the horizontalcomponent of lift. It will be found that the horizontalcomponent of lift is proportional to the angle ofbank—that is, it increases or decreases respectivelyas the angle of bank increases or decreases. It logi-cally follows then, that as the angle of bank isincreased the horizontal component of lift increases,thereby increasing the rate of turn. Consequently, atany given airspeed the rate of turn can be controlledby adjusting the angle of bank.

To provide a vertical component of lift sufficient tohold altitude in a level turn, an increase in the angleof attack is required. Since the drag of the airfoil isdirectly proportional to its angle of attack, induceddrag will increase as the lift is increased. This, inturn, causes a loss of airspeed in proportion to the

angle of bank; a small angle of bank results in asmall reduction in airspeed and a large angle ofbank results in a large reduction in airspeed.Additional thrust (power) must be applied to pre-vent a reduction in airspeed in level turns; therequired amount of additional thrust is proportionalto the angle of bank.

To compensate for added lift, which would result ifthe airspeed were increased during a turn, the angleof attack must be decreased, or the angle of bankincreased, if a constant altitude were to be main-tained. If the angle of bank were held constant andthe angle of attack decreased, the rate of turn woulddecrease. Therefore, in order to maintain a constantrate of turn as the airspeed is increased, the angle ofattack must remain constant and the angle of bankincreased.

It must be remembered that an increase in airspeedresults in an increase of the turn radius and that cen-trifugal force is directly proportional to the radius ofthe turn. In a correctly executed turn, the horizontalcomponent of lift must be exactly equal and oppositeto the centrifugal force. Therefore, as the airspeed isincreased in a constant rate level turn, the radius ofthe turn increases. This increase in the radius of turncauses an increase in the centrifugal force, whichmust be balanced by an increase in the horizontalcomponent of lift, which can only be increased byincreasing the angle of bank.

In a slipping turn, the airplane is not turning at therate appropriate to the bank being used, since the air-plane is yawed toward the outside of the turningflightpath. The airplane is banked too much for therate of turn, so the horizontal lift component isgreater than the centrifugal force. [Figure 3-21]Equilibrium between the horizontal lift componentand centrifugal force is reestablished either by

Figure 3-21. Normal, sloping, and skidding turns.

Lift Vertical Lift

HorizontalLift

CentrifugalForce

Weight Load

1. Normal Turn: Centrifugal Force Equals Horizontal Lift.

Weight Load

CentrifugalForce

Lift Vertical Lift

HorizontalLift

2. Slipping Turn: Centrifugal Force Less Than Horizontal Lift.

Vertical Lift

CentrifugalForce

LoadWeight

Lift

HorizontalLift

3. Skidding Turn: Centrifugal Force Greater Than Horizontal Lift.

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decreasing the bank, increasing the rate of turn, or acombination of the two changes.

A skidding turn results from an excess of centrifugalforce over the horizontal lift component, pulling theairplane toward the outside of the turn. The rate ofturn is too great for the angle of bank. Correction ofa skidding turn thus involves a reduction in the rateof turn, an increase in bank, or a combination of thetwo changes.

To maintain a given rate of turn, the angle of bankmust be varied with the airspeed. This becomes par-ticularly important in high-speed airplanes. Forinstance, at 400 miles per hour (m.p.h.), an airplanemust be banked approximately 44° to execute astandard rate turn (3° per second). At this angle ofbank, only about 79 percent of the lift of the airplanecomprises the vertical component of the lift; theresult is a loss of altitude unless the angle of attackis increased sufficiently to compensate for the lossof vertical lift.

FORCES IN CLIMBSFor all practical purposes, the wing’s lift in asteady state normal climb is the same as it is in asteady level flight at the same airspeed. Though theairplane’s flightpath has changed when the climbhas been established, the angle of attack of thewing with respect to the inclined flightpath revertsto practically the same values, as does the lift.There is an initial momentary change, however, asshown in figure 3-22. During the transition fromstraight-and-level flight to a climb, a change in liftoccurs when back elevator pressure is first applied.Raising the airplane’s nose increases the angle ofattack and momentarily increases the lift. Lift atthis moment is now greater than weight and startsthe airplane climbing. After the flightpath is stabi-lized on the upward incline, the angle of attack andlift again revert to about the level flight values.

If the climb is entered with no change in power set-ting, the airspeed gradually diminishes because the

thrust required to maintain a given airspeed in levelflight is insufficient to maintain the same airspeedin a climb. When the flightpath is inclined upward,a component of the airplane’s weight acts in thesame direction as, and parallel to, the total drag ofthe airplane, thereby increasing the total effectivedrag. Consequently, the total drag is greater than thepower, and the airspeed decreases. The reduction inairspeed gradually results in a correspondingdecrease in drag until the total drag (including thecomponent of weight acting in the same direction)equals the thrust. [Figure 3-23] Due to momentum,the change in airspeed is gradual, varying consider-ably with differences in airplane size, weight, totaldrag, and other factors.

Figure 3-23. Changes in speed during climb entry.

Generally, the forces of thrust and drag, and liftand weight, again become balanced when the air-speed stabilizes but at a value lower than instraight-and-level flight at the same power setting.Since in a climb the airplane’s weight is not onlyacting downward but rearward along with drag,additional power is required to maintain the sameairspeed as in level flight. The amount of powerdepends on the angle of climb. When the climb isestablished so steep that there is insufficient poweravailable, a slower speed results. It will be seenthen that the amount of reserve power determinesthe climb performance of the airplane.

FORCES IN DESCENTSAs in climbs, the forces acting on the airplane gothrough definite changes when a descent is enteredfrom straight-and-level flight. The analysis here isthat of descending at the same power as used instraight-and-level flight.

When forward pressure is applied to the elevatorcontrol to start descending, or the airplane’s nose isallowed to pitch down, the angle of attack isdecreased and, as a result, the lift of the airfoil isreduced. This reduction in total lift and angle ofattack is momentary and occurs during the time the

L

Steady ClimbNormal Lift

L

Climb EntryIncreased Lift

L

Level FlightNormal Lift

Figure 3-22. Changes in lift during climb entry.

TL

WD

D D

T

T

L

L

W

WLevel Flight

Forces BalancedConstant Speed

Climb EntryDrag GreaterThan Thrust

Speed Slowing

Steady ClimbForces BalancedConstant Speed

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flightpath changes downward. The change to adownward flightpath is due to the lift momentarilybecoming less than the weight of the airplane as theangle of attack is reduced. This imbalance betweenlift and weight causes the airplane to follow adescending flightpath with respect to the horizontalflightpath of straight-and-level flight. When theflightpath is in a steady descent, the airfoil’s angleof attack again approaches the original value, andlift and weight will again become stabilized. Fromthe time the descent is started until it is stabilized,the airspeed will gradually increase. This is due to acomponent of weight now acting forward along theflightpath, similar to the manner it acted rearwardin a climb. The overall effect is that of increasedpower or thrust, which in turn causes the increasein airspeed associated with descending at the samepower as used in level flight.

To descend at the same airspeed as used in straight-and-level flight, obviously, the power must be reducedas the descent is entered. The component of weightacting forward along the flightpath will increase asthe angle of rate of descent increases and conversely,will decrease as the angle of rate of descentdecreases. Therefore, the amount of power reductionrequired for a descent at the same speed as cruisewill be determined by the steepness of the descent.

STALLSAn airplane will fly as long as the wing is creatingsufficient lift to counteract the load imposed on it.When the lift is completely lost, the airplane stalls.

Remember, the direct cause of every stall is anexcessive angle of attack. There are any number offlight maneuvers which may produce an increase inthe angle of attack, but the stall does not occur untilthe angle of attack becomes excessive.

It must be emphasized that the stalling speed of aparticular airplane is not a fixed value for all flightsituations. However, a given airplane will alwaysstall at the same angle of attack regardless of air-speed, weight, load factor, or density altitude. Eachairplane has a particular angle of attack where theairflow separates from the upper surface of the wingand the stall occurs. This critical angle of attackvaries from 16° to 20° depending on the airplane’sdesign. But each airplane has only one specific angleof attack where the stall occurs.

There are three situations in which the critical angleof attack can be exceeded: in low-speed flying, inhigh-speed flying, and in turning flight.

The airplane can be stalled in straight-and-levelflight by flying too slowly. As the airspeed is being

decreased, the angle of attack must be increased toretain the lift required for maintaining altitude. Theslower the airspeed becomes, the more the angle ofattack must be increased. Eventually, an angle ofattack is reached which will result in the wing notproducing enough lift to support the airplane and itwill start settling. If the airspeed is reduced further,the airplane will stall, since the angle of attack hasexceeded the critical angle and the airflow over thewing is disrupted.

It must be reemphasized here that low speed is notnecessary to produce a stall. The wing can bebrought into an excessive angle of attack at anyspeed. For example, take the case of an airplanewhich is in a dive with an airspeed of 200 knotswhen suddenly the pilot pulls back sharply on theelevator control. [Figure 3-24] Because of gravityand centrifugal force, the airplane could not immedi-ately alter its flightpath but would merely change itsangle of attack abruptly from quite low to very high.Since the flightpath of the airplane in relation to theoncoming air determines the direction of the relativewind, the angle of attack is suddenly increased, andthe airplane would quickly reach the stalling angle ata speed much greater than the normal stall speed.

Figure 3-24. Forces exerted when pulling out of a dive.

Similarly, the stalling speed of an airplane is higherin a level turn than in straight-and-level flight.[Figure 3-25] This is because centrifugal force isadded to the airplane’s weight, and the wing mustproduce sufficient additional lift to counterbalancethe load imposed by the combination of centrifugalforce and weight. In a turn, the necessary additionallift is acquired by applying back pressure to the ele-vator control. This increases the wing’s angle ofattack, and results in increased lift. The angle ofattack must increase as the bank angle increases tocounteract the increasing load caused by centrifugalforce. If at any time during a turn the angle of attackbecomes excessive, the airplane will stall.

L

CF

WL

L

CF

CF CF

W

WW

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At this point, the action of the airplane during astall should be examined. To balance the airplaneaerodynamically, the center of lift is normallylocated aft of the center of gravity. Although thismakes the airplane inherently “nose heavy,”downwash on the horizontal stabilizer counteractsthis condition. It can be seen then, that at the pointof stall when the upward force of the wing’s liftand the downward tail force cease, an unbalancedcondition exists. This allows the airplane to pitchdown abruptly, rotating about its center of gravity.During this nose-down attitude, the angle of attackdecreases and the airspeed again increases; hence,the smooth flow of air over the wing begins again,lift returns, and the airplane is again flying.However, considerable altitude may be lost beforethis cycle is complete.

BASIC PROPELLER PRINCIPLESThe airplane propeller consists of two or more bladesand a central hub to which the blades are attached.Each blade of an airplane propeller is essentially arotating wing. As a result of their construction, thepropeller blades are like airfoils and produce forcesthat create the thrust to pull, or push, the airplanethrough the air.

The power needed to rotate the propeller blades isfurnished by the engine. The engine rotates the air-foils of the blades through the air at high speeds, andthe propeller transforms the rotary power of theengine into forward thrust.

An airplane moving through the air creates a dragforce opposing its forward motion. Consequently, ifan airplane is to fly, there must be a force applied toit that is equal to the drag, but acting forward. Thisforce is called “thrust.”

A cross section of a typical propeller blade is shownin figure 3-26. This section or blade element is an

airfoil comparable to a cross section of an airplanewing. One surface of the blade is cambered orcurved, similar to the upper surface of an airplanewing, while the other surface is flat like the bottomsurface of a wing. The chord line is an imaginaryline drawn through the blade from its leading edgeto its trailing edge. As in a wing, the leading edge isthe thick edge of the blade that meets the air as thepropeller rotates.

Figure 3-26. Airfoil sections of propeller blade.

Blade angle, usually measured in degrees, is theangle between the chord of the blade and the planeof rotation [figure 3-27] and is measured at a spe-cific point along the length of the blade. Becausemost propellers have a flat blade “face,” the chordline is often drawn along the face of the propellerblade. Pitch is not the same as blade angle, butbecause pitch is largely determined by blade angle,the two terms are often used interchangeably. Anincrease or decrease in one is usually associated withan increase or decrease in the other.

Figure 3-27. Propeller blade angle.

The pitch of a propeller may be designated ininches. A propeller designated as a “74-48” wouldbe 74 inches in length and have an effective pitchof 48 inches. The pitch in inches is the distancewhich the propeller would screw through the air inone revolution if there were no slippage.

Figure 3-25. Increase in stall speed and load factor.

Stall Speed Increase

Load Factor

100

90

80

60

40

20

00 10 20 30 40 50 60 70 80 90

13121110987654321

Per

cent

Incr

ease

InS

tall

Spe

ed

Loa

d F

acto

r or

"G

"

Bank Angle, Degrees

Thrust

Angle of Attack

Chord

Line

Rel

ativ

e W

ind

Forward Velocity

Pitch orBlade Angle

Rot

atio

nal V

eloc

ity

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When specifying a fixed-pitch propeller for a newtype of airplane, the manufacturer usually selectsone with a pitch that will operate efficiently at theexpected cruising speed of the airplane.Unfortunately, however, every fixed-pitch propellermust be a compromise, because it can be efficient atonly a given combination of airspeed and r.p.m.Pilots do not have it within their power to changethis combination in flight.

When the airplane is at rest on the ground with theengine operating, or moving slowly at the beginningof takeoff, the propeller efficiency is very lowbecause the propeller is restrained from advancingwith sufficient speed to permit its fixed-pitch bladesto reach their full efficiency. In this situation, eachpropeller blade is turning through the air at an angleof attack that produces relatively little thrust for theamount of power required to turn it.

To understand the action of a propeller, consider firstits motion, which is both rotational and forward.Thus, as shown by the vectors of propeller forces infigure 3-27, each section of a propeller blade movesdownward and forward. The angle at which this air(relative wind) strikes the propeller blade is its angleof attack. The air deflection produced by this anglecauses the dynamic pressure at the engine side of thepropeller blade to be greater than atmospheric, thuscreating thrust.

The shape of the blade also creates thrust, becauseit is cambered like the airfoil shape of a wing.Consequently, as the air flows past the propeller, thepressure on one side is less than that on the other. As ina wing, this produces a reaction force in the directionof the lesser pressure. In the case of a wing, the airflowover the wing has less pressure, and the force (lift) isupward. In the case of the propeller, which is mountedin a vertical instead of a horizontal plane, the areaof decreased pressure is in front of the propeller,and the force (thrust) is in a forward direction.Aerodynamically, then, thrust is the result of thepropeller shape and the angle of attack of the blade.

Another way to consider thrust is in terms of themass of air handled by the propeller. In theseterms, thrust is equal to the mass of air handled,times the slipstream velocity, minus the velocityof the airplane. The power expended in producingthrust depends on the rate of air mass movement.On the average, thrust constitutes approximately80 percent of the torque (total horsepowerabsorbed by the propeller). The other 20 percent islost in friction and slippage. For any speed of rota-tion, the horsepower absorbed by the propellerbalances the horsepower delivered by the engine.For any single revolution of the propeller, the

amount of air handled depends on the blade angle,which determines how big a “bite” of air the pro-peller takes. Thus, the blade angle is an excellentmeans of adjusting the load on the propeller tocontrol the engine r.p.m.

The blade angle is also an excellent method ofadjusting the angle of attack of the propeller. On con-stant-speed propellers, the blade angle must beadjusted to provide the most efficient angle of attackat all engine and airplane speeds. Lift versus dragcurves, which are drawn for propellers as well aswings, indicate that the most efficient angle of attackis a small one varying from 2° to 4° positive. Theactual blade angle necessary to maintain this smallangle of attack varies with the forward speed of theairplane.

Fixed-pitch and ground-adjustable propellers aredesigned for best efficiency at one rotation and for-ward speed. They are designed for a given airplaneand engine combination. A propeller may be usedthat provides the maximum propeller efficiency foreither takeoff, climb, cruise, or high-speed flight.Any change in these conditions results in loweringthe efficiency of both the propeller and the engine.Since the efficiency of any machine is the ratio ofthe useful power output to the actual power input,propeller efficiency is the ratio of thrust horsepowerto brake horsepower. Propeller efficiency variesfrom 50 to 87 percent, depending on how much thepropeller “slips.”

Propeller slip is the difference between the geometricpitch of the propeller and its effective pitch.[Figure 3-28] Geometric pitch is the theoreticaldistance a propeller should advance in one revolu-tion; effective pitch is the distance it actuallyadvances. Thus, geometric or theoretical pitch isbased on no slippage, but actual or effective pitchincludes propeller slippage in the air.

Figure 3-28. Propeller slippage.

The reason a propeller is “twisted” is that the outerparts of the propeller blades, like all things that turnabout a central point, travel faster than the portionsnear the hub. [Figure 3-29] If the blades had the

Slip

Effective PitchGeometric Pitch

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same geometric pitch throughout their lengths, atcruise speed the portions near the hub could havenegative angles of attack while the propeller tipswould be stalled. “Twisting,” or variations in thegeometric pitch of the blades, permits the propellerto operate with a relatively constant angle of attackalong its length when in cruising flight. To put itanother way, propeller blades are twisted to changethe blade angle in proportion to the differences inspeed of rotation along the length of the propellerand thereby keep thrust more nearly equalized alongthis length.

Figure 3-29. Propeller tips travel faster than hubs.

Usually 1° to 4° provides the most efficient lift/dragratio, but in flight the propeller angle of attack of afixed-pitch propeller will vary—normally from 0° to15°. This variation is caused by changes in the rela-tive airstream which in turn results from changes inairplane speed. In short, propeller angle of attack isthe product of two motions: propeller rotation aboutits axis and its forward motion.

A constant-speed propeller, however, automaticallykeeps the blade angle adjusted for maximum effi-ciency for most conditions encountered in flight.During takeoff, when maximum power and thrustare required, the constant-speed propeller is at a lowpropeller blade angle or pitch. The low blade anglekeeps the angle of attack small and efficient withrespect to the relative wind. At the same time, itallows the propeller to handle a smaller mass of airper revolution. This light load allows the engine toturn at high r.p.m. and to convert the maximumamount of fuel into heat energy in a given time. The

high r.p.m. also creates maximum thrust; for,although the mass of air handled per revolution issmall, the number of revolutions per minute is many,the slipstream velocity is high, and with the low air-plane speed, the thrust is maximum.

After liftoff, as the speed of the airplane increases,the constant-speed propeller automatically changesto a higher angle (or pitch). Again, the higher bladeangle keeps the angle of attack small and efficientwith respect to the relative wind. The higher bladeangle increases the mass of air handled per revolu-tion. This decreases the engine r.p.m., reducing fuelconsumption and engine wear, and keeps thrust at amaximum.

After the takeoff climb is established in an airplanehaving a controllable-pitch propeller, the pilotreduces the power output of the engine to climbpower by first decreasing the manifold pressure andthen increasing the blade angle to lower the r.p.m.

At cruising altitude, when the airplane is in levelflight and less power is required than is used in take-off or climb, the pilot again reduces engine power byreducing the manifold pressure and then increasingthe blade angle to decrease the r.p.m. Again, this pro-vides a torque requirement to match the reducedengine power; for, although the mass of air handledper revolution is greater, it is more than offset by adecrease in slipstream velocity and an increase inairspeed. The angle of attack is still small becausethe blade angle has been increased with an increasein airspeed.

TORQUE AND P FACTORTo the pilot, “torque” (the left turning tendency ofthe airplane) is made up of four elements whichcause or produce a twisting or rotating motionaround at least one of the airplane’s three axes. Thesefour elements are:

1. Torque Reaction from Engine and Propeller.

2. Corkscrewing Effect of the Slipstream.

3. Gyroscopic Action of the Propeller.

4. Asymmetric Loading of the Propeller (P Factor).

TORQUE REACTIONTorque reaction involves Newton’s Third Law ofPhysics—for every action, there is an equal andopposite reaction. As applied to the airplane, thismeans that as the internal engine parts and propellerare revolving in one direction, an equal force is try-ing to rotate the airplane in the opposite direction.[Figure 3-30]

Mod

erat

e

Travel Distance – Moderate Speed

Sho

rt

Travel Distance

Slow Speed

Gre

ater Travel Distance – Very High Speed

2500 r.p.m.

2500 r.p.m.

2500 r.p.m

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Figure 3-30.Torque reaction.

When the airplane is airborne, this force is actingaround the longitudinal axis, tending to make the air-plane roll. To compensate for this, some of the olderairplanes are rigged in a manner to create more lifton the wing that is being forced downward. Themore modern airplanes are designed with the engineoffset to counteract this effect of torque.

NOTE—Most United States built aircraft enginesrotate the propeller clockwise, as viewed from thepilot’s seat. The discussion here is with reference tothose engines.

Generally, the compensating factors are permanentlyset so that they compensate for this force at cruisingspeed, since most of the airplane’s operating lift is atthat speed. However, aileron trim tabs permit furtheradjustment for other speeds.

When the airplane’s wheels are on the ground duringthe takeoff roll, an additional turning momentaround the vertical axis is induced by torque reac-tion. As the left side of the airplane is being forceddown by torque reaction, more weight is beingplaced on the left main landing gear. This results inmore ground friction, or drag, on the left tire than onthe right, causing a further turning moment to theleft. The magnitude of this moment is dependent onmany variables. Some of these variables are: (1) sizeand horsepower of engine, (2) size of propeller andthe r.p.m., (3) size of the airplane, and (4) conditionof the ground surface.

This yawing moment on the takeoff roll is correctedby the pilot’s proper use of the rudder or rudder trim.

CORKSCREW EFFECTThe high-speed rotation of an airplane propellergives a corkscrew or spiraling rotation to the slip-stream. At high propeller speeds and low forwardspeed (as in the takeoffs and approaches to power-on stalls), this spiraling rotation is very compactand exerts a strong sideward force on the airplane’svertical tail surface. [Figure 3-31]

When this spiraling slipstream strikes the vertical finon the left, it causes a left turning moment about theairplane’s vertical axis. The more compact the spiral,the more prominent this force is. As the forwardspeed increases, however, the spiral elongates andbecomes less effective.

The corkscrew flow of the slipstream also causes arolling moment around the longitudinal axis.

Note that this rolling moment caused by thecorkscrew flow of the slipstream is to the right, whilethe rolling moment caused by torque reaction is tothe left—in effect one may be counteracting theother. However, these forces vary greatly and it is upto the pilot to apply proper correction action by useof the flight controls at all times. These forces mustbe counteracted regardless of which is the mostprominent at the time.

GYROSCOPIC ACTIONBefore the gyroscopic effects of the propeller can beunderstood, it is necessary to understand the basicprinciple of a gyroscope.

All practical applications of the gyroscope are basedupon two fundamental properties of gyroscopicaction: rigidity in space and precession. The one ofinterest for this discussion is precession.

Precession is the resultant action, or deflection, of aspinning rotor when a deflecting force is applied toits rim. As can be seen in figure 3-32, when a force isapplied, the resulting force takes effect 90° ahead ofand in the direction of rotation.

The rotating propeller of an airplane makes a verygood gyroscope and thus has similar properties. Anytime a force is applied to deflect the propeller out ofits plane of rotation, the resulting force is 90° aheadof and in the direction of rotation and in the directionof application, causing a pitching moment, a yawingmoment, or a combination of the two dependingupon the point at which the force was applied.

Action

Reaction

Prop Rotation

Yaw

Slipstream

Force

Figure 3-31. Corkscrewing slipstream.

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This element of torque effect has always beenassociated with and considered more prominent intailwheel-type airplanes, and most often occurswhen the tail is being raised during the takeoff roll.[Figure 3-33] This change in pitch attitude has thesame effect as applying a force to the top of thepropeller’s plane of rotation. The resultant forceacting 90° ahead causes a yawing moment to theleft around the vertical axis. The magnitude of thismoment depends on several variables, one ofwhich is the abruptness with which the tail israised (amount of force applied). However, preces-sion, or gyroscopic action, occurs when a force isapplied to any point on the rim of the propeller’splane of rotation; the resultant force will still be90° from the point of application in the directionof rotation. Depending on where the force isapplied, the airplane is caused to yaw left or right,to pitch up or down, or a combination of pitchingand yawing.

It can be said that as a result of gyroscopic action—any yawing around the vertical axis results in apitching moment, and any pitching around the lateralaxis results in a yawing moment.

To correct for the effect of gyroscopic action, it isnecessary for the pilot to properly use elevator andrudder to prevent undesired pitching and yawing.

ASYMMETRIC LOADING (P FACTOR)When an airplane is flying with a high angle ofattack, the “bite” of the downward moving blade is

greater than the “bite” of the upward movingblade; thus moving the center of thrust to theright of the prop disc area—causing a yawingmoment toward the left around the vertical axis.That explanation is correct; however, to provethis phenomenon, it would be necessary to workwind vector problems on each blade, which getsquite involved when considering both the angleof attack of the airplane and the angle of attack ofeach blade.

This asymmetric loading is caused by the resultantvelocity, which is generated by the combination ofthe velocity of the propeller blade in its plane ofrotation and the velocity of the air passing horizon-tally through the propeller “disc.” With the airplanebeing flown at positive angles of attack, the right(viewed from the rear) or downswinging blade, ispassing through an area of resultant velocity whichis greater than that affecting the left or upswingingblade. Since the propeller blade is an airfoil,increased velocity means increased lift. Therefore,the downswinging blade having more “lift” tendsto pull (yaw) the airplane’s nose to the left.

Simply stated, when the airplane is flying at a highangle of attack, the downward moving blade has ahigher resultant velocity; therefore creating more liftthan the upward moving blade. [Figure 3-34] Thismight be easier to visualize if the propeller shaft was

mounted perpendicular to the ground (like ahelicopter). If there were no air movement atall, except that generated by the propelleritself, identical sections of each blade wouldhave the same airspeed. However, with air mov-ing horizontally across this vertically mountedpropeller, the blade proceeding forward into theflow of air will have a higher airspeed than theblade retreating with the airflow. Thus, the bladeproceeding into the horizontal airflow is creatingmore lift, or thrust, moving the center of thrusttoward that blade. Visualize ROTATING thevertically mounted propeller shaft to shallower

Resultant force90°

Yaw

Effective Force

AppliedForce

Figure 3-32. Gyroscopic precession.

AppliedForce

ResultantForce

Yaw

EffectiveForce

Figure 3-33. Raising tail produces gyroscopic precession.

Low Angleof Attack

Load OnUpward Moving

Prop Blade

Load OnDownward Moving

Prop Blade

High Angleof Attack

Load OnUpward Moving

Prop Blade

Load OnDownward Moving

Prop Blade

Figure 3-34. Asymmetrical loading of propeller (P-factor).

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angles relative to the moving air (as on an airplane).This unbalanced thrust then becomes proportion-ately smaller and continues getting smaller until itreaches the value of zero when the propeller shaft isexactly horizontal in relation to the moving air.

Each of these four elements of torque effects varyin values with changes in flight situations. In onephase of flight, one of these elements may be moreprominent than another; whereas, in another phaseof flight, another element may be more prominent.The relationship of these values to each other willvary with different airplanes—depending on theAIRFRAME, ENGINE, AND PROPELLER com-binations as well as other design features.

To maintain positive control of the airplane in allflight conditions, the pilot must apply the flight con-trols as necessary to compensate for these varyingvalues.

LOAD FACTORSThe preceding sections only briefly considered someof the practical points of the principles of flight. Tobecome a pilot, a detailed technical course in thescience of aerodynamics is not necessary. However,with responsibilities for the safety of passengers, thecompetent pilot must have a well-founded concept ofthe forces which act on the airplane, and the advanta-geous use of these forces, as well as the operatinglimitations of the particular airplane. Any forceapplied to an airplane to deflect its flight from astraight line produces a stress on its structure; theamount of this force is termed “load factor.”

A load factor is the ratio of the total airload acting onthe airplane to the gross weight of the airplane. Forexample, a load factor of 3 means that the total loadon an airplane’s structure is three times its grossweight. Load factors are usually expressed in termsof “G”—that is, a load factor of 3 may be spoken ofas 3 G’s, or a load factor of 4 as 4 G’s.

It is interesting to note that in subjecting an airplaneto 3 G’s in a pullup from a dive, one will be presseddown into the seat with a force equal to three timesthe person’s weight. Thus, an idea of the magnitudeof the load factor obtained in any maneuver can bedetermined by considering the degree to which oneis pressed down into the seat. Since the operatingspeed of modern airplanes has increased signifi-cantly, this effect has become so pronounced that itis a primary consideration in the design of thestructure for all airplanes.

With the structural design of airplanes planned towithstand only a certain amount of overload, a

knowledge of load factors has become essential forall pilots. Load factors are important to the pilot fortwo distinct reasons:

1. Because of the obviously dangerous overload thatis possible for a pilot to impose on the airplanestructures; and

2. Because an increased load factor increases thestalling speed and makes stalls possible at seem-ingly safe flight speeds.

LOAD FACTORS IN AIRPLANE DESIGNThe answer to the question “how strong should anairplane be” is determined largely by the use to whichthe airplane will be subjected. This is a difficult prob-lem, because the maximum possible loads are muchtoo high for use in efficient design. It is true that anypilot can make a very hard landing or an extremelysharp pullup from a dive, which would result inabnormal loads. However, such extremely abnormalloads must be dismissed somewhat if airplanes arebuilt that will take off quickly, land slowly, and carrya worthwhile payload.

The problem of load factors in airplane design thenreduces to that of determining the highest load fac-tors that can be expected in normal operation undervarious operational situations. These load factorsare called “limit load factors.” For reasons ofsafety, it is required that the airplane be designed towithstand these load factors without any structuraldamage. Although the Code of Federal Regulationsrequires that the airplane structure be capable ofsupporting one and one-half times these limit loadfactors without failure, it is accepted that parts ofthe airplane may bend or twist under these loadsand that some structural damage may occur.

This 1.5 value is called the “factor of safety” andprovides, to some extent, for loads higher than thoseexpected under normal and reasonable operation.However, this strength reserve is not somethingwhich pilots should willfully abuse; rather it is therefor their protection when they encounter unexpectedconditions.

The above considerations apply to all loading con-ditions, whether they be due to gusts, maneuvers, orlandings. The gust load factor requirements now ineffect are substantially the same as those that havebeen in existence for years. Hundreds of thousandsof operational hours have proven them adequate forsafety. Since the pilot has little control over gustload factors (except to reduce the airplane’s speedwhen rough air is encountered), the gust loadingrequirements are substantially the same for mostgeneral aviation type airplanes regardless of their

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operational use. Generally, the gust load factors con-trol the design of airplanes which are intended forstrictly nonacrobatic usage.

An entirely different situation exists in airplanedesign with maneuvering load factors. It is necessaryto discuss this matter separately with respect to: (1)Airplanes which are designed in accordance with theCategory System (i.e., Normal, Utility, Acrobatic);and (2) Airplanes of older design which were built torequirements which did not provide for operationalcategories.

Airplanes designed under the Category System arereadily identified by a placard in the cockpit, whichstates the operational category (or categories) inwhich the airplane is certificated. The maximum safeload factors (limit load factors) specified for air-planes in the various categories are as follows:

CATEGORY LIMIT LOADNormal1 3.8 to –1.52Utility (mild acrobatics, including spins) 4.4 to –1.76Acrobatic 6.0 to –3.0

1 For airplanes with gross weight of more than 4,000pounds, the limit load factor is reduced. To the limitloads given above, a safety factor of 50 percent isadded.

There is an upward graduation in load factor with theincreasing severity of maneuvers. The CategorySystem provides for obtaining the maximum utilityof an airplane. If normal operation alone is intended,the required load factor (and consequently theweight of the airplane) is less than if the airplane isto be employed in training or acrobatic maneuvers asthey result in higher maneuvering loads.

Airplanes that do not have the category placard aredesigns that were constructed under earlier engineer-ing requirements in which no operational restrictionswere specifically given to the pilots. For airplanes ofthis type (up to weights of about 4,000 pounds) therequired strength is comparable to present-day util-ity category airplanes, and the same types of opera-tion are permissible. For airplanes of this type over4,000 pounds, the load factors decrease with weightso that these airplanes should be regarded as beingcomparable to the normal category airplanesdesigned under the Category System, and theyshould be operated accordingly.

LOAD FACTORS IN STEEP TURNSIn a constant altitude, coordinated turn in any air-plane, the load factor is the result of two forces:centrifugal force and gravity. [Figure 3-35] For anygiven bank angle, the rate of turn varies with the

airspeed; the higher the speed, the slower the rate ofturn. This compensates for added centrifugal force,allowing the load factor to remain the same.

Figure 3-35.Two forces cause load factor during turns.

Figure 3-36 reveals an important fact about turns—that the load factor increases at a terrific rate after abank has reached 45° or 50°. The load factor forany airplane in a 60° bank is 2 G’s. The load factorin an 80° bank is 5.76 G’s. The wing must producelift equal to these load factors if altitude is to bemaintained.

Figure 3-36. Angle of bank changes load factor.

It should be noted how rapidly the line denoting loadfactor rises as it approaches the 90° bank line, whichit reaches only at infinity. The 90° banked, constantaltitude turn mathematically is not possible. True, anairplane may be banked to 90° but not in a coordi-nated turn; an airplane which can be held in a 90°

60°

Centrifugal Force 1.73 G's

Load Factor 2 G'sGra

vity

1G

Load Factor Chart

7

6

5

4

3

2

1

0 10 20 30 40 50 60 70 80 90Bank Angle - In Degrees

Load

Fac

tor

- G

Uni

ts

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banked slipping turn is capable of straight knife-edged flight. At slightly more than 80°, the loadfactor exceeds the limit of 6 G’s, the limit loadfactor of an acrobatic airplane.

For a coordinated, constant altitude turn, theapproximate maximum bank for the average generalaviation airplane is 60°. This bank and its resultantnecessary power setting reach the limit of this typeof airplane. An additional 10° bank will increase theload factor by approximately 1 G, bringing it closeto the yield point established for these airplanes.[Figure 3-36]

LOAD FACTORS AND STALLING SPEEDSAny airplane, within the limits of its structure, maybe stalled at any airspeed. When a sufficiently highangle of attack is imposed, the smooth flow of airover an airfoil breaks up and separates, producing anabrupt change of flight characteristics and a suddenloss of lift, which results in a stall.

A study of this effect has revealed that the airplane’sstalling speed increases in proportion to the squareroot of the load factor. This means that an airplanewith a normal unaccelerated stalling speed of 50knots can be stalled at 100 knots by inducing a loadfactor of 4 G’s. If it were possible for this airplane towithstand a load factor of 9, it could be stalled at aspeed of 150 knots. Therefore, a competent pilotshould be aware of the following:

• The danger of inadvertently stalling the airplaneby increasing the load factor, as in a steep turn orspiral; and

• That in intentionally stalling an airplane above itsdesign maneuvering speed, a tremendous loadfactor is imposed.

Reference to the charts in figures 3-36 and 3-37 willshow that by banking the airplane to just beyond 72°in a steep turn produces a load factor of 3, and thestalling speed is increased significantly. If this turn ismade in an airplane with a normal unacceleratedstalling speed of 45 knots, the airspeed must be keptabove 75 knots to prevent inducing a stall. A similareffect is experienced in a quick pullup, or anymaneuver producing load factors above 1 G. Thishas been the cause of accidents resulting from a sud-den, unexpected loss of control, particularly in asteep turn or abrupt application of the back elevatorcontrol near the ground.

Since the load factor squares as the stalling speeddoubles, it may be realized that tremendous loadsmay be imposed on structures by stalling an airplaneat relatively high airspeeds.

The maximum speed at which an airplane may bestalled safely is now determined for all new designs.This speed is called the “design maneuveringspeed” (VA) and is required to be entered in theFAA-approved Airplane Flight Manual or Pilot’sOperating Handbook (AFM/POH) of all recentlydesigned airplanes. For older general aviation air-planes, this speed will be approximately 1.7 timesthe normal stalling speed. Thus, an older airplanewhich normally stalls at 60 knots must never bestalled at above 102 knots (60 knots x 1.7 = 102knots). An airplane with a normal stalling speed of

Figure 3-37. Load factor changes stall speed.

40

50

60

70

80

100

120

150

Load Factor vs. Stall Speed

0

1

2

3

4

5

Rat

io o

f Acc

el. V

to

Una

ccel

erat

ed V

ss

"G" Load Accelerated Stall Speed

Una

ccel

erat

ed S

tall

Spe

ed

0 1 2 3 4 5 6 7 8 20 40 60 80 100 120 140 160 180 200 220 240 260

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60 knots will undergo, when stalled at 102 knots, aload factor equal to the square of the increase inspeed or 2.89 G’s (1.7 x 1.7 = 2.89 G’s). (The abovefigures are an approximation to be considered as aguide and are not the exact answers to any set ofproblems. The design maneuvering speed should bedetermined from the particular airplane’s operatinglimitations when provided by the manufacturer.)

Since the leverage in the control system varieswith different airplanes and some types employ“balanced” control surfaces while others do not,the pressure exerted by the pilot on the controlscannot be accepted as an index of the load factorsproduced in different airplanes. In most cases, loadfactors can be judged by the experienced pilot fromthe feel of seat pressure. They can also be meas-ured by an instrument called an “accelerometer,”but since this instrument is not common in generalaviation training airplanes, the development of theability to judge load factors from the feel of theireffect on the body is important. A knowledge ofthe principles outlined above is essential to thedevelopment of this ability to estimate load factors.

A thorough knowledge of load factors induced byvarying degrees of bank, and the significance ofdesign maneuvering speed (VA) will aid in the pre-vention of two of the most serious types of accidents:

1. Stalls from steep turns or excessive maneuveringnear the ground; and

2. Structural failures during acrobatics or other vio-lent maneuvers resulting from loss of control.

LOAD FACTORS AND FLIGHT MANEUVERSCritical load factors apply to all flight maneuversexcept unaccelerated straight flight where a loadfactor of 1 G is always present. Certain maneuversconsidered in this section are known to involve rel-atively high load factors.

TURNS—Increased load factors are a characteristicof all banked turns. As noted in the section on loadfactors in steep turns and particularly figures 3-36and 3-37, load factors become significant both toflight performance and to the load on wing structureas the bank increases beyond approximately 45°.

The yield factor of the average light plane is reached ata bank of approximately 70° to 75°, and the stallingspeed is increased by approximately one-half at a bankof approximately 63°.

STALLS—The normal stall entered from straightlevel flight, or an unaccelerated straight climb, willnot produce added load factors beyond the 1 G of

straight-and-level flight. As the stall occurs, how-ever, this load factor may be reduced toward zero,the factor at which nothing seems to have weight;and the pilot has the feeling of “floating free inspace.” In the event recovery is effected by snappingthe elevator control forward, negative load factors,those which impose a down load on the wings andraise the pilot from the seat, may be produced.

During the pullup following stall recovery, significantload factors sometimes are induced. Inadvertentlythese may be further increased during excessive div-ing (and consequently high airspeed) and abruptpullups to level flight. One usually leads to the other,thus increasing the load factor. Abrupt pullups at highdiving speeds may impose critical loads on airplanestructures and may produce recurrent or secondarystalls by increasing the angle of attack to that ofstalling.

As a generalization, a recovery from a stall madeby diving only to cruising or design maneuveringairspeed, with a gradual pullup as soon as the air-speed is safely above stalling, can be effected witha load factor not to exceed 2 or 2.5 G’s. A higherload factor should never be necessary unlessrecovery has been effected with the airplane’s nosenear or beyond the vertical attitude, or at extremelylow altitudes to avoid diving into the ground.

SPINS—Since a stabilized spin is not essentiallydifferent from a stall in any element other thanrotation, the same load factor considerations applyas those that apply to stall recovery. Since spinrecoveries usually are effected with the nose muchlower than is common in stall recoveries, higherairspeeds and consequently higher load factors areto be expected. The load factor in a proper spinrecovery will usually be found to be about 2.5 G’s.

The load factor during a spin will vary with the spincharacteristics of each airplane but is usually foundto be slightly above the 1 G of level flight. There aretwo reasons this is true:

1. The airspeed in a spin is very low, usually within2 knots of the unaccelerated stalling speeds; and

2. The airplane pivots, rather than turns, while it isin a spin.

HIGH-SPEED STALLS—The average light planeis not built to withstand the repeated application ofload factors common to high-speed stalls. The loadfactor necessary for these maneuvers produces astress on the wings and tail structure, which does notleave a reasonable margin of safety in most light air-planes.

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The only way this stall can be induced at an airspeedabove normal stalling involves the imposition of anadded load factor, which may be accomplished by asevere pull on the elevator control. A speed of 1.7times stalling speed (about 102 knots in a light air-plane with a stalling speed of 60 knots) will producea load factor of 3 G’s. Further, only a very narrowmargin for error can be allowed for acrobatics inlight airplanes. To illustrate how rapidly the loadfactor increases with airspeed, a high-speed stall at112 knots in the same airplane would produce a loadfactor of 4 G’s.

CHANDELLES AND LAZY EIGHTS—It wouldbe difficult to make a definite statement concerningload factors in these maneuvers as both involvesmooth, shallow dives and pullups. The load factorsincurred depend directly on the speed of the divesand the abruptness of the pullups.

Generally, the better the maneuver is performed, theless extreme will be the load factor induced. A chan-delle or lazy eight, in which the pullup produces aload factor greater than 2 G’s will not result in asgreat a gain in altitude, and in low-powered airplanesit may result in a net loss of altitude.

The smoothest pullup possible, with a moderateload factor, will deliver the greatest gain in altitudein a chandelle and will result in a better overallperformance in both chandelles and lazy eights.Further, it will be noted that recommended entryspeed for these maneuvers is generally near themanufacturer’s design maneuvering speed, therebyallowing maximum development of load factorswithout exceeding the load limits.

ROUGH AIR—All certificated airplanes aredesigned to withstand loads imposed by gusts ofconsiderable intensity. Gust load factors increasewith increasing airspeed and the strength used fordesign purposes usually corresponds to the highestlevel flight speed. In extremely rough air, as in thun-derstorms or frontal conditions, it is wise to reducethe speed to the design maneuvering speed.Regardless of the speed held, there may be gusts thatcan produce loads which exceed the load limits.

Most airplane flight manuals now include turbulentair penetration information. Operators of modernairplanes, capable of a wide range of speeds andaltitudes, are benefited by this added feature both incomfort and safety. In this connection, it is to benoted that the maximum “never-exceed” placarddive speeds are determined for smooth air only.High-speed dives or acrobatics involving speedabove the known maneuvering speed should neverbe practiced in rough or turbulent air.

In summary, it must be remembered that load factorsinduced by intentional acrobatics, abrupt pullupsfrom dives, high-speed stalls, and gusts at high air-speeds all place added stress on the entire structureof an airplane.

Stress on the structure involves forces on any part ofthe airplane. There is a tendency for the uninformedto think of load factors only in terms of their effecton spars and struts. Most structural failures due toexcess load factors involve rib structure within theleading and trailing edges of wings and tail group.The critical area of fabric-covered airplanes is thecovering about one-third of the chord aft on the topsurface of the wing.

The cumulative effect of such loads over a longperiod of time may tend to loosen and weaken vitalparts so that actual failure may occur later when theairplane is being operated in a normal manner.

VG DIAGRAMThe flight operating strength of an airplane is pre-sented on a graph whose horizontal scale is based onload factor. [Figure 3-38] The diagram is called a Vgdiagram—velocity versus “g” loads or load factor.Each airplane has its own Vg diagram which is validat a certain weight and altitude.

The lines of maximum lift capability (curved lines)are the first items of importance on the Vg diagram.The subject airplane in the illustration is capable ofdeveloping no more than one positive “g” at 62m.p.h., the wing level stall speed of the airplane.Since the maximum load factor varies with thesquare of the airspeed, the maximum positive liftcapability of this airplane is 2 “g” at 92 m.p.h., 3“g” at 112 m.p.h., 4.4 “g” at 137 m.p.h., and soforth. Any load factor above this line is unavailableaerodynamically; i.e., the subject airplane cannotfly above the line of maximum lift capability (itwill stall). Essentially the same situation exists fornegative lift flight with the exception that the speednecessary to produce a given negative load factor ishigher than that to produce the same positive loadfactor.

If the subject airplane is flown at a positive loadfactor greater than the positive limit load factor of4.4, structural damage will be possible. When theairplane is operated in this region, objectionablepermanent deformation of the primary structuremay take place and a high rate of fatigue damage isincurred. Operation above the limit load factormust be avoided in normal operation.

There are two other points of importance on the Vgdiagram. First, is the intersection of the positive limit

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load factor and the line of maximum positive liftcapability. The airspeed at this point is the minimumairspeed at which the limit load can be developedaerodynamically. Any airspeed greater than this pro-vides a positive lift capability sufficient to damagethe airplane; any airspeed less does not providepositive lift capability sufficient to cause damagefrom excessive flight loads. The usual term given tothis speed is “maneuvering speed,” since considerationof subsonic aerodynamics would predict minimumusable turn radius to occur at this condition. Themaneuver speed is a valuable reference point, since anairplane operating below this point cannot produce adamaging positive flight load. Any combination ofmaneuver and gust cannot create damage due to excessairload when the airplane is below the maneuverspeed.

Next, is the intersection of the negative limit loadfactor and line of maximum negative lift capability.Any airspeed greater than this provides a negativelift capability sufficient to damage the airplane; anyairspeed less does not provide negative lift capabil-ity sufficient to damage the airplane from excessiveflight loads.

The limit airspeed (or redline speed) is a design ref-erence point for the airplane—the subject airplane islimited to 225 m.p.h. If flight is attempted beyond

the limit airspeed, structural damage or structuralfailure may result from a variety of phenomena.

Thus, the airplane in flight is limited to a regime ofairspeeds and g’s which do not exceed the limit (orredline) speed, do not exceed the limit load factor,and cannot exceed the maximum lift capability. Theairplane must be operated within this “envelope” toprevent structural damage and ensure that the antic-ipated service lift of the airplane is obtained. Thepilot must appreciate the Vg diagram as describingthe allowable combination of airspeeds and loadfactors for safe operation. Any maneuver, gust, orgust plus maneuver outside the structural envelopecan cause structural damage and effectively shortenthe service life of the airplane.

WEIGHT AND BALANCEOften a pilot regards the airplane’s weight and balancedata as information of interest only to engineers,dispatchers, and operators of scheduled and non-scheduled air carriers. Along with this idea, thereasoning is that the airplane was weighed duringthe certification process and that this data is validindefinitely, regardless of equipment changes ormodifications. Further, this information is mistakenlyreduced to a workable routine or “rule of thumb”such as: “If I have three passengers, I can load only100 gallons of fuel; four passengers—70 gallons.”

Figure 3-38.Typical Vg diagram.

Structural Damage

Structural Damage

Caution Range

NormalStall Speed

Level Flight at One "G"

Accelerated Stall

NormalOperating

Range

Maneuvering Speed

Max.Structural

Cruise Speed

Structural

Failure

7

6

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3

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1

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-1

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-3

Load

Fac

tor

20 40 60 80 100 120 140 160 180 200 220 240

Indicated Airspeed MPH

Never ExceedSpeed

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Admittedly, this rule of thumb is adequate in manycases, but as the subject “Weight and Balance” sug-gests, the concern is not only with the weight of theairplane but also the location of its center of gravity(CG). The importance of the CG should have becomeapparent in the discussion of stability, controllability,and performance. If all pilots understood andrespected the effect of CG on an airplane, then onetype of accident would be eliminated from therecords: “PRIMARY CAUSE OF ACCIDENT—AIRPLANE CENTER OF GRAVITY OUT OFREARWARD LIMITS AND UNEQUAL LOADDISTRIBUTION RESULTING IN AN UNSTABLEAIRPLANE. PILOT LOST CONTROL OF AIR-PLANE ON TAKEOFF AND CRASHED.”

The reasons airplanes are so certificated are obviouswhen one gives it a little thought. For instance, it isof added value to the pilot to be able to carry extrafuel for extended flights when the full complementof passengers is not to be carried. Further, it is unrea-sonable to forbid the carriage of baggage when it isonly during spins that its weight will adversely affectthe airplane’s flight characteristics. Weight and bal-ance limits are placed on airplanes for two principalreasons:

1. Because of the effect of the weight on the air-plane’s primary structure and its performancecharacteristics; and

2. Because of the effect the location of this weighthas on flight characteristics, particularly in stalland spin recovery and stability.

EFFECTS OF WEIGHT ON FLIGHT PERFORMANCEThe takeoff/climb and landing performance of anairplane are determined on the basis of its maximumallowable takeoff and landing weights. A heaviergross weight will result in a longer takeoff run andshallower climb, and a faster touchdown speed andlonger landing roll. Even a minor overload maymake it impossible for the airplane to clear an obsta-cle that normally would not have been seriouslyconsidered during takeoffs under more favorableconditions.

The detrimental effects of overloading on performanceare not limited to the immediate hazards involvingtakeoffs and landings. Overloading has an adverseeffect on all climb and cruise performance which leadsto overheating during climbs, added wear on engineparts, increased fuel consumption, slower cruisingspeeds, and reduced range.

The manufacturers of modern airplanes furnishweight and balance data with each airplane produced.

Generally, this information may be found in the FAA-approved Airplane Flight Manual or Pilot’s OperatingHandbook (AFM/POH). With the advancements inairplane design and construction in recent years hascome the development of “easy to read charts” fordetermining weight and balance data. Increasedperformance and load carrying capability of theseairplanes require strict adherence to the operatinglimitations prescribed by the manufacturer.Deviations from the recommendations can result instructural damage or even complete failure of theairplane’s structure. Even if an airplane is loadedwell within the maximum weight limitations, it isimperative that weight distribution be within thelimits of center of gravity location. The precedingbrief study of aerodynamics and load factors pointsout the reasons for this precaution. The followingdiscussion is background information into some ofthe reasons why weight and balance conditions areimportant to the safe flight of an airplane.

The pilot is often completely unaware of the weightand balance limitations of the airplane being flownand of the reasons for these limitations. In some air-planes, it is not possible to fill all seats, baggagecompartments, and fuel tanks, and still remainwithin approved weight or balance limits. As anexample, in several popular four-place airplanes thefuel tanks may not be filled to capacity when fouroccupants and their baggage are carried. In a certaintwo-place airplane, no baggage may be carried inthe compartment aft of the seats when spins are tobe practiced.

EFFECT OF WEIGHT ON AIRPLANE STRUCTUREThe effect of additional weight on the wing structureof an airplane is not readily apparent. Airworthinessrequirements prescribe that the structure of an air-plane certificated in the normal category (in whichacrobatics are prohibited) must be strong enoughto withstand a load factor of 3.8 to take care ofdynamic loads caused by maneuvering and gusts.This means that the primary structure of the airplanecan withstand a load of 3.8 times the approved grossweight of the airplane without structural failureoccurring. If this is accepted as indicative of theload factors that may be imposed during operationsfor which the airplane is intended, a 100-poundoverload imposes a potential structural overloadof 380 pounds. The same consideration is evenmore impressive in the case of utility and acrobaticcategory airplanes, which have load factorrequirements of 4.4 and 6.0 respectively.

Structural failures which result from overloadingmay be dramatic and catastrophic, but more oftenthey affect structural components progressively in a

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manner which is difficult to detect and expensive torepair. One of the most serious results of habitualoverloading is that its results tend to be cumulative,and may result in structural failure later during com-pletely normal operations. The additional stressplaced on structural parts by overloading is believed toaccelerate the occurrence of metallic fatigue failures.

A knowledge of load factors imposed by flightmaneuvers and gusts will emphasize the conse-quences of an increase in the gross weight of anairplane. The structure of an airplane about toundergo a load factor of 3 G’s, as in the recoveryfrom a steep dive, must be prepared to withstandan added load of 300 pounds for each 100-poundincrease in weight. It should be noted that thiswould be imposed by the addition of about 16 gallonsof unneeded fuel in a particular airplane. The FAAcertificated civil airplane has been analyzed struc-turally, and tested for flight at the maximum grossweight authorized and within the speeds posted forthe type of flights to be performed. Flights at weightsin excess of this amount are quite possible and oftenare well within the performance capabilities of anairplane. Nonetheless, this fact should not beallowed to mislead the pilot, as the pilot may notrealize that loads for which the airplane was notdesigned are being imposed on all or some part ofthe structure.

In loading an airplane with either passengers orcargo, the structure must be considered. Seats,baggage compartments, and cabin floors aredesigned for a certain load or concentration ofload and no more. As an example, a light planebaggage compartment may be placarded for 20pounds because of the limited strength of its sup-porting structure even though the airplane may notbe overloaded or out of center-of-gravity limitswith more weight at that location.

EFFECTS OF WEIGHT ON STABILITY AND CONTROLLABILITYThe effects that overloading has on stability also arenot generally recognized. An airplane, which isobserved to be quite stable and controllable whenloaded normally, may be discovered to have verydifferent flight characteristics when it is overloaded.Although the distribution of weight has the mostdirect effect on this, an increase in the airplane’sgross weight may be expected to have an adverseeffect on stability, regardless of location of thecenter of gravity.

The stability of many certificated airplanes is com-pletely unsatisfactory if the gross weight isexceeded.

EFFECT OF LOAD DISTRIBUTIONThe effect of the position of the center of gravityon the load imposed on an airplane’s wing in flightis not generally realized, although it may be verysignificant to climb and cruising performance.Contrary to the beliefs of some pilots, an airplanewith forward loading is “heavier” and conse-quently, slower than the same airplane with thecenter of gravity further aft.

Figure 3-39 illustrates the reason for this. With forwardloading, “nose-up” trim is required in most airplanesto maintain level cruising flight. Nose-up triminvolves setting the tail surfaces to produce agreater down load on the aft portion of the fuselage,which adds to the wing loading and the total liftrequired from the wing if altitude is to be main-tained. This requires a higher angle of attack ofthe wing, which results in more drag and, in turn,produces a higher stalling speed.

Figure 3-39. Load distribution affects balance.

With aft loading and “nose-down” trim, the tail sur-faces will exert less down load, relieving the wing ofthat much wing loading and lift required to maintainaltitude. The required angle of attack of the wing isless, so the drag is less, allowing for a faster cruisespeed. Theoretically, a neutral load on the tail surfacesin cruising flight would produce the most efficientoverall performance and fastest cruising speed, butwould also result in instability. Consequently, modernairplanes are designed to require a down load on thetail for stability and controllability.

Remember that a zero indication on the trim tab con-trol is not necessarily the same as “neutral trim”because of the force exerted by downwash from thewings and the fuselage on the tail surfaces.

The effects of the distribution of the airplane’s use-ful load have a significant influence on its flight

Centerof Lift

Centerof Lift

Load Imposedby Tail

Gross Weight

ForwardCG

Load Imposedby TailGross Weight

AFT CG

StrongerDown Load

on Tail

LighterDown Load

on Tail

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characteristics, even when the load is within the cen-ter-of-gravity limits and the maximum permissiblegross weight. Important among these effects arechanges in controllability, stability, and the actualload imposed on the wing.

Generally, an airplane becomes less controllable,especially at slow flight speeds, as the center ofgravity is moved further aft. An airplane whichcleanly recovers from a prolonged spin with thecenter of gravity at one position may fail completelyto respond to normal recovery attempts when thecenter of gravity is moved aft by 1 or 2 inches.

It is common practice for airplane designers toestablish an aft center-of-gravity limit that is within1 inch of the maximum which will allow normalrecovery from a one-turn spin. When certificatingan airplane in the utility category to permit inten-tional spins, the aft center-of-gravity limit is usuallyestablished at a point several inches forward of thatwhich is permissible for certification in the normalcategory.

Another factor affecting controllability, which isbecoming more important in current designs of largeairplanes, is the effect of long moment arms to thepositions of heavy equipment and cargo. The sameairplane may be loaded to maximum gross weightwithin its center-of-gravity limits by concentratingfuel, passengers, and cargo near the design center ofgravity; or by dispersing fuel and cargo loads inwingtip tanks and cargo bins forward and aft of thecabin.

With the same total weight and center of gravity,maneuvering the airplane or maintaining level flightin turbulent air will require the application of greatercontrol forces when the load is dispersed. This is truebecause of the longer moment arms to the positions ofthe heavy fuel and cargo loads which must be over-come by the action of the control surfaces. An airplanewith full outboard wing tanks or tip tanks tends to besluggish in roll when control situations are marginal,while one with full nose and aft cargo bins tends to beless responsive to the elevator controls.

The rearward center-of-gravity limit of an airplane isdetermined largely by considerations of stability.The original airworthiness requirements for a typecertificate specify that an airplane in flight at a cer-tain speed will dampen out vertical displacementof the nose within a certain number of oscillations.An airplane loaded too far rearward may not dothis; instead when the nose is momentarily pulledup, it may alternately climb and dive becomingsteeper with each oscillation. This instability is notonly uncomfortable to occupants, but it could even

become dangerous by making the airplane unman-ageable under certain conditions.

The recovery from a stall in any airplane becomesprogressively more difficult as its center of gravitymoves aft. This is particularly important in spinrecovery, as there is a point in rearward loading ofany airplane at which a “flat” spin will develop. Aflat spin is one in which centrifugal force, actingthrough a center of gravity located well to the rear,will pull the tail of the airplane out away from theaxis of the spin, making it impossible to get the nosedown and recover.

An airplane loaded to the rear limit of its permissiblecenter-of-gravity range will handle differently inturns and stall maneuvers and have different landingcharacteristics than when it is loaded near the for-ward limit.

The forward center-of-gravity limit is determined bya number of considerations. As a safety measure, itis required that the trimming device, whether tab oradjustable stabilizer, be capable of holding the air-plane in a normal glide with the power off. A con-ventional airplane must be capable of a full stall,power-off landing in order to ensure minimum land-ing speed in emergencies. A tailwheel-type airplaneloaded excessively nose heavy will be difficult totaxi, particularly in high winds. It can be nosed overeasily by use of the brakes, and it will be difficult toland without bouncing since it tends to pitch downon the wheels as it is slowed down and flared forlanding. Steering difficulties on the ground mayoccur in nosewheel-type airplanes, particularly dur-ing the landing roll and takeoff.

• The CG position influences the lift and angle ofattack of the wing, the amount and direction offorce on the tail, and the degree of deflection ofthe stabilizer needed to supply the proper tailforce for equilibrium. The latter is very importantbecause of its relationship to elevator controlforce.

• The airplane will stall at a higher speed with aforward CG location. This is because the stallingangle of attack is reached at a higher speed due toincreased wing loading.

• Higher elevator control forces normally existwith a forward CG location due to the increasedstabilizer deflection required to balance the air-plane.

• The airplane will cruise faster with an aft CGlocation because of reduced drag. The drag isreduced because a smaller angle of attack and less

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downward deflection of the stabilizer arerequired to support the airplane and overcome thenose-down pitching tendency.

• The airplane becomes less stable as the CG ismoved rearward. This is because when the CG ismoved rearward it causes an increase in the angleof attack. Therefore, the wing contribution to theairplane’s stability is now decreased, while thetail contribution is still stabilizing. When thepoint is reached that the wing and tail contribu-tions balance, then neutral stability exists. AnyCG movement further aft will result in an unsta-ble airplane.

• A forward CG location increases the need forgreater back elevator pressure. The elevator mayno longer be able to oppose any increase innose-down pitching. Adequate elevator controlis needed to control the airplane throughout theairspeed range down to the stall.

HIGH-SPEED FLIGHT

SUPERSONIC VS. SUBSONIC FLOWIn subsonic aerodynamics, the theory of lift is basedupon the forces generated on a body and a movinggas (air) in which it is immersed. At speeds belowabout 260 knots, air can be considered incompress-ible, in that at a fixed altitude, its density remainsnearly constant while its pressure varies. Underthis assumption, air acts the same as water and isclassified as a fluid. Subsonic aerodynamic theoryalso assumes the effects of viscosity (the propertyof a fluid that tends to prevent motion of one partof the fluid with respect to another) are negligible,and classifies air as an ideal fluid, conforming tothe principles of ideal-fluid aerodynamics such ascontinuity, Bernoulli’s principle, and circulation.

In reality, air is compressible and viscous. While theeffects of these properties are negligible at lowspeeds, compressibility effects in particular becomeincreasingly important as speed increases.Compressibility (and to a lesser extent viscosity) isof paramount importance at speeds approaching thespeed of sound. In these speed ranges, compressibil-ity causes a change in the density of the air aroundan airplane.

During flight, a wing produces lift by acceleratingthe airflow over the upper surface. This acceleratedair can, and does, reach sonic speeds even though theairplane itself may be flying subsonic. At someextreme angles of attack, in some airplanes, thespeed of the air over the top surface of the wing maybe double the airplane’s speed. It is therefore entirely

possible to have both supersonic and subsonic air-flow on an airplane at the same time. When flowvelocities reach sonic speeds at some location on anairplane (such as the area of maximum camber onthe wing), further acceleration will result in theonset of compressibility effects such as shock waveformation, drag increase, buffeting, stability, andcontrol difficulties. Subsonic flow principles areinvalid at all speeds above this point. [Figure 3-40]

Figure 3-40. Wing airflow.

SPEED RANGESThe speed of sound varies with temperature. Understandard temperature conditions of 15°C, the speedof sound at sea level is 661 knots. At 40,000 feet,where the temperature is –55°C, the speed of sounddecreases to 574 knots. In high-speed flight and/orhigh-altitude flight, the measurement of speed isexpressed in terms of a “Mach number”—the ratioof the true airspeed of the airplane to the speed ofsound in the same atmospheric conditions. An air-plane traveling at the speed of sound is traveling atMach 1.0. Airplane speed regimes are defined asfollows:

Subsonic—Mach numbers below 0.75

Transonic—Mach numbers from .075 to 1.20

Supersonic—Mach numbers from 1.20 to 5.00

Hypersonic—Mach numbers above 5.00

While flights in the transonic and supersonic rangesare common occurrences for military airplanes,civilian jet airplanes normally operate in a cruisespeed range of Mach 0.78 to Mach 0.90.

The speed of an airplane in which airflow over anypart of the wing first reaches (but does not exceed)Mach 1.0 is termed that airplane’s critical Machnumber or “Mach Crit.” Thus, critical Mach number

M=.50

M=.72(Critical Mach Number)

SupersonicFlow

M=.77

Normal Shock WaveSubsonic Possible Separation

Maximum Local VelocityIs Less Than Sonic

Maximum Local VelocityEqual To Sonic

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is the boundary between subsonic and transonicflight and is an important point of reference for allcompressibility effects encountered in transonicflight. Shock waves, buffet, and airflow separationtake place above critical Mach number. A jet airplanetypically is most efficient when cruising at or near itscritical Mach number. At speeds 5 – 10 percent abovethe critical Mach number, compressibility effectsbegin. Drag begins to rise sharply. Associated withthe “drag rise” are buffet, trim and stability changes,and a decrease in control surface effectiveness. Thisis the point of “drag divergence,” and is typically thespeed chosen for high-speed cruise operations. Atsome point beyond high-speed cruise are the turbinepowered airplane’s maximum operating limitspeeds: VMO/MMO. [Figure 3-41]

Figure 3-41. Critical Mach.

VMO is the maximum operating speed expressed interms of knots. VMO limits ram air pressure actingagainst the structure and prevents flutter. MMO is themaximum operating speed expressed in terms of Machnumber. An airplane should not be flown in excess ofthis speed. Doing so risks encountering the full effectsof compressibility, including possible loss of control.

MACH NUMBER VS. AIRSPEEDSpeeds such as Mach Crit and MMO for a specific air-plane occur at a given Mach number. The true airspeed(TAS), however, varies with outside air temperature.Therefore, true airspeeds corresponding to a specificMach number can vary considerably (as much as 75 –100 knots). When an airplane cruising at a constantMach number enters an area of higher outside air tem-peratures, true airspeed and required fuel increases, andrange decreases. Conversely, when entering an area ofcolder outside air temperatures, true airspeed and fuelflow decreases, and range increases.

In a jet airplane operating at high altitude, the indicatedairspeed (IAS) for any given Mach number decreaseswith an increase in altitude above a certain level. Thereverse occurs during descent. Normally, climbs and

descents are accomplished using indicated airspeed inthe lower altitudes and Mach number in the higheraltitudes.

Unlike operations in the lower altitudes, the indi-cated airspeed (IAS) at which a jet airplane stallsincreases significantly with altitude. This is due tothe fact that true airspeed (TAS) increases withaltitude. At high true airspeeds, air compressioncauses airflow distortion over the wings and in thepitot system. At the same time, the indicated airspeed(IAS) representing MMO decreases with altitude.Eventually, the airplane can reach an altitude wherethere is little or no difference between the two.

BOUNDARY LAYERAir has viscosity, and will encounter resistance toflow over a surface. The viscous nature of airflowreduces the local velocities on a surface and isresponsible for skin friction drag. As the air passesover the wing’s surface, the air particles nearest thesurface come to rest. The next layer of particles isslowed down but not stopped. Some small but meas-urable distance from the surface, the air particles aremoving at free stream velocity. The layer of air overthe wing’s surface, which is slowed down or stoppedby viscosity, is termed the “boundary layer.” Typicalboundary layer thicknesses on an airplane rangefrom small fractions of an inch near the leading edgeof a wing to the order of 12 inches at the aft end of alarge airplane such as a Boeing 747.

There are two different types of boundary layer flow:laminar and turbulent. The laminar boundary layer is avery smooth flow, while the turbulent boundary layercontains swirls or “eddies.” The laminar flow createsless skin friction drag than the turbulent flow, but isless stable. Boundary layer flow over a wing surfacebegins as a smooth laminar flow. As the flow continuesback from the leading edge, the laminar boundary layerincreases in thickness. At some distance back from theleading edge, the smooth laminar flow breaks downand transitions to a turbulent flow. From a dragstandpoint, it is advisable to have the transition fromlaminar to turbulent flow as far aft on the wing aspossible, or have a large amount of the wing surfacewithin the laminar portion of the boundary layer. Thelow energy laminar flow, however, tends to breakdown more suddenly than the turbulent layer.

Another phenomenon associated with viscous flowis separation. Separation occurs when the airflowbreaks away from an airfoil. The natural progressionis from laminar boundary layer to turbulent bound-ary layer and then to airflow separation. Airflowseparation produces high drag and ultimatelydestroys lift. The boundary layer separation point

CDrag

Coefficient

DForce Divergence

Mach NumberCritical

Mach NumberC = 0.3L

0.5 1.0

M, Mach Number

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moves forward on the wing as the angle of attack isincreased. [Figure 3-42]

“Vortex Generators” are used to delay or preventshock wave induced boundary layer separationencountered in transonic flight. Vortex generatorsare small low aspect ratio airfoils placed at a 12° to15° angle of attack to the airstream. They are usu-ally spaced a few inches apart along the wingahead of the ailerons or other control surfaces.Vortex generators create a vortex which mixes theboundary airflow with the high energy airflow justabove the surface. This produces higher surfacevelocities and increases the energy of the boundarylayer. Thus, a stronger shock wave will be necessaryto produce airflow separation.

SHOCK WAVESWhen an airplane flies at subsonic speeds, the airahead is “warned” of the airplane’s coming by apressure change transmitted ahead of the airplane atthe speed of sound. Because of this warning, the airbegins to move aside before the airplane arrives andis prepared to let it pass easily. When the airplane’sspeed reaches the speed of sound, the pressurechange can no longer warn the air ahead because theairplane is keeping up with its own pressure waves.Rather, the air particles pile up in front of the air-plane causing a sharp decrease in the flow velocitydirectly in front of the airplane with a correspondingincrease in air pressure and density.

As the airplane’s speed increases beyond the speedof sound, the pressure and density of the compressedair ahead of it increase, the area of compressionextending some distance ahead of the airplane. Atsome point in the airstream, the air particles arecompletely undisturbed, having had no advancedwarning of the airplane’s approach, and in the nextinstant the same air particles are forced to undergosudden and drastic changes in temperature, pressure,density, and velocity. The boundary between theundisturbed air and the region of compressed air iscalled a shock or “compression” wave.

This same type of wave is formed whenever a super-sonic airstream is slowed to subsonic without a change

in direction, such as when the airstream is acceleratedto sonic speed over the cambered portion of a wing, andthen decelerates to subsonic speed as the area of maxi-mum camber is passed. A shock wave will form as aboundary between the supersonic and subsonic ranges.

Whenever a shock wave forms perpendicular to theairflow, it is termed a “normal” shock wave, and theflow immediately behind the wave is subsonic. Asupersonic airstream passing through a normal shockwave will experience these changes:

• The airstream is slowed to subsonic.

• The airflow immediately behind the shock wavedoes not change direction.

• The static pressure and density of the airstreambehind the wave is greatly increased.

• The energy of the airstream (indicated by totalpressure—dynamic plus static) is greatlyreduced.

Shock wave formation causes an increase in drag. Oneof the principal effects of a shock wave is the formationof a dense high pressure region immediately behind thewave. The instability of the high pressure region, andthe fact that part of the velocity energy of the airstreamis converted to heat as it flows through the wave is acontributing factor in the drag increase, but the dragresulting from airflow separation is much greater. If theshock wave is strong, the boundary layer may not havesufficient kinetic energy to withstand airflow separa-tion. The drag incurred in the transonic region due toshock wave formation and airflow separation is knownas “wave drag.” When speed exceeds the critical Machnumber by about 10 percent, wave drag increasessharply. A considerable increase in thrust (power) isrequired to increase flight speed beyond this point intothe supersonic range where, depending on the airfoilshape and the angle of attack, the boundary layer mayreattach.

Normal shock waves form on the wing’s uppersurface first. Further increases in Mach number,however, can enlarge the supersonic area on the

LaminarSub-Layer

TurbulentBoundary

LayerTransitionRegion

LaminarBoundary

Layer

Figure 3-42. Boundary layer.

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upper surface and form an additional area ofsupersonic flow and a normal shock wave on thelower surface. As flight speed approaches thespeed of sound, the areas of supersonic flowenlarge and the shock waves move nearer thetrailing edge. [Figure 3-43]

Figure 3-43. Shock waves.

Associated with “drag rise”are buffet (known as Machbuffet), trim and stabilitychanges, and a decrease incontrol force effectiveness.The loss of lift due to airflowseparation results in a loss ofdownwash, and a change inthe position of the centerpressure on the wing.Airflow separation producesa turbulent wake behind thewing which causes the tailsurfaces to buffet (vibrate).The nose-up and nose-downpitch control provided by thehorizontal tail is dependenton the downwash behind thewing. Thus, a decrease indownwash decreases thehorizontal tail’s pitch controleffectiveness. Movement ofthe wing center of pressureaffects the wing pitchingmoment. If the center ofpressure moves aft, a divingmoment referred to as“Mach tuck” or “tuck under”is produced, and if it movesforward, a nose-up momentis produced. This is the primaryreason for the development of

the T-tail configuration on many turbine-poweredairplanes, which places the horizontal stabilizer asfar as practical from the turbulence of the wings.

SWEEPBACKMost of the difficulties of transonic flight are asso-ciated with shock wave induced flow separation.Therefore, any means of delaying or alleviating theshock induced separation will improve aerodynamicperformance. One method is wing sweepback.Sweepback theory is based upon the concept that itis only the component of the airflow perpendicularto the leading edge of the wing that affects pressuredistribution and formation of shock waves. [Figure3-44]

On a straight wing airplane, the airflow strikes thewing leading edge at 90°, and its full impact producespressure and lift. A wing with sweepback is struck bythe same airflow at an angle smaller than 90°. Thisairflow on the swept wing has the effect of persuadingthe wing into believing that it is flying slower than itreally is; thus the formation of shock waves isdelayed. Advantages of wing sweep include anincrease in critical Mach number, force divergence

SupersonicFlow

M=.82

Normal Shock

Normal Shock

Separation

SupersonicFlow

M=.95

Normal Shock

Normal Shock

M=1.05

"Bow Wave"Subsonic

Airflow

Spanwise Flow

True AirspeedMach 0.85

AirspeedSensed By Wing

Mach 0.70

Figure 3-44. Sweepback effect.

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Mach number, and the Mach number at which drag risewill peak. In other words, sweep will delay the onset ofcompressibility effects.

The Mach number, which produces a sharp change indrag coefficient, is termed the “force divergence”Mach number and, for most airfoils, usually exceedsthe critical Mach number by 5 to 10 percent. At thisspeed, the airflow separation induced by shock waveformation can create significant variations in the drag,lift, or pitching moment coefficients. In addition tothe delay of the onset of compressibility effects,sweepback reduces the magnitude in the changes ofdrag, lift or moment coefficients. In other words, theuse of sweepback will “soften” the force divergence.

A disadvantage of swept wings is that they tend tostall at the wingtips rather than at the wing roots.[Figure 3-45] This is because the boundary layer tendsto flow spanwise toward the tips and to separate nearthe leading edges. Because the tips of a swept wingare on the aft part of the wing (behind the center oflift), a wingtip stall will cause the center of lift tomove forward on the wing, forcingthe nose to rise further. The tendencyfor tip stall is greatest when wingsweep and taper are combined.

Figure 3-45 Wingtip stall.

The stall situation can be aggravated by a T-tail config-uration, which affords little or no pre-stall warning inthe form of tail control surface buffet. [Figure 3-46]The T-tail, being above the wing wake remainseffective even after the wing has begun to stall,allowing the pilot to inadvertently drive the winginto a deeper stall at a much greater angle of attack.If the horizontal tail surfaces then become buried in the

wing’s wake, the elevator may lose all effectiveness,making it impossible to reduce pitch attitude and breakthe stall. In the pre-stall and immediate post-stallregimes, the lift/drag qualities of a swept wing airplane(specifically the enormous increase in drag at lowspeeds) can cause an increasingly descending flight-path with no change in pitch attitude, further increasingthe angle of attack. In this situation, without reliableangle of attack information, a nose-down pitch attitudewith an increasing airspeed is no guarantee that recoveryhas been effected, and up-elevator movement at thisstage may merely keep the airplane stalled.

It is a characteristic of T-tail airplanes to pitchup viciously when stalled in extreme nose-highattitudes, making recovery difficult or violent. Thestick pusher inhibits this type of stall. At approxi-mately one knot above stall speed, pre-programmedstick forces automatically move the stick forward,preventing the stall from developing. A “g” limitermay also be incorporated into the system to preventthe pitch down generated by the stick pusher fromimposing excessive loads on the airplane. A “stick

shaker,” on the other hand provides stall warningwhen the airspeed is 5 to 7 percent above stall speed.

MACH BUFFET BOUNDARIESThus far, only the Mach buffet that results fromexcessive speed has been addressed. It must beremembered that Mach buffet is a function of thespeed of the airflow over the wing—not necessarilythe speed of the airplane. Any time that too great a

IntitialStall Area

Pre-Stall

Stalled

Figure 3-46.T-tail stall.

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lift demand is made on the wing, whether from toofast an airspeed or from too high an angle of attacknear the MMO, the “high-speed” buffet will occur.However, there are also occasions when the buffetcan be experienced at much lower speeds known asthe “low-speed Mach buffet.”

The most likely situation that could cause the low-speed buffet would be when the airplane is flownat too slow a speed for its weight and altitudenecessitating a high angle of attack. This very highangle of attack would have the effect of increasingairflow velocity over the upper surface of the wingto the point that all of the same effects of the shockwaves and buffet would occur as in the high-speedbuffet situation. The angle of attack of the winghas the greatest effect on inducing the Mach buffetat either the high-speed or low-speed boundariesfor the airplane. The conditions that increase theangle of attack, hence the speed of the airflow overthe wing and chances of Mach buffet are as follows:

• High Altitudes—The higher an airplane flies, thethinner the air and the greater the angle of attackrequired to produce the lift needed to maintainlevel flight.

• Heavy Weights—The heavier the airplane, thegreater the lift required of the wing, and all otherthings being equal, the greater the angle of attack.

• “G” Loading—An increase in the “G” loadingon the airplane has the same effect as increasingthe weight of the airplane. Whether the increasein “G” forces is caused by turns, rough controlusage, or turbulence, the effect of increasing thewing’s angle of attack is the same.

FLIGHT CONTROLSOn high-speed airplanes, flight controls are dividedinto primary flight controls and secondary or auxil-iary flight controls. The primary flight controlsmaneuver the airplane about the pitch, roll, and yawaxes. They include the ailerons, elevator, and rudder.Secondary or auxiliary flight controls include tabs,leading edge flaps, trailing edge flaps, spoilers, andslats.

Spoilers are used on the upper surface of the wing tospoil or reduce lift. High-speed airplanes, due totheir clean low drag design use spoilers as speedbrakes to slow them down. Spoilers are extendedimmediately after touchdown to dump lift and thustransfer the weight of the airplane from the wingsonto the wheels for better braking performance.[Figure 3-47]

Jet transport airplanes have small ailerons. The spacefor ailerons is limited because as much of the wing

trailing edge as possible is needed for flaps. Anotherreason is that a conventional size aileron would causewing twist at high speed. Because the ailerons are nec-essarily small, spoilers are used in unison with aileronsto provide additional roll control.

Some jet transports have two sets of ailerons; a pair ofoutboard low-speed ailerons, and a pair of high-speedinboard ailerons. When the flaps are fully retractedafter takeoff, the outboard ailerons are automaticallylocked out in the faired position.

When used for roll control, the spoiler on the side ofthe up-going aileron extends and reduces the lift on thatside, causing the wing to drop. If the spoilers areextended as speed brakes, they can still be used for rollcontrol. If they are the Differential type, they willextend further on one side and retract on the other side.If they are the Non-Differential type, they will extendfurther on one side but will not retract on the other side.When fully extended as speed brakes, the Non-Differential spoilers remain extended and do not sup-plement the ailerons.

To obtain a smooth stall and a higher angle of attackwithout airflow separation, an airplane’s wing leadingedge should have a well-rounded almost blunt shapethat the airflow can adhere to at the higher angle ofattack. With this shape, the airflow separation will startat the trailing edge and progress forward gradually asangle of attack is increased.

The pointed leading edge necessary for high-speedflight results in an abrupt stall and restricts the useof trailing edge flaps because the airflow cannotfollow the sharp curve around the wing leadingedge. The airflow tends to tear loose rather suddenlyfrom the upper surface at a moderate angle of attack.To utilize trailing edge flaps, and thus increase themaximum lift coefficient, the wing must go to ahigher angle of attack without airflow separation.Therefore, leading edge slots, slats, and flaps areused to improve the low-speed characteristics duringtakeoff, climb, and landing. Although these devicesare not as powerful as trailing edge flaps, they areeffective when used full span in combination withhigh-lift trailing edge flaps. With the aid of thesesophisticated high-lift devices, airflow separation isdelayed and the maximum lift coefficient (CLmax) isincreased considerably. In fact, a 50-knot reductionin stall speed is not uncommon.

The operational requirements of a large jet transportairplane necessitate large pitch trim changes. Someof these requirements are:

• The requirement for a large CG range.

• The need to cover a large speed range.

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• The need to cope with possibly large trimchanges due to wing leading edge and trailingedge high-lift devices without limiting theamount of elevator remaining.

• The need to reduce trim drag to a minimum.

These requirements are met by the use of a variableincidence horizontal stabilizer. Large trim changeson a fixed-tail airplane require large elevator deflec-tions. At these large deflections, little further elevatormovement remains in the same direction. A variableincidence horizontal stabilizer is designed to take outthe trim changes. The stabilizer is larger than theelevator, and consequently does not need to bemoved through as large an angle. This leaves theelevator streamlining the tail plane with a full rangeof movement up and down. The variable incidencehorizontal stabilizer can be set to handle the bulk ofthe pitch control demand, with the elevator handlingthe rest. On airplanes equipped with a variable

incidence horizontal stabilizer, the elevator issmaller and less effective in isolation than it is ona fixed-tail airplane. In comparison to other flightcontrols, the variable incidence horizontal stabi-lizer is enormously powerful in its effect. Its useand effect must be fully understood and appreci-ated by flight crewmembers.

Because of the size and high speeds of jet transportairplanes, the forces required to move the controlsurfaces can be beyond the strength of the pilot.Consequently, the control surfaces are actuated byhydraulic or electrical power units. Moving the con-trols in the cockpit signals the control anglerequired, and the power unit positions the actualcontrol surface. In the event of complete power unitfailure, movement of the control surface can beeffected by manually controlling the control tabs.Moving the control tab upsets the aerodynamicbalance which causes the control surface to move.

Landing Flaps

Inbd Wing

Outbd Wing

ForeflapMidflap

Aftflap

Aileron

Takeoff FlapsInbd Wing

Outbd Wing

Aileron

Flaps RetractedInbd Wing

Outbd Wing

ForeflapMidflap

AftflapLeading EdgeFlap

Leading EdgeSlat

Aileron

737

LeadingEdge Flaps

Leading Edge Slats Tab Aileron

OutbdFlap

FlightSpoilers

Stabilizer

TabElevator

Rudder

Inbd Flap

Ground Spoiler

727

Control Surfaces

Fence

Balance Tab

Outboard Aileron

Slats

Inboard Aileron

Leading EdgeFlaps

Ground SpoilersInboard Flaps

Flight Spoilers

Lower Rudder

Anti-BalanceTabs

Upper Rudder

ControlTab

ElevatorStabilizer

Vortex Generators

Pitot TubesOutboard Flap

Control Tab

Figure 3-47. Control surfaces.

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4-1

Aircraft flight control systems are classified as primaryand secondary. The primary control systems consist ofthose that are required to safely control an airplane during flight. These include the ailerons, elevator (orstabilator), and rudder. Secondary control systemsimprove the performance characteristics of the airplane, or relieve the pilot of excessive control forces.Examples of secondary control systems are wing flapsand trim systems.

PRIMARY FLIGHT CONTROLSAirplane control systems are carefully designed to provide a natural feel, and at the same time, allow adequate responsiveness to control inputs. At low airspeeds, the controls usually feel soft and sluggish,and the airplane responds slowly to control applications.At high speeds, the controls feel firm and the responseis more rapid.

Movement of any of the three primary flight controlsurfaces changes the airflow and pressure distributionover and around the airfoil. These changes affect thelift and drag produced by the airfoil/control surfacecombination, and allow a pilot to control the airplaneabout its three axes of rotation.

Design features limit the amount of deflection of flight control surfaces. For example, control-stop mechanisms may be incorporated into the flight controls, or movement of the control column and/orrudder pedals may be limited. The purpose of thesedesign limits is to prevent the pilot from inadvertently overcontrolling and overstressing the aircraft duringnormal maneuvers.

A properly designed airplane should be stable and easily controlled during maneuvering. Control surfaceinputs cause movement about the three axes of rota-tion. The types of stability an airplane exhibits alsorelate to the three axes of rotation. [Figure 4-1]

AILERONSAilerons control roll about the longitudinal axis. Theailerons are attached to the outboard trailing edge of

Vertical Axis(Directional Stability)

Rudder-YawMovement

Elevator-PitchMovement

Lateral Axis(Longitudinal Stability)

Longitudinal Axis(Lateral Stability)

Aileron-RollMovement

PRIMARYCONTROLSURFACE

AIRPLANEMOVEMENT

AXES OFROTATION

TYPE OFSTABILITY

Aileron Roll Longitudinal Lateral

Rudder Yaw Vertical Directional

Elevator/Stabilator Pitch Lateral Longitudinal

Figure 4-1. Airplane controls, movement, axes of rotation, andtype of stability.

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each wing and move in the opposite direction from each other. Ailerons are connected by cables, bellcranks, pulleys or push-pull tubes to each other and to the control wheel.

Moving the control wheel to the right causes the rightaileron to deflect upward and the left aileron to deflectdownward. The upward deflection of the right ailerondecreases the camber resulting in decreased lift on theright wing. The corresponding downward deflection ofthe left aileron increases the camber resulting inincreased lift on the left wing. Thus, the increased lifton the left wing and the decreased lift on the right wingcauses the airplane to roll to the right.

ADVERSE YAWSince the downward deflected aileron produces morelift, it also produces more drag. This added dragattempts to yaw the airplane’s nose in the direction ofthe raised wing. This is called adverse yaw. [Figure 4-2]

The rudder is used to counteract adverse yaw, and theamount of rudder control required is greatest at low airspeeds, high angles of attack, and with large ailerondeflections. However, with lower airspeeds, the verticalstabilizer/rudder combination becomes less effective,and magnifies the control problems associated withadverse yaw.

All turns are coordinated by use of ailerons, rudder, andelevator. Applying aileron pressure is necessary toplace the airplane in the desired angle of bank, whilesimultaneously applying rudder pressure to counteractthe resultant adverse yaw. During a turn, the angle ofattack must be increased by applying elevator pressurebecause more lift is required than when in straight-and-level flight. The steeper the turn, the more back elevator pressure is needed.

As the desired angle of bank is established, aileron andrudder pressures should be relaxed. This will stop thebank from increasing because the aileron and ruddercontrol surfaces will be neutral in their streamlinedposition. Elevator pressure should be held constant tomaintain a constant altitude.

The rollout from a turn is similar to the roll-in exceptthe flight controls are applied in the opposite direction.Aileron and rudder are applied in the direction of therollout or toward the high wing. As the angle of bankdecreases, the elevator pressure should be relaxed asnecessary to maintain altitude.

DIFFERENTIAL AILERONSWith differential ailerons, one aileron is raised a greaterdistance than the other aileron is lowered for a givenmovement of the control wheel. This produces anincrease in drag on the descending wing. The greater drag results from deflecting the up aileron on the descending wing to a greater angle than the downaileron on the rising wing. While adverse yaw isreduced, it is not eliminated completely. [Figure 4-3]

FRISE-TYPE AILERONSWith a Frise-type aileron, when pressure is applied tothe control wheel, the aileron that is being raised pivotson an offset hinge. This projects the leading edge of theaileron into the airflow and creates drag. This helpsequalize the drag created by the lowered aileron on theopposite wing and reduces adverse yaw. [Figure 4-4]

AdverseYaw

Figure 4-2. Adverse yaw is caused by higher drag on the outside wing, which is producing more lift.

Neutral

Raised

Lowered

Figure 4-4. Frise-type ailerons.

Figure 4-3. Differential ailerons.

Up AileronDeflection

Down AileronDeflection

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The Frise-type aileron also forms a slot so that air flowssmoothly over the lowered aileron, making it moreeffective at high angles of attack. Frise-type aileronsalso may be designed to function differentially. Likethe differential aileron, the Frise-type aileron does noteliminate adverse yaw entirely. Coordinated rudderapplication is still needed wherever ailerons areapplied.

COUPLED AILERONS AND RUDDERCoupled ailerons and rudder means these controls arelinked. This is accomplished with rudder-aileron interconnect springs, which help correct for ailerondrag by automatically deflecting the rudder at the sametime the ailerons are deflected. For example, when thecontrol yoke is moved to produce a left roll, the interconnect cable and spring pulls forward on the leftrudder pedal just enough to prevent the nose of the airplane from yawing to the right. The force applied tothe rudder by the springs can be overridden if itbecomes necessary to slip the airplane. [Figure 4-5]

ELEVATORThe elevator controls pitch about the lateral axis. Likethe ailerons on small airplanes, the elevator is connected to the control column in the cockpit by aseries of mechanical linkages. Aft movement of thecontrol column deflects the trailing edge of the elevatorsurface up. This is usually referred to as up elevator.[Figure 4-6]

The up-elevator position decreases the camber of theelevator and creates a downward aerodynamic force,which is greater than the normal tail-down force thatexists in straight-and-level flight. The overall effectcauses the tail of the airplane to move down and thenose to pitch up. The pitching moment occurs about thecenter of gravity (CG). The strength of the pitchingmoment is determined by the distance between the CGand the horizontal tail surface, as well as by the aerodynamic effectiveness of the horizontal tail surface.

Moving the control column forward has the oppositeeffect. In this case, elevator camber increases, creatingmore lift (less tail-down force) on the horizontal stabilizer/elevator. This moves the tail upward andpitches the nose down. Again, the pitching momentoccurs about the CG.

As mentioned earlier in the coverage on stability,power, thrustline, and the position of the horizontal tailsurfaces on the empennage are factors in how effectivethe elevator is in controlling pitch. For example, thehorizontal tail surfaces may be attached near the lowerpart of the vertical stabilizer, at the midpoint, or at thehigh point, as in the T-tail design.

T-TAILIn a T-tail configuration, the elevator is above most ofthe effects of downwash from the propeller as well asairflow around the fuselage and/or wings during normal flight conditions. Operation of the elevators inthis undisturbed air makes for control movements thatare consistent throughout most flight regimes. T-taildesigns have become popular on many light airplanesand on large aircraft, especially those with aft-fuselagemounted engines since the T-tail configuration removesthe tail from the exhaust blast of the engines. Seaplanesand amphibians often have T-tails in order to keep thehorizontal surfaces as far from the water as possible.An additional benefit is reduced vibration and noiseinside the aircraft.

At slow speeds, the elevator on a T-tail aircraft must bemoved through a larger number of degrees of travel to raise the nose a given amount as compared to a conventional-tail aircraft. This is because the

Control ColumnAft Up

Elevator

DownwardAerodynamic Force

CGNose

UpTail

Down

Figure 4-6. The elevator is the primary control for changingthe pitch attitude of an airplane.

Rudder Deflects with Ailerons

Rudder/AileronInterconnectingSprings

Figure 4-5. Coupled ailerons and rudder.

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conventional-tail aircraft has the downwash from thepropeller pushing down on the tail to assist in raisingthe nose. Since controls on aircraft are rigged in such amanner as to require increasing control forces forincreased control travel, the forces required to raise thenose of a T-tail aircraft are greater than for a conventional-tail aircraft. Longitudinal stability of atrimmed aircraft is the same for both types of configuration, but the pilot must be aware that at slowspeeds during takeoffs and landings or stalls, the control forces will be greater than for similar size airplanes equipped with conventional tails.

T-tail airplanes also require additional designconsiderations to counter the problem of flutter. Sincethe weight of the horizontal surfaces is at the top of thevertical stabilizer, the moment arm created causes highloads on the vertical stabilizer which can result influtter. Engineers must compensate for this by increasing the design stiffness of the vertical stabilizer,usually resulting in a weight penalty over conventionaltail designs.

When flying at a very high angle of attack with a lowairspeed and an aft CG, the T-tail airplane may be susceptible to a deep stall. In a deep stall, the airflowover the horizontal tail is blanketed by the disturbedairflow from the wings and fuselage. In these circumstances, elevator or stabilator control could bediminished, making it difficult to recover from the stall. It should be noted that an aft CG could be a contributing factor in these incidents since similar recovery problems are also found with conventional-tail aircraft with an aft CG. [Figure 4-7]

Since flight at a high angle of attack with a low airspeed and an aft CG position can be dangerous,many airplanes have systems to compensate for this situation. The systems range from control stops to elevator down springs. An elevator down spring assistsin lowering the nose to prevent a stall caused by the aftCG position. The stall occurs because the properlytrimmed airplane is flying with the elevator in a trailingedge down position, forcing the tail up and the nosedown. In this unstable condition, if the airplaneencounters turbulence and slows down further, the trim

tab no longer positions the elevator in the nose-downposition. The elevator then streamlines, and the nose ofthe aircraft pitches upward. This aggravates the situation and can possibly result in a stall.

The elevator down spring produces a mechanical loadon the elevator, causing it to move toward the nose-down position if not otherwise balanced. The elevatortrim tab balances the elevator down spring to positionthe elevator in a trimmed position. When the trim tabbecomes ineffective, the down spring drives the elevator to a nose down position. The nose of the aircraft lowers, speed builds up, and a stall is prevented. [Figure 4-8]

The elevator must also have sufficient authority to holdthe nose of the airplane up during the roundout for alanding. In this case, a forward CG may cause a problem. During the landing flare, power normally isreduced, which decreases the airflow over the empennage. This, coupled with the reduced landingspeed, makes the elevator less effective.

From this discussion, it should be apparent that pilotsmust understand and follow proper loading procedures,particularly with regard to the CG position. More information on aircraft loading, as well as weight andbalance, is included in Chapter 8.

STABILATORAs mentioned in Chapter 1, a stabilator is essentially aone-piece horizontal stabilizer with the same type ofcontrol system. Because stabilators pivot around a central hinge point, they are extremely sensitive to control inputs and aerodynamic loads.

Antiservo tabs are incorporated on the trailing edge todecrease sensitivity. In addition, a balance weight isusually incorporated ahead of the main spar. Thebalance weight may project into the empennage or maybe incorporated on the forward portion of thestabilator tips. [Figure 4-9]

Down Spring Elevator

Stabilizer

Figure 4-8.When the aerodynamic efficiency of the horizontaltail surface is inadequate due to an aft center of gravity con-dition, an elevator down spring may be used to supply amechanical load to lower the nose.

Figure 4-7. Airplane with a T-tail design at a high angle ofattack and an aft CG.

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When the control column is pulled back, it raises thestabilator’s trailing edge, rotating the airplane’s noseup. Pushing the control column forward lowers thetrailing edge of the stabilator and pitches the nose ofthe airplane down. Without an antiservo tab, the airplane would be prone to overcontrolling frompilot-induced control inputs.

CANARDThe term canard refers to a control surface that functions as a horizontal stabilizer but is located infront of the main wings. The term also is used todescribe an airplane equipped with a canard. In effect,it is an airfoil similar to the horizontal surface on a conventional aft-tail design. The difference is that thecanard actually creates lift and holds the nose up, asopposed to the aft-tail design which exerts downwardforce on the tail to prevent the nose from rotating downward. [Figure 4-10]

Although the Wright Flyer was configured as a canard with the horizontal surfaces in front of the lifting surface, it was not until recently that thecanard configuration began appearing on newer airplanes. Canard designs include two types—one witha horizontal surface of about the same size as a normalaft-tail design, and the other with a surface of the sameapproximate size and airfoil of the aft-mounted wingknown as a tandem wing configuration. Theoretically,the canard is considered more efficient because usingthe horizontal surface to help lift the weight of the aircraft should result in less drag for a given amount of lift.

The canard’s main advantage is in the area of stall characteristics. A properly designed canard or tandemwing will run out of authority to raise the nose of theaircraft at a point before the main wing will stall. Thismakes the aircraft stall-proof and results only in adescent rate that can be halted by adding power.Ailerons on the main wing remain effective throughoutthe recovery. Other canard configurations are designedso the canard stalls before the main wing, automaticallylowering the nose and recovering the aircraft to a safeflying speed. Again, the ailerons remain effectivethroughout the stall.

The canard design has several limitations. First, it isimportant that the forward lifting surface of a canarddesign stalls before the main wing. If the main wingstalls first, the lift remaining from the forward wing orcanard would be well ahead of the CG, and the airplanewould pitch up uncontrollably. Second, when the for-ward surface stalls first, or is limited in its ability toincrease the angle of attack, the main wing neverreaches a point where its maximum lift is created, sacrificing some performance. Third, use of flaps onthe main wing causes design problems for the forwardwing or canard. As lift on the main wing is increasedby extension of flaps, the lift requirement of the canardis also increased. The forward wing or canard must belarge enough to accommodate flap use, but not so largethat it creates more lift than the main wing.

Finally, the relationship of the main wing to the forward surface also makes a difference. When positioned closely in the vertical plane, downwashfrom the forward wing can have a negative effect onthe lift of the main wing. Increasing vertical separationincreases efficiency of the design. Efficiency is alsoincreased as the size of the two surfaces grows closer tobeing equal.

RUDDERThe rudder controls movement of the airplane about itsvertical axis. This motion is called yaw. Like the

Pivot Point

Antiservo TabStabilator

BalanceWeight

Figure 4-9. The stabilator is a one-piece horizontal tail sur-face that pivots up and down about a central hinge point.

Canard—A horizontal surface mounted ahead of the main wing to pro-vide longitudinal stability and control. It may be a fixed, movable, orvariable geometry surface, with or without control surfaces.

Canard Configuration—A configuration in which the span of the for-ward wings is substantially less than that of the main wing.

Figure 4-10.This advanced aircraft includes a variable-sweepcanard design, which provides longitudinal stability aboutthe lateral axis.

Courtesy of Raytheon Corporation

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other primary control surfaces, the rudder is a movable surface hinged to a fixed surface, in this case, to the vertical stabilizer, or fin. Moving the left or right rudder pedal controls the rudder. When the rudder is deflected into the airflow, a horizontal force is exerted in the opposite direction.[Figure 4-11]

By pushing the left pedal, the rudder moves left. Thisalters the airflow around the vertical stabilizer/rudder,and creates a sideward lift that moves the tail to theright and yaws the nose of the airplane to the left.Rudder effectiveness increases with speed, so largedeflections at low speeds and small deflections at highspeeds may be required to provide the desired reaction.In propeller-driven aircraft, any slipstream flowingover the rudder increases its effectiveness.

V-TAILThe V-tail design utilizes two slanted tail surfaces toperform the same functions as the surfaces of a con-ventional elevator and rudder configuration. The fixedsurfaces act as both horizontal and vertical stabilizers.[Figure 4-12]

The movable surfaces, which are usually called ruddervators, are connected through a special linkagethat allows the control wheel to move both surfacessimultaneously. On the other hand, displacement of therudder pedals moves the surfaces differentially, therebyproviding directional control.

When both rudder and elevator controls are moved bythe pilot, a control mixing mechanism moves each surface the appropriate amount. The control system forthe V-tail is more complex than that required for a conventional tail. In addition, the V-tail design is moresusceptible to Dutch roll tendencies than a conven-tional tail and total reduction in drag is only minimal.

SECONDARY FLIGHT CONTROLSSecondary flight control systems may consist of theflaps, leading edge devices, spoilers, and trim devices.

FLAPSFlaps are the most common high-lift devices used onpractically all airplanes. These surfaces, which areattached to the trailing edge of the wing, increase bothlift and induced drag for any given angle of attack.Flaps allow a compromise between high cruising speedand low landing speed, because they may be extendedwhen needed, and retracted into the wing’s structurewhen not needed. There are four common typesof flaps: plain, split, slotted, and Fowler flaps.[Figure 4-13]

The plain flap is the simplest of the four types. Itincreases the airfoil camber, resulting in a significantincrease in the coefficient of lift at a given angle ofattack. At the same time, it greatly increases drag andmoves the center of pressure aft on the airfoil, resultingin a nose-down pitching moment.

Ruddervator—A pair of control surfaces on the tail of an aircraftarranged in the form of a V. These surfaces, when moved together bythe control wheel, serve as elevators, and when moved differentially bythe rudder pedals, serve as a rudder.

Yaw

Left RudderForward

AerodynamicForce

LeftRudder

CG

Figure 4-11.The effect of left rudder pressure.

Figure 4-12. V-tail design.

Plain Flap

Slotted Flap

Split Flap

Fowler Flap

Figure 4-13. Four common types of flaps

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The split flap is deflected from the lower surface of theairfoil and produces a slightly greater increase in liftthan does the plain flap. However, more drag is createdbecause of the turbulent air pattern produced behindthe airfoil. When fully extended, both plain and splitflaps produce high drag with little additional lift.

The most popular flap on airplanes today is the slottedflap. Variations of this design are used for small airplanes as well as for large ones. Slotted flapsincrease the lift coefficient significantly more thanplain or spilt flaps. On small airplanes, the hinge islocated below the lower surface of the flap, and whenthe flap is lowered, it forms a duct between the flapwell in the wing and the leading edge of the flap.

When the slotted flap is lowered, high-energy air fromthe lower surface is ducted to the flap’s upper surface.The high-energy air from the slot accelerates the uppersurface boundary layer and delays airflow separation,providing a higher coefficient of lift. Thus, the slottedflap produces much greater increases in CLmax than theplain or split flap. While there are many types of slotted flaps, large airplanes often have double- and eventriple-slotted flaps. These allow the maximum increase indrag without the airflow over the flaps separating anddestroying the lift they produce.

Fowler flaps are a type of slotted flap. This flap designnot only changes the camber of the wing, it alsoincreases the wing area. Instead of rotating down on ahinge, it slides backwards on tracks. In the first portionof its extension, it increases the drag very little, butincreases the lift a great deal as it increases both thearea and camber. As the extension continues, the flapdeflects downward, and during the last portion of itstravel, it increases the drag with little additionalincrease in lift.

LEADING EDGE DEVICESHigh-lift devices also can be applied to the leading edgeof the airfoil. The most common types are fixed slots,movable slats, and leading edge flaps. [Figure 4-14]

Fixed slots direct airflow to the upper wing surface anddelay airflow separation at higher angles of attack. Theslot does not increase the wing camber, but allows ahigher maximum coefficient of lift because the stall isdelayed until the wing reaches a greater angle of attack.

Movable slats consist of leading edge segments, whichmove on tracks. At low angles of attack, each slat is held flush against the wing’s leading edge by the highpressure that forms at the wing’s leading edge. As theangle of attack increases, the high-pressure area movesaft below the lower surface of the wing, allowing the slats to move forward. Some slats, however, arepilot operated and can be deployed at any angle ofattack. Opening a slat allows the air below the wingto flow over the wing’s upper surface, delaying airflow separation.

Leading edge flaps, like trailing edge flaps, are used toincrease both CLmax and the camber of the wings. Thistype of leading edge device is frequently used in conjunction with trailing edge flaps and can reduce thenose-down pitching movement produced by the latter.As is true with trailing edge flaps, a small increment ofleading edge flaps increases lift to a much greaterextent than drag. As greater amounts of flaps areextended, drag increases at a greater rate than lift.

SPOILERSOn some airplanes, high-drag devices called spoilersare deployed from the wings to spoil the smooth airflow, reducing lift and increasing drag. Spoilers areused for roll control on some aircraft, one of theadvantages being the elimination of adverse yaw. Toturn right, for example, the spoiler on the right wing israised, destroying some of the lift and creating moredrag on the right. The right wing drops, and the airplanebanks and yaws to the right. Deploying spoilers on bothwings at the same time allows the aircraft to descendwithout gaining speed. Spoilers are also deployed tohelp shorten ground roll after landing. By destroyinglift, they transfer weight to the wheels, improving braking effectiveness. [Figure 4-15]

Fixed Slot

Movable Slat

Leading Edge Flap

Figure 4-14. Leading edge high lift devices.Figure 4-15. Spoilers reduce lift and increase drag duringdescent and landing.

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4-8

TRIM SYSTEMSAlthough the airplane can be operated throughout a wide range of attitudes, airspeeds, and power settings, it can only be designed to fly hands off within a very limited combination of these variables.Therefore, trim systems are used to relieve the pilot of the need to maintain constant pressure on the flightcontrols. Trim systems usually consist of cockpit controls and small hinged devices attached to the trailing edge of one or more of the primary flight control surfaces. They are designed to help minimize a pilot’s workload by aerodynamically assisting movement and position of the flight control surface towhich they are attached. Common types of trim systems include trim tabs, balance tabs, antiservo tabs, ground adjustable tabs, and an adjustable stabilizer.

TRIM TABSThe most common installation on small airplanes is asingle trim tab attached to the trailing edge of the elevator. Most trim tabs are manually operated by asmall, vertically mounted control wheel. However, a trim crank may be found in some airplanes. The cockpit control includes a tab position indicator.Placing the trim control in the full nose-down position moves the tab to its full up position. With thetab up and into the airstream, the airflow over the horizontal tail surface tends to force the trailing edge ofthe elevator down. This causes the tail of the airplaneto move up, and results in a nose-down pitch change.[Figure 4-16]

If you set the trim tab to the full nose-up position, thetab moves to its full-down position. In this case, the airflowing under the horizontal tail surface hits the taband tends to force the trailing edge of the elevator up,reducing the elevator’s angle of attack. This causes atail-down movement of the airplane and a nose-uppitch change.

In spite of the opposite direction movement of the trimtab and the elevator, control of trim is natural to a pilot.If you have to exert constant back pressure on the control column, the need for nose-up trim is indicated.The normal trim procedure is to continue trimminguntil the airplane is balanced and the nose-heavy condition is no longer apparent. Pilots normally establish the desired power, pitch attitude, andconfiguration first, and then trim the airplane to relievecontrol pressures that may exist for that flight condition. Any time power, pitch attitude, orconfiguration is changed, expect that retrimming willbe necessary to relieve the control pressures for thenew flight condition.

BALANCE TABSThe control forces may be excessively high in some airplanes, and in order to decrease them, the manufacturer may use balance tabs. They look like trimtabs and are hinged in approximately the same placesas trim tabs. The essential difference between the twois that the balancing tab is coupled to the control surface rod so that when the primary control surface ismoved in any direction, the tab automatically moves inthe opposite direction. In this manner, the airflow striking the tab counter-balances some of the air pressure against the primary control surface, andenables the pilot to more easily move and hold the control surface in position.

If the linkage between the tab and the fixed surface isadjustable from the cockpit, the tab acts as a combination trim and balance tab, which can beadjusted to any desired deflection. Any time the controlsurface is deflected, the tab moves in the oppositedirection and eases the load on the pilot.

ANTISERVO TABSIn addition to decreasing the sensitivity of the stabilator, an antiservo tab also functions as a trimdevice to relieve control pressure and maintain the stabilator in the desired position. The fixed end of the linkage is on the opposite side of the surface from the horn on the tab, and when the trailing edge ofthe stabilator moves up, the linkage forces the trailingedge of the tab up. When the stabilator moves down,the tab also moves down. This is different than trimtabs on elevators, which move opposite of the control surface. [Figure 4-17]

Nose-Down Trim

Nose-Up Trim

Tab Up; Elevator Down

Tab Down; Elevator Up

Figure 4-16. The movement of the elevator is opposite to thedirection of movement of the elevator trim tab.

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This tab works in the same manner as the balance tabexcept that, instead of moving in the opposite direc-tion, it moves in the same direction as the trailing edgeof the stabilator. For example, when the trailing edgeof the stabilator moves up, the linkage forces the trailing edge of the tab up. When the stabilator movesdown, the tab also moves down.

GROUND ADJUSTABLE TABSMany small airplanes have a non-moveable metal trimtab on the rudder. This tab is bent in one direction orthe other while on the ground to apply a trim force tothe rudder. The correct displacement is determined bytrial-and-error process. Usually, small adjustments arenecessary until you are satisfied that the airplane is nolonger skidding left or right during normal cruisingflight. [Figure 4-18]

ADJUSTABLE STABILIZERRather than using a movable tab on the trailing edge ofthe elevator, some airplanes have an adjustable stabilizer. With this arrangement, linkages pivot the horizontal stabilizer about its rear spar. This isaccomplished by use of a jackscrew mounted on theleading edge of the stabilator. [Figure 4-19]

On small airplanes, the jackscrew is cable-operatedwith a trim wheel or crank, and on larger airplanes, it is motor driven. The trimming effect and cockpitindications for an adjustable stabilizer are similar tothose of a trim tab.

Since the primary and secondary flight control systems vary extensively between aircraft, you need tobe familiar with the systems in your aircraft. A goodsource of information is the Airplane Flight Manual(AFM) or the Pilot’s Operating Handbook (POH).

Figure 4-18. A ground-adjustable tab is used on the rudder ofmany small airplanes to correct for a tendency to fly with thefuselage slightly misaligned with the relative wind.

PivotJackscrew

Nose DownNose Up

Trim Motoror Trim Cable

Figure 4-19. Some airplanes, including most jet transports,use an adjustable stabilizer to provide the required pitch trimforces.

Stabilator

AntiservoTab

Pivot Point

Figure 4-17. An antiservo tab attempts to streamline the con-trol surface and is used to make the stabilator less sensitiveby opposing the force exerted by the pilot.

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5-1

This chapter covers the main systems found on smallairplanes. These include the engine, propeller, andinduction systems, as well as the ignition, fuel,lubrication, cooling, electrical, landing gear, autopilot,and environmental control systems. A comprehensiveintroduction to gas turbine engines is included at theend of this chapter.

POWERPLANTThe airplane engine and propeller, often referred to as apowerplant, work in combination to produce thrust.The powerplant propels the airplane and drives thevarious systems that support the operation ofan airplane.

RECIPROCATING ENGINESMost small airplanes are designed with reciprocatingengines. The name is derived from the back-and-forth,or reciprocating, movement of the pistons. It is thismotion that produces the mechanical energy needed toaccomplish work. Two common means of classifyingreciprocating engines are:

1. by cylinder arrangement with respect to thecrankshaft—radial, in-line, v-type or opposed, or

2. by the method of cooling—liquid or air-cooled.

Radial engines were widely used during World War II,and many are still in service today. With these engines,a row or rows of cylinders are arranged in a circularpattern around the crankcase. The main advantage of aradial engine is the favorable power-to-weight ratio.

In-line engines have a comparatively small frontal area,but their power-to-weight ratios are relatively low. Inaddition, the rearmost cylinders of an air-cooled,in-line engine receive very little cooling air, so theseengines are normally limited to four or six cylinders.V-type engines provide more horsepower than in-lineengines and still retain a small frontal area. Furtherimprovements in engine design led to the developmentof the horizontally-opposed engine.

Opposed-type engines are the most popular reciprocat-ing engines used on small airplanes. These enginesalways have an even number of cylinders, since acylinder on one side of the crankcase “opposes” acylinder on the other side. The majority of theseengines are air cooled and usually are mounted in ahorizontal position when installed on fixed-wingairplanes. Opposed-type engines have high power-to-weight ratios because they have a comparatively small,lightweight crankcase. In addition, the compactcylinder arrangement reduces the engine’s frontal areaand allows a streamlined installation that minimizesaerodynamic drag.

Powerplant—A complete engine and propeller combinationwith accessories.

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The main parts of a reciprocating engine include thecylinders, crankcase, and accessory housing. Theintake/exhaust valves, spark plugs, and pistons arelocated in the cylinders. The crankshaft and connectingrods are located in the crankcase. [Figure 5-1] Themagnetos are normally located on the engine accessoryhousing.

The basic principle for reciprocating engines involvesthe conversion of chemical energy, in the form of fuel,into mechanical energy. This occurs within thecylinders of the engine through a process known as thefour-stroke operating cycle. These strokes are calledintake, compression, power, and exhaust. [Figure 5-2]

1. The intake stroke begins as the piston starts itsdownward travel. When this happens, the intakevalve opens and the fuel/air mixture is drawn intothe cylinder.

2. The compression stroke begins when the intakevalve closes and the piston starts moving back tothe top of the cylinder. This phase of the cycle isused to obtain a much greater power output fromthe fuel/air mixture once it is ignited.

3. The power stroke begins when the fuel/airmixture is ignited. This causes a tremendouspressure increase in the cylinder, and forces thepiston downward away from the cylinder head,creating the power that turns the crankshaft.

4. The exhaust stroke is used to purge the cylinderof burned gases. It begins when the exhaust valveopens and the piston starts to move toward thecylinder head once again.

Even when the engine is operated at a fairly low speed,the four-stroke cycle takes place several hundred timeseach minute. In a four-cylinder engine, each cylinderoperates on a different stroke. Continuous rotation of acrankshaft is maintained by the precise timing of thepower strokes in each cylinder. Continuous operationof the engine depends on the simultaneous function ofauxiliary systems, including the induction, ignition,fuel, oil, cooling, and exhaust systems.

PROPELLERThe propeller is a rotating airfoil, subject to induceddrag, stalls, and other aerodynamic principles thatapply to any airfoil. It provides the necessary thrust topull, or in some cases push, the airplane through the air.The engine power is used to rotate the propeller, which

Intake Valve Exhaust Valve

Cylinder

Piston

Crankshaft

Connecting Rod

Crankcase

Figure 5-1. Main components of a reciprocating engine.

Intake Compression

Power Exhaust

IntakeValve

ExhaustValve

SparkPlug

Piston

ConnectingRod

Crankshaft

1 2

3 4

Figure 5-2. The arrows in this illustration indicate thedirection of motion of the crankshaft and piston during thefour-stroke cycle.

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in turn generates thrust very similar to the manner inwhich a wing produces lift. The amount of thrustproduced depends on the shape of the airfoil, the angleof attack of the propeller blade, and the r.p.m. of theengine. The propeller itself is twisted so the blade anglechanges from hub to tip. The greatest angle ofincidence, or the highest pitch, is at the hub while thesmallest pitch is at the tip. [Figure 5-3]

The reason for the twist is to produce uniform lift fromthe hub to the tip. As the blade rotates, there is adifference in the actual speed of the various portions ofthe blade. The tip of the blade travels faster than thatpart near the hub, because the tip travels a greaterdistance than the hub in the same length of time.Changing the angle of incidence (pitch) from the hubto the tip to correspond with the speed producesuniform lift throughout the length of the blade. If thepropeller blade was designed with the same angle ofincidence throughout its entire length, it would beinefficient, because as airspeed increases in flight, theportion near the hub would have a negative angle ofattack while the blade tip would be stalled. [Figure 5-4]

Small airplanes are equipped with either one of twotypes of propellers. One is the fixed-pitch, and the otheris the controllable-pitch.

FIXED-PITCH PROPELLERThe pitch of this propeller is set by the manufacturer,and cannot be changed. With this type of propeller, thebest efficiency is achieved only at a given combinationof airspeed and r.p.m.

There are two types of fixed-pitch propellers—theclimb propeller and the cruise propeller. Whether theairplane has a climb or cruise propeller installeddepends upon its intended use:

• The climb propeller has a lower pitch, thereforeless drag. Less drag results in higher r.p.m. andmore horsepower capability, which increasesperformance during takeoffs and climbs, butdecreases performance during cruising flight.

• The cruise propeller has a higher pitch, thereforemore drag. More drag results in lower r.p.m. andless horsepower capability, which decreasesperformance during takeoffs and climbs, butincreases efficiency during cruising flight.

The propeller is usually mounted on a shaft, which maybe an extension of the engine crankshaft. In this case,the r.p.m. of the propeller would be the same as thecrankshaft r.p.m. On some engines, the propeller ismounted on a shaft geared to the engine crankshaft. Inthis type, the r.p.m. of the propeller is different thanthat of the engine. In a fixed-pitch propeller, thetachometer is the indicator of engine power.[Figure 5-5]

A tachometer is calibrated in hundreds of r.p.m., andgives a direct indication of the engine and propellerr.p.m. The instrument is color-coded, with a green arcdenoting the maximum continuous operating r.p.m.Some tachometers have additional markings to reflect

Angle of Incidence—For a propeller, it is the angle formed by the chordline and the reference plane containing the propeller hub. For a wing, itis the angle formed by the chord line of the wing and the longitudinalaxis of the airplane.

Figure 5-3. Changes in propeller blade angle from hub to tip.

Mod

erat

e

Travel Distance – Moderate Speed

Sho

rt

Travel Distance

Slow Speed

Gre

ater Travel Distance – Very High Speed

2500 r.p.m.

2500 r.p.m.

2500 r.p.m

Figure 5-4. Relationship of travel distance and speed ofvarious portions of propeller blade.

Figure 5-5. Engine r.p.m. is indicated on the tachometer.

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engine and/or propeller limitations. Therefore, themanufacturer’s recommendations should be used as areference to clarify any misunderstanding oftachometer markings.

The revolutions per minute are regulated by thethrottle, which controls the fuel/air flow to the engine.At a given altitude, the higher the tachometer reading,the higher the power output of the engine.

When operating altitude increases, the tachometer maynot show correct power output of the engine. Forexample, 2,300 r.p.m. at 5,000 feet produce lesshorsepower than 2,300 r.p.m. at sea level. The reasonfor this is that power output depends on air density. Airdensity decreases as altitude increases. Therefore, adecrease in air density (higher density altitude)decreases the power output of the engine. As altitudechanges, the position of the throttle must be changed tomaintain the same r.p.m. As altitude is increased, thethrottle must be opened further to indicate the samer.p.m. as at a lower altitude.

ADJUSTABLE-PITCH PROPELLERAlthough some older adjustable-pitch propellers couldonly be adjusted on the ground, most modernadjustable-pitch propellers are designed so that you canchange the propeller pitch in flight. The firstadjustable-pitch propeller systems provided only twopitch settingsa low-pitch setting and a high-pitchsetting. Today, however, nearly all adjustable-pitchpropeller systems are capable of a range ofpitch settings.

A constant-speed propeller is the most common type ofadjustable-pitch propeller. The main advantage of aconstant-speed propeller is that it converts a highpercentage of brake horsepower (BHP) into thrusthorsepower (THP) over a wide range of r.p.m. andairspeed combinations. A constant-speed propeller ismore efficient than other propellers because it allowsselection of the most efficient engine r.p.m. for thegiven conditions.

An airplane with a constant-speed propeller has twocontrols—the throttle and the propeller control. Thethrottle controls power output, and the propellercontrol regulates engine r.p.m. and, in turn, propellerr.p.m., which is registered on the tachometer.

Once a specific r.p.m. is selected, a governorautomatically adjusts the propeller blade angle asnecessary to maintain the selected r.p.m. For example,after setting the desired r.p.m. during cruising flight, anincrease in airspeed or decrease in propeller load willcause the propeller blade angle to increase as necessaryto maintain the selected r.p.m. A reduction in airspeedor increase in propeller load will cause the propellerblade angle to decrease.

The range of possible blade angles for a constant-speedpropeller is the propeller’s constant-speed range and isdefined by the high and low pitch stops. As long as thepropeller blade angle is within the constant-speed rangeand not against either pitch stop, a constant enginer.p.m. will be maintained. However, once the propellerblades contact a pitch stop, the engine r.p.m. willincrease or decrease as appropriate, with changes inairspeed and propeller load. For example, once aspecific r.p.m. has been selected, if aircraft speeddecreases enough to rotate the propeller blades untilthey contact the low pitch stop, any further decrease inairspeed will cause engine r.p.m. to decrease the sameway as if a fixed-pitch propeller were installed. Thesame holds true when an airplane equipped with aconstant-speed propeller accelerates to a fasterairspeed. As the aircraft accelerates, the propeller bladeangle increases to maintain the selected r.p.m. until thehigh pitch stop is reached. Once this occurs, the bladeangle cannot increase any further and enginer.p.m. increases.

On airplanes that are equipped with a constant-speedpropeller, power output is controlled by the throttle andindicated by a manifold pressure gauge. The gaugemeasures the absolute pressure of the fuel/air mixtureinside the intake manifold and is more correctly ameasure of manifold absolute pressure (MAP). At aconstant r.p.m. and altitude, the amount of powerproduced is directly related to the fuel/air flow beingdelivered to the combustion chamber. As you increasethe throttle setting, more fuel and air is flowing to theengine; therefore, MAP increases. When the engine isnot running, the manifold pressure gauge indicatesambient air pressure (i.e., 29.92 in. Hg). When theengine is started, the manifold pressure indication willdecrease to a value less than ambient pressure (i.e., idleat 12 in. Hg). Correspondingly, engine failure or powerloss is indicated on the manifold gauge as an increasein manifold pressure to a value corresponding to theambient air pressure at the altitude where the failureoccurred. [Figure 5-6]

The manifold pressure gauge is color-coded to indicatethe engine’s operating range. The face of the manifoldpressure gauge contains a green arc to show the normaloperating range, and a red radial line to indicate theupper limit of manifold pressure.

For any given r.p.m., there is a manifold pressure thatshould not be exceeded. If manifold pressure isexcessive for a given r.p.m., the pressure within thecylinders could be exceeded, thus placing undue stresson the cylinders. If repeated too frequently, this stresscould weaken the cylinder components, and eventuallycause engine failure.

Manifold Absolute Pressure (MAP)—The absolute pressure of thefuel/air mixture within the intake manifold, usually indicated in inchesof mercury.

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You can avoid conditions that could overstress thecylinders by being constantly aware of the r.p.m.,especially when increasing the manifold pressure.Conform to the manufacturer’s recommendations forpower settings of a particular engine so as to maintainthe proper relationship between manifold pressureand r.p.m.

When both manifold pressure and r.p.m. need to bechanged, avoid engine overstress by making poweradjustments in the proper order:

• When power settings are being decreased, reducemanifold pressure before reducing r.p.m. If r.p.m. isreduced before manifold pressure, manifoldpressure will automatically increase and possiblyexceed the manufacturer’s tolerances.

• When power settings are being increased,reverse the order—increase r.p.m. first, thenmanifold pressure.

• To prevent damage to radial engines, operating timeat maximum r.p.m. and manifold pressure must beheld to a minimum, and operation at maximumr.p.m. and low manifold pressure must be avoided.

Under normal operating conditions, the most severewear, fatigue, and damage to high performancereciprocating engines occurs at high r.p.m. and lowmanifold pressure.

INDUCTION SYSTEMSThe induction system brings in air from the outside,mixes it with fuel, and delivers the fuel/air mixture tothe cylinder where combustion occurs. Outside airenters the induction system through an intake port onthe front of the engine cowling. This port normally con-tains an air filter that inhibits the entry of dust and otherforeign objects. Since the filter may occasionally

become clogged, an alternate source of air must beavailable. Usually, the alternate air comes from insidethe engine cowling, where it bypasses a cloggedair filter. Some alternate air sources functionautomatically, while others operate manually.

Two types of induction systems are commonly used insmall airplane engines:

1. the carburetor system, which mixes the fuel andair in the carburetor before this mixture enters theintake manifold, and

2. the fuel injection system, which mixes the fueland air just before entry into each cylinder.

CARBURETOR SYSTEMSCarburetors are classified as either float-type orpressure-type. Pressure carburetors are usually notfound on small airplanes. The basic difference betweena pressure carburetor and a float-type is the pressurecarburetor delivers fuel under pressure by a fuel pump.

In the operation of the float-type carburetor system, theoutside air first flows through an air filter, usuallylocated at an air intake in the front part of the enginecowling. This filtered air flows into the carburetor andthrough a venturi, a narrow throat in the carburetor.When the air flows through the venturi, a low-pressurearea is created, which forces the fuel to flow through amain fuel jet located at the throat. The fuel then flowsinto the airstream, where it is mixed with the flowingair. See figure 5-7 on page 5-6.

The fuel/air mixture is then drawn through the intakemanifold and into the combustion chambers, where it isignited. The “float-type carburetor” acquires its namefrom a float, which rests on fuel within the floatchamber. A needle attached to the float opens andcloses an opening at the bottom of the carburetor bowl.This meters the correct amount of fuel into thecarburetor, depending upon the position of the float,which is controlled by the level of fuel in the floatchamber. When the level of the fuel forces the float torise, the needle valve closes the fuel opening and shutsoff the fuel flow to the carburetor. The needle valveopens again when the engine requires additional fuel.The flow of the fuel/air mixture to the combustionchambers is regulated by the throttle valve, which iscontrolled by the throttle in the cockpit.

MIXTURE CONTROLCarburetors are normally calibrated at sea-levelpressure, where the correct fuel-to-air mixture ratio isestablished with the mixture control set in the FULLRICH position. However, as altitude increases, thedensity of air entering the carburetor decreases, whilethe density of the fuel remains the same. This creates a

Figure 5-6. Engine power output is indicated on the manifoldpressure gauge.

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progressively richer mixture, which can result inengine roughness and an appreciable loss of power. Theroughness normally is due to spark plug fouling fromexcessive carbon buildup on the plugs. Carbon buildupoccurs because the excessively rich mixture lowers thetemperature inside the cylinder, inhibiting completecombustion of the fuel. This condition may occurduring the pretakeoff runup at high-elevation airportsand during climbs or cruise flight at high altitudes. Tomaintain the correct fuel/air mixture, you must lean themixture using the mixture control. Leaning the mixturedecreases fuel flow, which compensates for thedecreased air density at high altitude.

During a descent from high altitude, the opposite istrue. The mixture must be enriched, or it may becometoo lean. An overly lean mixture causes detonation,which may result in rough engine operation,overheating, and a loss of power. The best way tomaintain the proper mixture is to monitor the enginetemperature and enrichen the mixture as needed.Proper mixture control and better fuel economy forfuel-injected engines can be achieved by use of anexhaust gas temperature gauge. Since the process ofadjusting the mixture can vary from one airplane toanother, it is important to refer to the Airplane FlightManual (AFM) or the Pilot’s Operating Handbook(POH) to determine the specific procedures for agiven airplane.

CARBURETOR ICINGOne disadvantage of the float-type carburetor is itsicing tendency. Carburetor ice occurs due to the effectof fuel vaporization and the decrease in air pressure inthe venturi, which causes a sharp temperature drop inthe carburetor. If water vapor in the air condenses whenthe carburetor temperature is at or below freezing, icemay form on internal surfaces of the carburetor,including the throttle valve. [Figure 5-8]

FUEL/AIR MIXTUREThe blend of fuel and air is routedto the combustion chambers to beburned.

THROTTLE VALVEThe flow of the fuel/air mixture iscontrolled by the throttle valve. Thethrottle valve is adjusted from thecockpit by the throttle.

DISCHARGE NOZZLEFuel is forced through thedischarge nozzle into the venturiby greater atmosphericpressure in the float chamber.

VENTURIThe shape of the venturi createsan area of low pressure.

AIR INLETAir enters the carburetorthrough the air inlet.

AIR BLEEDThe air bleed allows air to be mixed with fuel beingdrawn out of the discharge nozzle to decreasefuel density and promote fuel vaporization.

FUEL INLETFuel is received intothe carburetor throughthe fuel inlet.

FLOAT CHAMBERFuel level is maintainedby a float-type device.

FUEL

MIXTURE NEEDLEThe mixture needle controls fuel tothe discharge nozzle. Mixture needleposition can be adjusted using themixture control.

Figure 5-7. Float-type carburetor.

To Engine

Incoming Air

Ice

Ice

Venturi

Fuel/AirMixture

Ice

Figure 5-8. The formation of carburetor ice may reduce orblock fuel/air flow to the engine.

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The reduced air pressure, as well as the vaporization offuel, contributes to the temperature decrease in thecarburetor. Ice generally forms in the vicinity of thethrottle valve and in the venturi throat. This restrictsthe flow of the fuel/air mixture and reduces power. Ifenough ice builds up, the engine may cease to operate.

Carburetor ice is most likely to occur whentemperatures are below 70°F (21°C) and the relativehumidity is above 80 percent. However, due to thesudden cooling that takes place in the carburetor, icingcan occur even with temperatures as high as 100°F(38°C) and humidity as low as 50 percent. Thistemperature drop can be as much as 60 to 70°F.Therefore, at an outside air temperature of 100°F, atemperature drop of 70°F results in an air temperaturein the carburetor of 30°F. [Figure 5-9]

The first indication of carburetor icing in an airplanewith a fixed-pitch propeller is a decrease in enginer.p.m., which may be followed by engine roughness. Inan airplane with a constant-speed propeller, carburetoricing usually is indicated by a decrease in manifoldpressure, but no reduction in r.p.m. Propeller pitch isautomatically adjusted to compensate for loss ofpower. Thus, a constant r.p.m. is maintained. Althoughcarburetor ice can occur during any phase of flight, it isparticularly dangerous when using reduced powerduring a descent. Under certain conditions, carburetorice could build unnoticed until you try to add power. Tocombat the effects of carburetor ice, engines withfloat-type carburetors employ a carburetor heat system.

CARBURETOR HEATCarburetor heat is an anti-icing system that preheatsthe air before it reaches the carburetor. Carburetor heatis intended to keep the fuel/air mixture above thefreezing temperature to prevent the formationof carburetor ice. Carburetor heat can be used to meltice that has already formed in the carburetorprovided that the accumulation is not too great. The

emphasis, however, is on using carburetor heat as apreventative measure.

The carburetor heat should be checked during theengine runup. When using carburetor heat, follow themanufacturer’s recommendations.

When conditions are conducive to carburetor icingduring flight, periodic checks should be made to detectits presence. If detected, full carburetor heat should beapplied immediately, and it should be left in the ONposition until you are certain that all the ice has beenremoved. If ice is present, applying partial heat orleaving heat on for an insufficient time might aggravatethe situation. In extreme cases of carburetor icing, evenafter the ice has been removed, full carburetor heatshould be used to prevent further ice formation. Acarburetor temperature gauge, if installed, is veryuseful in determining when to use carburetor heat.

Whenever the throttle is closed during flight, theengine cools rapidly and vaporization of the fuel is lesscomplete than if the engine is warm. Also, in thiscondition, the engine is more susceptible to carburetoricing. Therefore, if you suspect carburetor icingconditions and anticipate closed-throttle operation,adjust the carburetor heat to the full ON position beforeclosing the throttle, and leave it on during theclosed-throttle operation. The heat will aid invaporizing the fuel, and help prevent the formation ofcarburetor ice. Periodically, open the throttle smoothlyfor a few seconds to keep the engine warm, otherwisethe carburetor heater may not provide enough heat toprevent icing.

The use of carburetor heat causes a decrease in enginepower, sometimes up to 15 percent, because the heatedair is less dense than the outside air that had beenentering the engine. This enriches the mixture. Whenice is present in an airplane with a fixed-pitch propellerand carburetor heat is being used, there is a decrease inr.p.m., followed by a gradual increase in r.p.m. as theice melts. The engine also should run more smoothlyafter the ice has been removed. If ice is not present, ther.p.m. will decrease, then remain constant. Whencarburetor heat is used on an airplane with aconstant-speed propeller, and ice is present, a decreasein the manifold pressure will be noticed, followed by agradual increase. If carburetor icing is not present, thegradual increase in manifold pressure will not beapparent until the carburetor heat is turned off.

It is imperative that a pilot recognizes carburetor icewhen it forms during flight. In addition, a loss ofpower, altitude, and/or airspeed will occur. Thesesymptoms may sometimes be accompanied byvibration or engine roughness. Once a power loss isnoticed, immediate action should be taken to eliminate

Rel

ativ

e H

umid

ity

20°F(-7°C)

32°F(0°C)

70°F(21°C)

100°F(38°C)

100%

50%

80%

60%

70%

90%High Carburetor

Icing Potential

Carburetor Icing Possible

Outside Air Temperature

Figure 5-9. Although carburetor ice is most likely to formwhen the temperature and humidity are in ranges indicatedby this chart, carburetor ice is possible under conditionsnot depicted.

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ice already formed in the carburetor, and to preventfurther ice formation. This is accomplished byapplying full carburetor heat, which will cause afurther reduction in power, and possibly engineroughness as melted ice goes through the engine. Thesesymptoms may last from 30 seconds to severalminutes, depending on the severity of the icing. Duringthis period, the pilot must resist the temptation todecrease the carburetor heat usage. Carburetor heatmust remain in the full-hot position until normalpower returns.

Since the use of carburetor heat tends to reduce theoutput of the engine and also to increase the operatingtemperature, carburetor heat should not be used whenfull power is required (as during takeoff) or duringnormal engine operation, except to check for thepresence or to remove carburetor ice.

CARBURETOR AIR TEMPERATURE GAUGESome airplanes are equipped with a carburetor airtemperature gauge, which is useful in detectingpotential icing conditions. Usually, the face of thegauge is calibrated in degrees Celsius (°C), with ayellow arc indicating the carburetor air temperatureswhere icing may occur. This yellow arc typicallyranges between -15°C and +5°C (5°F and 41°F). If theair temperature and moisture content of the air are suchthat carburetor icing is improbable, the engine can beoperated with the indicator in the yellow range with noadverse effects. However, if the atmospheric conditionsare conducive to carburetor icing, the indicator must bekept outside the yellow arc by application ofcarburetor heat.

Certain carburetor air temperature gauges have a redradial, which indicates the maximum permissiblecarburetor inlet air temperature recommended by theengine manufacturer; also, a green arc may be includedto indicate the normal operating range.

OUTSIDE AIR TEMPERATURE GAUGEMost airplanes also are equipped with an outside airtemperature (OAT) gauge calibrated in both degreesCelsius and Fahrenheit. It provides the outside orambient air temperature for calculating trueairspeed, and also is useful in detecting potentialicing conditions.

FUEL INJECTION SYSTEMSIn a fuel injection system, the fuel is injected eitherdirectly into the cylinders, or just ahead of the intakevalve. A fuel injection system is considered to be lesssusceptible to icing than the carburetor system. Impacticing on the air intake, however, is a possibility ineither system. Impact icing occurs when ice forms onthe exterior of the airplane, and blocks openings suchas the air intake for the injection system.

The air intake for the fuel injection system is similar tothat used in the carburetor system, with an alternate airsource located within the engine cowling. This sourceis used if the external air source is obstructed. Thealternate air source is usually operated automatically,with a backup manual system that can be used if theautomatic feature malfunctions.

A fuel injection system usually incorporates these basiccomponents—an engine-driven fuel pump, a fuel/aircontrol unit, fuel manifold (fuel distributor), dischargenozzles, an auxiliary fuel pump, and fuel pressure/flowindicators. [Figure 5-10]

The auxiliary fuel pump provides fuel under pressureto the fuel/air control unit for engine starting and/oremergency use. After starting, the engine-driven fuelpump provides fuel under pressure from the fuel tankto the fuel/air control unit. This control unit, whichessentially replaces the carburetor, meters fuel basedon the mixture control setting, and sends it to the fuelmanifold valve at a rate controlled by the throttle. Afterreaching the fuel manifold valve, the fuel is distributedto the individual fuel discharge nozzles. The dischargenozzles, which are located in each cylinder head, injectthe fuel/air mixture directly into each cylinderintake port.

Some of the advantages of fuel injection are:

• Reduction in evaporative icing.

• Better fuel flow.

• Faster throttle response.

• Precise control of mixture.

• Better fuel distribution.

• Easier cold weather starts.

FuelManifoldValve

FuelDischargeNozzle

Fuel/AirControlUnit

Engine-DrivenFuel Pump

Fuel Tank

AuxiliaryFuel Pump

Figure 5-10. Fuel injection system.

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Disadvantages usually include:

• Difficulty in starting a hot engine.

• Vapor locks during ground operations onhot days.

• Problems associated with restarting an enginethat quits because of fuel starvation.

SUPERCHARGERS ANDTURBOSUPERCHARGERSTo increase an engine’s horsepower, manufacturershave developed supercharger and turbosuperchargersystems that compress the intake air to increase itsdensity. Airplanes with these systems have a manifoldpressure gauge, which displays manifold absolutepressure (MAP) within the engine’s intake manifold.

On a standard day at sea level with the engine shutdown, the manifold pressure gauge will indicate theambient absolute air pressure of 29.92 in. Hg. Becauseatmospheric pressure decreases approximately 1 in. Hgper 1,000 feet of altitude increase, the manifoldpressure gauge will indicate approximately 24.92 in.Hg at an airport that is 5,000 feet above sea level withstandard day conditions.

As a normally aspirated aircraft climbs, it eventuallyreaches an altitude where the MAP is insufficient for anormal climb. That altitude limit is the aircraft’sservice ceiling, and it is directly affected by theengine’s ability to produce power. If the induction airentering the engine is pressurized, or boosted, by eithera supercharger or a turbosupercharger, the aircraft’sservice ceiling can be increased. With these systems,you can fly at higher altitudes with the advantage ofhigher true airspeeds and the increased ability tocircumnavigate adverse weather.

SUPERCHARGERSA supercharger is an engine-driven air pump orcompressor that increases manifold pressure and forcesthe fuel/air mixture into the cylinders. The higher themanifold pressure, the more dense the fuel/air mixture,and the more power an engine can produce. With anormally aspirated engine, it is not possible to havemanifold pressure higher than the existing atmosphericpressure. A supercharger is capable of boostingmanifold pressure above 30 in. Hg.

The components in a supercharged induction systemare similar to those in a normally aspirated system, withthe addition of a supercharger between the fuelmetering device and intake manifold. A supercharger isdriven by the engine through a gear train at one speed,two speeds, or variable speeds. In addition,superchargers can have one or more stages. Each stageprovides an increase in pressure. Therefore,superchargers may be classified as single stage, twostage, or multistage, depending on the number of timescompression occurs.

An early version of a single-stage, single-speedsupercharger may be referred to as a sea-levelsupercharger. An engine equipped with this type ofsupercharger is called a sea-level engine. With thistype of supercharger, a single gear-driven impeller isused to increase the power produced by an engine at allaltitudes. The drawback, however, is that with this typeof supercharger, engine power output still decreaseswith an increase in altitude, in the same way that it doeswith a normally aspirated engine.

Single-stage, single-speed superchargers are found onmany high-powered radial engines, and use an airintake that faces forward so the induction system cantake full advantage of the ram air. Intake air passesthrough ducts to a carburetor, where fuel is metered inproportion to the airflow. The fuel/air charge is thenducted to the supercharger, or blower impeller, whichaccelerates the fuel/air mixture outward. Onceaccelerated, the fuel/air mixture passes through adiffuser, where air velocity is traded for pressureenergy. After compression, the resulting high pressurefuel/air mixture is directed to the cylinders.

Some of the large radial engines developed duringWorld War II have a single-stage, two-speedsupercharger. With this type of supercharger, a singleimpeller may be operated at two speeds. The lowimpeller speed is often referred to as the low blowersetting, while the high impeller speed is called the highblower setting. On engines equipped with a two-speedsupercharger, a lever or switch in the cockpit activatesan oil-operated clutch that switches from one speed tothe other.

Under normal operations, takeoff is made with thesupercharger in the low blower position. In this mode,the engine performs as a ground-boosted engine, andthe power output decreases as the aircraft gainsaltitude. However, once the aircraft reaches a specifiedaltitude, a power reduction is made, and thesupercharger control is switched to the high blowerposition. The throttle is then reset to the desired

Service Ceiling—The maximum density altitude where the best rate-of-climb airspeed will produce a 100 feet-per-minute climb at maximumweight while in a clean configuration with maximum continuous power.

Supercharger—An engine-driven air compressor used to provideadditional pressure to the induction air so the engine can produce addi-tional power.

Sea-Level Engine—A reciprocating aircraft engine having a ratedtakeoff power that is producible only at sea level.

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manifold pressure. An engine equipped with this typeof supercharger is called an altitude engine.[Figure 5-11]

TURBOSUPERCHARGERSThe most efficient method of increasing horsepower ina reciprocating engine is by use of a turbosupercharger,or turbocharger, as it is usually called. A drawback ofgear-driven superchargers is that they use a largeamount of the engine’s power output for the amount ofpower increase they produce. This problem is avoidedwith a turbocharger, because turbochargers arepowered by an engine’s exhaust gases. This means aturbocharger recovers energy from hot exhaust gasesthat would otherwise be lost.

Another advantage of turbochargers is that they can becontrolled to maintain an engine’s rated sea-levelhorsepower from sea level up to the engine’s criticalaltitude. Critical altitude is the maximum altitude atwhich a turbocharged engine can produce its ratedhorsepower. Above the critical altitude, power outputbegins to decrease like it does for a normally aspiratedengine.

Turbochargers increase the pressure of the engine’sinduction air, which allows the engine to develop sealevel or greater horsepower at higher altitudes. Aturbocharger is comprised of two main elements—a

turbine and a compressor. The compressor sectionhouses an impeller that turns at a high rate of speed. Asinduction air is drawn across the impeller blades, theimpeller accelerates the air, allowing a large volume ofair to be drawn into the compressor housing. Theimpeller’s action subsequently produces high-pressure,high-density air, which is delivered to the engine. Toturn the impeller, the engine’s exhaust gases are used todrive a turbine wheel that is mounted on the oppositeend of the impeller’s drive shaft. By directing differentamounts of exhaust gases to flow over the turbine,more energy can be extracted, causing the impeller todeliver more compressed air to the engine. The wastegate is used to vary the mass of exhaust gas flowinginto the turbine. A waste gate is essentially anadjustable butterfly valve that is installed in the exhaustsystem. When closed, most of the exhaust gases fromthe engine are forced to flow through the turbine. Whenopen, the exhaust gases are allowed to bypass theturbine by flowing directly out through the engine’sexhaust pipe. [Figure 5-12]

Since the temperature of a gas rises when it iscompressed, turbocharging causes the temperature ofthe induction air to increase. To reduce thistemperature and lower the risk of detonation, manyturbocharged engines use an intercooler. Anintercooler is a small heat exchanger that uses outsideair to cool the hot compressed air before it enters thefuel metering device.

SYSTEM OPERATIONOn most modern turbocharged engines, the position ofthe waste gate is governed by a pressure-sensingcontrol mechanism coupled to an actuator. Engine oildirected into or away from this actuator moves thewaste gate position. On these systems, the actuator isautomatically positioned to produce the desired MAPsimply by changing the position of the throttle control.

Other turbocharging system designs use a separatemanual control to position the waste gate. With manualcontrol, you must closely monitor the manifoldpressure gauge to determine when the desired MAP hasbeen achieved. Manual systems are often found onaircraft that have been modified with aftermarketturbocharging systems. These systems require specialoperating considerations. For example, if the wastegate is left closed after descending from a high altitude,it is possible to produce a manifold pressure thatexceeds the engine’s limitations. This condition is

Brake

Horsepower

S.L. Density Altitude

Normally Aspirated Engine

High BlowerLow Blower

Two SpeedSuperchargedEngine

Figure 5-11. Power output of normally aspirated engine com-pared to a single-stage, two-speed supercharged engine.

Altitude Engine—A reciprocating aircraft engine having a rated takeoffpower that is producible from sea level to an established higher altitude.

Turbocharger—An air compressor driven by exhaust gases, whichincreases the pressure of the air going into the engine through thecarburetor or fuel injection system.

Waste Gate—A controllable valve in the exhaust system of areciprocating engine equipped with a turbocharger. The valve iscontrolled to vary the amount of exhaust gases forced through theturbocharger turbine.

Intercooler—A device used to remove heat from air or liquid.

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referred to as an overboost, and it may produce severedetonation because of the leaning effect resulting fromincreased air density during descent.

Although an automatic waste gate system is less likelyto experience an overboost condition, it can still occur.If you try to apply takeoff power while the engine oiltemperature is below its normal operating range, thecold oil may not flow out of the waste gate actuatorquickly enough to prevent an overboost. To helpprevent overboosting, you should advance the throttlecautiously to prevent exceeding the maximummanifold pressure limits.

There are system limitations that you should be awareof when flying an aircraft with a turbocharger. Forinstance, a turbocharger turbine and impeller canoperate at rotational speeds in excess of 80,000 r.p.m.while at extremely high temperatures. To achieve highrotational speed, the bearings within the system mustbe constantly supplied with engine oil to reduce thefrictional forces and high temperature. To obtainadequate lubrication, the oil temperature should be inthe normal operating range before high throttle settingsare applied. In addition, you should allow theturbocharger to cool and the turbine to slow downbefore shutting the engine down. Otherwise, the oilremaining in the bearing housing will boil, causinghard carbon deposits to form on the bearings and shaft.These deposits rapidly deteriorate the turbocharger’sefficiency and service life. For further limitations, referto the AFM/POH.

HIGH ALTITUDE PERFORMANCEAs an aircraft equipped with a turbocharging systemclimbs, the waste gate is gradually closed to maintainthe maximum allowable manifold pressure. At somepoint, however, the waste gate will be fully closed, andwith further increases in altitude, the manifold pressurewill begin to decrease. This is the critical altitude,which is established by the airplane or enginemanufacturer. When evaluating the performance of theturbocharging system, if the manifold pressure beginsdecreasing before the specified critical altitude, theengine and turbocharging system should be inspectedby a qualified aviation maintenance technician toverify the system’s proper operation.

IGNITION SYSTEMThe ignition system provides the spark that ignites thefuel/air mixture in the cylinders and is made up ofmagnetos, spark plugs, high-tension leads, and theignition switch. [Figure 5-13]

Exhaust GasDischarge Waste Gate

This controls the amount of exhaustthrough the turbine. Waste gate positionis actuated by engine oil pressure.

TurbochargerThe turbocharger incorporates aturbine, which is driven by exhaustgases, and a compressor thatpressurizes the incoming air.

Throttle BodyThis regulates airflow to the engine.

Intake ManifoldPressurized air from theturbocharger is suppliedto the cylinders.

Exhaust ManifoldExhaust gas is ducted throughthe exhaust manifold and isused to turn the turbine whichdrives the compressor.

Air IntakeIntake air is ducted to the turbochargerwhere it is compressed.

Figure 5-12.Turbocharger components.

Overboost—A condition in which a reciprocating engine has exceededthe maximum manifold pressure allowed by the manufacturer.

OFFR L BOTH

START

UpperMagneto Wires

LowerMagneto Wires

UpperSparkPlugs

Lower SparkPlugs

Left Magneto Right Magneto

LowerSparkPlugs

UpperSparkPlugs

2

3

1

4

Figure 5-13. Ignition system components.

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A magneto uses a permanent magnet to generate anelectrical current completely independent of theaircraft’s electrical system. The magneto generatessufficiently high voltage to jump a spark across thespark plug gap in each cylinder. The system begins tofire when you engage the starter and the crankshaftbegins to turn. It continues to operate whenever thecrankshaft is rotating.

Most standard certificated airplanes incorporate a dualignition system with two individual magnetos, separatesets of wires, and spark plugs to increase reliabilityof the ignition system. Each magneto operatesindependently to fire one of the two spark plugs in eachcylinder. The firing of two spark plugs improvescombustion of the fuel/air mixture and results in aslightly higher power output. If one of the magnetosfails, the other is unaffected. The engine will continueto operate normally, although you can expect a slightdecrease in engine power. The same is true if one of thetwo spark plugs in a cylinder fails.

The operation of the magneto is controlled in thecockpit by the ignition switch. The switch hasfive positions:

1. OFF

2. R—Right

3. L—Left

4. BOTH

5. START

With RIGHT or LEFT selected, only the associatedmagneto is activated. The system operates on bothmagnetos with BOTH selected.

You can identify a malfunctioning ignition systemduring the pretakeoff check by observing the decreasein r.p.m. that occurs when you first move the ignitionswitch from BOTH to RIGHT, and then from BOTH toLEFT. A small decrease in engine r.p.m. is normalduring this check. The permissible decrease is listed inthe AFM or POH. If the engine stops running when youswitch to one magneto or if the r.p.m. drop exceeds theallowable limit, do not fly the airplane until theproblem is corrected. The cause could be fouled plugs,broken or shorted wires between the magneto and theplugs, or improperly timed firing of the plugs. It shouldbe noted that “no drop” in r.p.m. is not normal, and inthat instance, the airplane should not be flown.

Following engine shutdown, turn the ignition switch tothe OFF position. Even with the battery and masterswitches OFF, the engine can fire and turn over if youleave the ignition switch ON and the propeller ismoved because the magneto requires no outside sourceof electrical power. The potential for serious injury inthis situation is obvious.

Loose or broken wires in the ignition system also cancause problems. For example, if the ignition switch isOFF, the magneto may continue to fire if the ignitionswitch ground wire is disconnected. If this occurs, theonly way to stop the engine is to move themixture lever to the idle cutoff position, then have thesystem checked by a qualified aviation maintenancetechnician.

COMBUSTIONDuring normal combustion, the fuel/air mixture burnsin a very controlled and predictable manner. Althoughthe process occurs in a fraction of a second, the mixtureactually begins to burn at the point where it is ignitedby the spark plugs, then burns away from the plugsuntil it is all consumed. This type of combustion causesa smooth buildup of temperature and pressure andensures that the expanding gases deliver the maximumforce to the piston at exactly the right time in the powerstroke. [Figure 5-14]

Detonation is an uncontrolled, explosive ignition ofthe fuel/air mixture within the cylinder’s combustionchamber. It causes excessive temperatures andpressures which, if not corrected, can quickly lead tofailure of the piston, cylinder, or valves. In less severecases, detonation causes engine overheating,roughness, or loss of power.

Detonation is characterized by high cylinder headtemperatures, and is most likely to occur whenoperating at high power settings. Some commonoperational causes of detonation include:

Magneto—A self-contained, engine-driven unit that supplies electricalcurrent to the spark plugs; completely independent of the airplane’selectrical system. Normally, there are two magnetos per engine.

Normal Combustion Explosion

Figure 5-14. Normal combustion and explosive combustion.

Detonation—An uncontrolled, explosive ignition of the fuel/air mixturewithin the cylinder’s combustion chamber.

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• Using a lower fuel grade than that specified bythe aircraft manufacturer.

• Operating with extremely high manifoldpressures in conjunction with low r.p.m.

• Operating the engine at high power settings withan excessively lean mixture.

• Detonation also can be caused by extendedground operations, or steep climbs where cylinder cooling is reduced.

Detonation may be avoided by following these basicguidelines during the various phases of ground andflight operations:

• Make sure the proper grade of fuel is being used.

• While on the ground, keep the cowl flaps (ifavailable) in the full-open position to provide themaximum airflow through the cowling.

• During takeoff and initial climb, the onset ofdetonation can be reduced by using an enrichedfuel mixture, as well as using a shallower climbangle to increase cylinder cooling.

• Avoid extended, high power, steep climbs.

• Develop a habit of monitoring the engineinstruments to verify proper operation accordingto procedures established by the manufacturer.

Preignition occurs when the fuel/air mixture ignitesprior to the engine’s normal ignition event. Prematureburning is usually caused by a residual hot spot in thecombustion chamber, often created by a small carbondeposit on a spark plug, a cracked spark plug insulator,or other damage in the cylinder that causes a part toheat sufficiently to ignite the fuel/air charge.Preignition causes the engine to lose power, andproduces high operating temperature. As withdetonation, preignition may also cause severeengine damage, because the expanding gases exertexcessive pressure on the piston while still on itscompression stroke.

Detonation and preignition often occur simultaneouslyand one may cause the other. Since either conditioncauses high engine temperature accompanied by adecrease in engine performance, it is often difficult todistinguish between the two. Using the recommendedgrade of fuel and operating the engine within its propertemperature, pressure, and r.p.m. ranges reduce thechance of detonation or preignition.

FUEL SYSTEMSThe fuel system is designed to provide an uninterruptedflow of clean fuel from the fuel tanks to the engine. Thefuel must be available to the engine under allconditions of engine power, altitude, attitude, andduring all approved flight maneuvers. Two commonclassifications apply to fuel systems in smallairplanesgravity-feed and fuel-pump systems.

The gravity-feed system utilizes the force of gravity totransfer the fuel from the tanks to the engineforexample, on high-wing airplanes where the fuel tanksare installed in the wings. This places the fuel tanksabove the carburetor, and the fuel is gravity fed throughthe system and into the carburetor. If the design of theairplane is such that gravity cannot be used to transferfuel, fuel pumps are installedfor example, onlow-wing airplanes where the fuel tanks in the wingsare located below the carburetor. [Figure 5-15]

Preignition—The uncontrolled combustion of the fuel/air mixture inadvance of the normal ignition.

Selector Valve

Strainer

Carburetor

Primer

Left Tank Right Tank

Left Tank Right Tank

Selector Valve

FUEL-PUMP SYSTEM

GRAVITY-FEED SYSTEM

Vent

Strainer

Engine-Driven Pump

Electric Pump

Primer

Carburetor

Figure 5-15. Gravity-feed and fuel-pump systems.

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FUEL PUMPSAirplanes with fuel pump systems have two fuelpumps. The main pump system is engine driven, and anelectrically driven auxiliary pump is provided for usein engine starting and in the event the engine pumpfails. The auxiliary pump, also known as a boost pump,provides added reliability to the fuel system. Theelectrically driven auxiliary pump is controlled by aswitch in the cockpit.

FUEL PRIMERBoth gravity fed and pump systems may incorporate afuel primer into the system. The primer is used to drawfuel from the tanks to vaporize it directly into thecylinders prior to starting the engine. This isparticularly helpful during cold weather, when enginesare hard to start because there is not enough heatavailable to vaporize the fuel in the carburetor. It isimportant to lock the primer in place when it is not inuse. If the knob is free to move, it may vibrate outduring flight and can cause an excessively richmixture. To avoid overpriming, read the priminginstructions for your airplane.

FUEL TANKSThe fuel tanks, normally located inside the wings of anairplane, have a filler opening on top of the wingthrough which they can be filled. A filler cap coversthis opening. The tanks are vented to the outside tomaintain atmospheric pressure inside the tank. Theymay be vented through the filler cap or through a tubeextending through the surface of the wing. Fuel tanksalso include an overflow drain that may stand alone orbe collocated with the fuel tank vent. This allows fuelto expand with increases in temperature withoutdamage to the tank itself. If the tanks have been filledon a hot day, it is not unusual to see fuel coming fromthe overflow drain.

FUEL GAUGESThe fuel quantity gauges indicate the amount of fuelmeasured by a sensing unit in each fuel tank and isdisplayed in gallons or pounds. Aircraft certificationrules only require accuracy in fuel gauges when theyread “empty.” Any reading other than “empty” shouldbe verified. Do not depend solely on the accuracy ofthe fuel quantity gauges. Always visually check thefuel level in each tank during the preflight inspection,and then compare it with the corresponding fuelquantity indication.

If a fuel pump is installed in the fuel system, a fuelpressure gauge is also included. This gauge indicatesthe pressure in the fuel lines. The normal operatingpressure can be found in the AFM/POH, or on thegauge by color coding.

FUEL SELECTORSThe fuel selector valve allows selection of fuel fromvarious tanks. A common type of selector valvecontains four positions: LEFT, RIGHT, BOTH, andOFF. Selecting the LEFT or RIGHT position allowsfuel to feed only from that tank, while selecting theBOTH position feeds fuel from both tanks. The LEFTor RIGHT position may be used to balance the amountof fuel remaining in each wing tank. [Figure 5-16]

Fuel placards will show any limitations on fuel tankusage, such as “level flight only” and/or “both” forlandings and takeoffs.

Regardless of the type of fuel selector in use, fuelconsumption should be monitored closely to ensurethat a tank does not run completely out of fuel. Runninga fuel tank dry will not only cause the engine to stop,but running for prolonged periods on one tank causesan unbalanced fuel load between tanks. Running a tankcompletely dry may allow air to enter the fuel system,which may cause vapor lock. When this situationdevelops, it may be difficult to restart the engine. Onfuel-injected engines, the fuel may become so hot itvaporizes in the fuel line, not allowing fuel to reachthe cylinders.

FUEL STRAINERS, SUMPS, AND DRAINSAfter the fuel selector valve, the fuel passes through astrainer before it enters the carburetor. This strainerremoves moisture and other sediments that might be inthe system. Since these contaminants are heavier thanaviation fuel, they settle in a sump at the bottom of thestrainer assembly. A sump is defined as a low point in afuel system and/or fuel tank. The fuel system maycontain sump, fuel strainer, and fuel tank drains, someof which may be collocated.

The fuel strainer should be drained before each flight.Fuel samples should be drained and checked visually

Figure 5-16. Fuel selector valve.

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for water and contaminants. Water in the sump ishazardous because in cold weather the water can freezeand block fuel lines. In warm weather, it can flow intothe carburetor and stop the engine. If water is present inthe sump, it is likely there is more water in the fueltanks, and you should continue to drain them until thereis no evidence of water. In any event, never take offuntil you are certain that all water and contaminantshave been removed from the engine fuel system.

Because of the variation in fuel systems, you shouldbecome thoroughly familiar with the systems that applyto your airplane. Consult the AFM or POH for specificoperating procedures.

FUEL GRADESAviation gasoline, or AVGAS, is identified by anoctane or performance number (grade), whichdesignates the antiknock value or knock resistance ofthe fuel mixture in the engine cylinder. The higher thegrade of gasoline, the more pressure the fuel canwithstand without detonating. Lower grades of fuel areused in lower-compression engines because these fuelsignite at a lower temperature. Higher grades are used inhigher-compression engines, because they must igniteat higher temperatures, but not prematurely. If theproper grade of fuel is not available, use the next highergrade as a substitute. Never use a lower grade. This cancause the cylinder head temperature and engine oiltemperature to exceed their normal operating range,which may result in detonation.

Several grades of aviation fuel are available. Care mustbe exercised to ensure that the correct aviation grade isbeing used for the specific type of engine. The properfuel grade is stated in the AFM or POH, on placards inthe cockpit, and next to the filler caps. Due to its leadcontent, auto gas should NEVER be used in aircraftengines unless the aircraft has been modified with aSupplemental Type Certificate (STC) issued by theFederal Aviation Administration.

The current method to identify aviation gasoline foraircraft with reciprocating engines is by the octane andperformance number, along with the abbreviationAVGAS. These aircraft use AVGAS 80, 100, and100LL. Although AVGAS 100LL performs the same asgrade 100, the “LL” indicates it has a low lead content.Fuel for aircraft with turbine engines is classified asJET A, JET A-1, and JET B. Jet fuel is basicallykerosene and has a distinctive kerosene smell.

Since use of the correct fuel is critical, dyes areadded to help identify the type and grade of fuel.[Figure 5-17]

In addition to the color of the fuel itself, thecolor-coding system extends to decals and variousairport fuel handling equipment. For example, allaviation gasolines are identified by name, using whiteletters on a red background. In contrast, turbine fuelsare identified by white letters on a black background.

FUEL CONTAMINATIONOf the accidents attributed to powerplant failure fromfuel contamination, most have been traced to:

• Inadequate preflight inspection by the pilot.

• Servicing aircraft with improperly filtered fuelfrom small tanks or drums.

• Storing aircraft with partially filled fuel tanks.

• Lack of proper maintenance.

Fuel should be drained from the fuel strainer quickdrain and from each fuel tank sump into a transparentcontainer, and then checked for dirt and water. Whenthe fuel strainer is being drained, water in the tank maynot appear until all the fuel has been drained from thelines leading to the tank. This indicates that waterremains in the tank, and is not forcing the fuel out ofthe fuel lines leading to the fuel strainer. Therefore,drain enough fuel from the fuel strainer to be certainthat fuel is being drained from the tank. The amountwill depend on the length of fuel line from the tank tothe drain. If water or other contaminants are found inthe first sample, drain further samples until notrace appears.

80AVGAS

100AVGAS

RED

GREEN

AVGAS 80

AVGAS 100

100LLAVGAS

BLUEAVGAS 100LL

JETA

COLORLESSOR STRAW

JET A

FUEL TYPEAND GRADE

COLOR OFFUEL

EQUIPMENTCOLOR

Figure 5-17. Aviation fuel color-coding system.

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Water may also remain in the fuel tanks after thedrainage from the fuel strainer had ceased to show anytrace of water. This residual water can be removed onlyby draining the fuel tank sump drains.

Water is the principal fuel contaminant. Suspendedwater droplets in the fuel can be identified by a cloudyappearance of the fuel or by the clear separation ofwater from the colored fuel, which occurs after thewater has settled to the bottom of the tank. As a safetymeasure, the fuel sumps should be drained before everyflight during the preflight inspection.

Fuel tanks should be filled after each flight, or at leastafter the last flight of the day to prevent moisturecondensation within the tank. Another way to preventfuel contamination is to avoid refueling from cans anddrums. Refueling from cans or drums may result in fuelcontamination.

The use of a funnel and chamois skin when refuelingfrom cans or drums is hazardous under any conditions,and should be discouraged. In remote areas or inemergency situations, there may be no alternative torefueling from sources with inadequate anticontamina-tion systems, and a chamois and funnel may be the onlypossible means of filtering fuel. However, the use of achamois will not always ensure decontaminated fuel.Worn-out chamois will not filter water; neither will anew, clean chamois that is already water-wet or damp.Most imitation chamois skins will not filter water.

REFUELING PROCEDURESStatic electricity is formed by the friction of air passingover the surfaces of an airplane in flight and by the flowof fuel through the hose and nozzle during refueling.Nylon, dacron, or wool clothing is especially prone toaccumulate and discharge static electricity from theperson to the funnel or nozzle. To guard against thepossibility of static electricity igniting fuel fumes, aground wire should be attached to the aircraft beforethe fuel cap is removed from the tank. The refuelingnozzle then should be grounded to the aircraft beforerefueling is begun, and should remain groundedthroughout the refueling process. When a fuel truck isused, it should be grounded prior to the fuel nozzlecontacting the aircraft.

If fueling from drums or cans is necessary, properbonding and grounding connections are important.Drums should be placed near grounding posts, and thefollowing sequence of connections observed:

1. Drum to ground.

2. Ground to aircraft.

3. Drum to aircraft.

4. Nozzle to aircraft before the fuel cap is removed.

When disconnecting, reverse the order.

The passage of fuel through a chamois increases thecharge of static electricity and the danger of sparks. Theaircraft must be properly grounded and the nozzle,chamois filter, and funnel bonded to the aircraft. If acan is used, it should be connected to either thegrounding post or the funnel. Under no circumstancesshould a plastic bucket or similar nonconductivecontainer be used in this operation.

STARTING SYSTEMMost small aircraft use a direct-cranking electric startersystem. This system consists of a source of electricity,wiring, switches, and solenoids to operate the starterand a starter motor. Most aircraft have starters thatautomatically engage and disengage when operated,but some older aircraft have starters that aremechanically engaged by a lever actuated by the pilot.The starter engages the aircraft flywheel, rotating theengine at a speed that allows the engine to start andmaintain operation.

Electrical power for starting is usually supplied by anon-board battery, but can also be supplied by externalpower through an external power receptacle. When thebattery switch is turned on, electricity is supplied to themain power bus through the battery solenoid. Both thestarter and the starter switch draw current from the mainbus, but the starter will not operate until the startingsolenoid is energized by the starter switch being turnedto the “start” position. When the starter switch is releasedfrom the “start” position, the solenoid removes powerfrom the starter motor. The starter motor is protectedfrom being driven by the engine through a clutch in thestarter drive that allows the engine to run faster than thestarter motor. [Figure 5-18]

When starting an engine, the rules of safety andcourtesy should be strictly observed. One of the mostimportant is to make sure there is no one near thepropeller. In addition, the wheels should be chockedand the brakes set, to avoid hazards caused byunintentional movement. To avoid damage to thepropeller and property, the airplane should be in an areawhere the propeller will not stir up gravel or dust.

OIL SYSTEMSThe engine oil system performs several importantfunctions, including:

• Lubrication of the engine’s moving parts.

• Cooling of the engine by reducing friction.

• Removing heat from the cylinders.

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• Providing a seal between the cylinder wallsand pistons.

• Carrying away contaminants.

Reciprocating engines use either a wet-sump ordry-sump oil system. In a dry-sump system, the oil iscontained in a separate tank, and circulated through theengine by pumps. In a wet-sump system, the oil islocated in a sump, which is an integral part of theengine. [Figure 5-19]

The main component of a wet-sump system is the oilpump, which draws oil from the sump and routes it tothe engine. After the oil passes through the engine, itreturns to the sump. In some engines, additionallubrication is supplied by the rotating crankshaft,which splashes oil onto portions of the engine.

An oil pump also supplies oil pressure in a dry-sumpsystem, but the source of the oil is a separate oil tank,located external to the engine. After oil is routedthrough the engine, it is pumped from the variouslocations in the engine back to the oil tank by scavengepumps. Dry sump systems allow for a greater volumeof oil to be supplied to the engine, which makes themmore suitable for very large reciprocating engines.

The oil pressure gauge provides a direct indication ofthe oil system operation. It measures the pressure in

BatteryContactor(Solenoid)

StarterContactor

ExternalPowerRelay

++

To groundthroughmasterswitch

OFF

LR

B

S

STARTER

IGNITIONSWITCH

ExternalPowerPlug

MAIN

BUS

+

-

Figure 5-18.Typical starting circuit.

Pressure Oil From Oil Pump

OIL TEMP

Sump Oil and Return OilFrom Relief Valve

Low PressureOil Screen

Oil Coolerand Filter

Oil Sump

Engine and Accessory Bearings

Oil Filler Capand Dipstick

Oil Pressure Relief Valve

Oil PressureGauge

Oil Temperature Gauge

High Pressure Oil Screen

OIL PRESS

TOP VIEW

Oil Pump

OIL TEMP

100 245

OIL PRESS

0 60 115

Figure 5-19. Wet-sump oil system.

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pounds per square inch (p.s.i.) of the oil supplied to theengine. Green indicates the normal operating range,while red indicates the minimum and maximumpressures. There should be an indication of oil pressureduring engine start. Refer to the AFM/POH formanufacturer limitations.

The oil temperature gauge measures the temperature ofoil. A green area shows the normal operating range andthe red line indicates the maximum allowabletemperature. Unlike oil pressure, changes in oiltemperature occur more slowly. This is particularlynoticeable after starting a cold engine, when it may takeseveral minutes or longer for the gauge to show anyincrease in oil temperature.

Check oil temperature periodically during flightespecially when operating in high or low ambient airtemperature. High temperature indications mayindicate a plugged oil line, a low oil quantity, a blockedoil cooler, or a defective temperature gauge. Lowtemperature indications may indicate improper oilviscosity during cold weather operations.

The oil filler cap and dipstick (for measuring the oilquantity) are usually accessible through a panel in theengine cowling. If the quantity does not meet themanufacturer’s recommended operating levels, oilshould be added. The AFM, POH, or placards near theaccess panel provide information about the correct oiltype and weight, as well as the minimum and maximumoil quantity. [Figure 5-20]

ENGINE COOLING SYSTEMSThe burning fuel within the cylinders produces intenseheat, most of which is expelled through the exhaustsystem. Much of the remaining heat, however, must beremoved, or at least dissipated, to prevent the enginefrom overheating. Otherwise, the extremely highengine temperatures can lead to loss of power,excessive oil consumption, detonation, and seriousengine damage.

While the oil system is vital to internal cooling of theengine, an additional method of cooling is necessaryfor the engine’s external surface. Most small airplanesare air cooled, although some are liquid cooled.

Air cooling is accomplished by air flowing into theengine compartment through openings in front of theengine cowling. Baffles route this air over fins attachedto the engine cylinders, and other parts of the engine,where the air absorbs the engine heat. Expulsion of thehot air takes place through one or more openings in thelower, aft portion of the engine cowling. [Figure 5-21]

The outside air enters the engine compartment throughan inlet behind the propeller hub. Baffles direct it to thehottest parts of the engine, primarily the cylinders,which have fins that increase the area exposed tothe airflow.

The air cooling system is less effective during groundoperations, takeoffs, go-arounds, and other periods ofhigh-power, low-airspeed operation. Conversely,high-speed descents provide excess air and canshock-cool the engine, subjecting it to abrupttemperature fluctuations.

Figure 5-20. Always check the engine oil level during the pre-flight inspection.

Fixed CowlOpening

Baffle

Cylinders

AirInlet

Baffle

Figure 5-21. Outside air aids in cooling the engine.

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Operating the engine at higher than its designedtemperature can cause loss of power, excessive oilconsumption, and detonation. It will also lead toserious permanent damage, such as scoring thecylinder walls, damaging the pistons and rings, andburning and warping the valves. Monitoring thecockpit engine temperature instruments will aid inavoiding high operating temperature.

Under normal operating conditions in airplanes notequipped with cowl flaps, the engine temperature canbe controlled by changing the airspeed or the poweroutput of the engine. High engine temperatures can bedecreased by increasing the airspeed and/or reducingthe power.

The oil temperature gauge gives an indirect anddelayed indication of rising engine temperature, butcan be used for determining engine temperature if thisis the only means available.

Many airplanes are equipped with a cylinder-headtemperature gauge. This instrument indicates a directand immediate cylinder temperature change. Thisinstrument is calibrated in degrees Celsius orFahrenheit, and is usually color-coded with a green arcto indicate the normal operating range. A red line onthe instrument indicates maximum allowable cylinderhead temperature.

To avoid excessive cylinder head temperatures,increase airspeed, enrich the mixture, and/or reducepower. Any of these procedures help in reducing theengine temperature. On airplanes equipped withcowl flaps, use the cowl flap positions to control thetemperature. Cowl flaps are hinged covers that fit overthe opening through which the hot air is expelled. If theengine temperature is low, the cowl flaps can be closed,thereby restricting the flow of expelled hot air andincreasing engine temperature. If the enginetemperature is high, the cowl flaps can be opened topermit a greater flow of air through the system, therebydecreasing the engine temperature.

EXHAUST SYSTEMSEngine exhaust systems vent the burned combustiongases overboard, provide heat for the cabin, and defrostthe windscreen. An exhaust system has exhaust pipingattached to the cylinders, as well as a muffler and amuffler shroud. The exhaust gases are pushed out ofthe cylinder through the exhaust valve and then throughthe exhaust pipe system to the atmosphere.

For cabin heat, outside air is drawn into the air inletand is ducted through a shroud around the muffler. Themuffler is heated by the exiting exhaust gases and, inturn, heats the air around the muffler. This heated air isthen ducted to the cabin for heat and defrostapplications. The heat and defrost are controlled in thecockpit, and can be adjusted to the desired level.

Exhaust gases contain large amounts of carbonmonoxide, which is odorless and colorless. Carbonmonoxide is deadly, and its presence is virtuallyimpossible to detect. The exhaust system must be ingood condition and free of cracks.

Some exhaust systems have an exhaust gastemperature probe. This probe transmits the exhaustgas temperature (EGT) to an instrument in the cockpit.The EGT gauge measures the temperature of the gasesat the exhaust manifold. This temperature varies withthe ratio of fuel to air entering the cylinders and can beused as a basis for regulating the fuel/air mixture. TheEGT gauge is highly accurate in indicating the correctmixture setting. When using the EGT to aid in leaningthe fuel/air mixture, fuel consumption can be reduced.For specific procedures, refer to the manufacturer’srecommendations for leaning the mixture.

ELECTRICAL SYSTEMAirplanes are equipped with either a 14- or 28-voltdirect-current electrical system. A basic airplaneelectrical system consists of the following components:

• Alternator/generator

• Battery

• Master/battery switch

• Alternator/generator switch

• Bus bar, fuses, and circuit breakers

• Voltage regulator

• Ammeter/loadmeter

• Associated electrical wiring

Engine-driven alternators or generators supply electriccurrent to the electrical system. They also maintain asufficient electrical charge in the battery. Electricalenergy stored in a battery provides a source ofelectrical power for starting the engine and a limitedsupply of electrical power for use in the event thealternator or generator fails.

Cowl Flaps—Shutter-like devices arranged around certain air-cooledengine cowlings, which may be opened or closed to regulate the flow ofair around the engine.

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Most direct current generators will not produce asufficient amount of electrical current at low enginer.p.m. to operate the entire electrical system. Therefore,during operations at low engine r.p.m., the electricalneeds must be drawn from the battery, which canquickly be depleted.

Alternators have several advantages over generators.Alternators produce sufficient current to operate theentire electrical system, even at slower engine speeds,by producing alternating current, which is converted todirect current. The electrical output of an alternator ismore constant throughout a wide range of enginespeeds.

Some airplanes have receptacles to which an externalground power unit (GPU) may be connected to provideelectrical energy for starting. These are very useful,especially during cold weather starting. Follow themanufacturer’s recommendations for engine startingusing a GPU.

The electrical system is turned on or off with a masterswitch. Turning the master switch to the ON positionprovides electrical energy to all the electricalequipment circuits with the exception of the ignitionsystem. Equipment that commonly uses the electricalsystem for its source of energy includes:

• Position lights

• Anticollision lights

• Landing lights

• Taxi lights

• Interior cabin lights

• Instrument lights

• Radio equipment

• Turn indicator

• Fuel gauges

• Electric fuel pump

• Stall warning system

• Pitot heat

• Starting motor

Many airplanes are equipped with a battery switch thatcontrols the electrical power to the airplane in amanner similar to the master switch. In addition, analternator switch is installed which permits the pilot toexclude the alternator from the electrical system in theevent of alternator failure. [Figure 5-22]

With the alternator half of the switch in the OFFposition, the entire electrical load is placed on thebattery. Therefore, all nonessential electrical equipmentshould be turned off to conserve battery power.

A bus bar is used as a terminal in the airplane electricalsystem to connect the main electrical system to theequipment using electricity as a source of power. Thissimplifies the wiring system and provides a commonpoint from which voltage can be distributed throughoutthe system. [Figure 5-23]

Fuses or circuit breakers are used in the electrical systemto protect the circuits and equipment from electricaloverload. Spare fuses of the proper amperage limitshould be carried in the airplane to replace defective orblown fuses. Circuit breakers have the same function asa fuse but can be manually reset, rather than replaced, ifan overload condition occurs in the electrical system.Placards at the fuse or circuit breaker panel identify thecircuit by name and show the amperage limit.

An ammeter is used to monitor the performance of theairplane electrical system. The ammeter shows if thealternator/generator is producing an adequate supply ofelectrical power. It also indicates whether or not thebattery is receiving an electrical charge.

Ammeters are designed with the zero point in thecenter of the face and a negative or positive indicationon either side. [Figure 5-24] When the pointer of theammeter on the left is on the plus side, it shows thecharging rate of the battery. A minus indication meansmore current is being drawn from the battery than isbeing replaced. A full-scale minus deflection indicatesa malfunction of the alternator/generator. A full-scalepositive deflection indicates a malfunction of theregulator. In either case, consult the AFM or POH forappropriate action to be taken.

Figure 5-22. On this master switch, the left half is for thealternator and the right half is for the battery.

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Not all airplanes are equipped with an ammeter. Somehave a warning light that, when lighted, indicates adischarge in the system as a generator/alternatormalfunction. Refer to the AFM or POH for appropriateaction to be taken.

Another electrical monitoring indicator is a loadmeter.This type of gauge, illustrated on the right in figure5-24, has a scale beginning with zero and shows the loadbeing placed on the alternator/generator. The loadmeterreflects the total percentage of the load placed on the

PRIMARY

BUS

AVIONICS

BUS

To Fuel Quantity Indicators

To Flashing Beacon

To Pitot Heat

To Radio Cooling Fan

To Strobe Lights

To Landing and Taxi Lights

To Ignition Switch

To Wing Flap System

To Red Doorpost Maplight

To Low-Voltage Warning Light

To Instrument, Radio, Compassand Post Lights

To Oil Temperature Gauge

To Turn Coordinator

To Low-Vacuum Warning Light

Switch/Circuit Breaker toStandby Vacuum Pump

To White Doorpost Light

To Audio Muting Relay

To Control Wheel Maplight

To Navigation Lights

To Dome Light

To Radio

To Radio

To Radio or Transponderand Encoding Altimeter

To Radio

FUELIND.

BCNPITOT

PULLOFF STROBE

RADIOFAN

LDGLTS

FLAP

INSTLTS

STBY VAC

RADIO 1

RADIO 2

RADIO 3

NAVDOME

RADIO 4

Low-VoltageWarning Light

Low Volt Out

Power In

Sense (+)

Field

Sense (-)

GroundG

F

B

Pull Off

ALT

AlternatorControl

Unit

ALT

BAT

+

-

ToWingFlap

CircuitBreaker

RL

Battery

BatteryContactor

Magnetos

Ground ServicePlug Receptacle

StarterContactor

Starter

Flight HourRecorder

Oil PressureSwitch

Clock orDigital Clock

Ammeter

AlternatorField Circuit

Breaker

Master Switch

Alternator

CODE

Circuit Breaker (Auto-Reset) Fuse Diode Resistor

Circuit Breaker (Pull-Off,Push-to-Reset)

To Inst LTS Circuit Breaker

Capacitor (Noise Filter)

Circuit Breaker (Push to Reset)

Figure 5-23. Electrical system schematic.

Figure 5-24. Ammeter and loadmeter.

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generating capacity of the electrical system by the elec-trical accessories and battery. When all electrical com-ponents are turned off, it reflects only the amount ofcharging current demanded by the battery.

A voltage regulator controls the rate of charge to thebattery by stabilizing the generator or alternatorelectrical output. The generator/alternator voltageoutput should be higher than the battery voltage. Forexample, a 12-volt battery would be fed by agenerator/alternator system of approximately 14 volts.The difference in voltage keeps the battery charged.

HYDRAULIC SYSTEMSThere are multiple applications for hydraulic use inairplanes, depending on the complexity of the airplane.For example, hydraulics are often used on smallairplanes to operate wheel brakes, retractable landinggear, and some constant-speed propellers. On largeairplanes, hydraulics are used for flight controlsurfaces, wing flaps, spoilers, and other systems.

A basic hydraulic system consists of a reservoir, pump(either hand, electric, or engine driven), a filter to keepthe fluid clean, selector valve to control the direction offlow, relief valve to relieve excess pressure, and anactuator.

The hydraulic fluid is pumped through the system to anactuator or servo. Servos can be either single-acting ordouble-acting servos based on the needs of the system.This means that the fluid can be applied to one or bothsides of the servo, depending on the servo type, andtherefore provides power in one direction with asingle-acting servo. A servo is a cylinder with a pistoninside that turns fluid power into work and creates thepower needed to move an aircraft system or flightcontrol. The selector valve allows the fluid direction tobe controlled. This is necessary for operations like theextension and retraction of landing gear where the fluidmust work in two different directions. The relief valveprovides an outlet for the system in the event ofexcessive fluid pressure in the system. Each systemincorporates different components to meet theindividual needs of different aircraft.

A mineral-based fluid is the most widely used type forsmall airplanes. This type of hydraulic fluid, which is akerosene-like petroleum product, has good lubricatingproperties, as well as additives to inhibit foaming andprevent the formation of corrosion. It is quite stablechemically, has very little viscosity change withtemperature, and is dyed for identification. Sinceseveral types of hydraulic fluids are commonly used,

make sure your airplane is serviced with the typespecified by the manufacturer. Refer to the AFM, POH,or the Maintenance Manual. [Figure 5-25]

LANDING GEARThe landing gear forms the principal support of theairplane on the surface. The most common type oflanding gear consists of wheels, but airplanes can alsobe equipped with floats for water operations, or skis forlanding on snow. [Figure 5-26]

The landing gear on small airplanes consists of threewheelstwo main wheels, one located on each side ofthe fuselage, and a third wheel, positioned either at thefront or rear of the airplane. Landing gear employing arear-mounted wheel is called a conventional landinggear. Airplanes with conventional landing gear areoften referred to as tailwheel airplanes. When the thirdwheel is located on the nose, it is called a nosewheel,and the design is referred to as a tricycle gear. Asteerable nosewheel or tailwheel permits the airplaneto be controlled throughout all operations while onthe ground.

TRICYCLE LANDING GEAR AIRPLANESA tricycle gear airplane has three main advantages:

Servo—A motor or other form of actuator which receives a small signalfrom the control device and exerts a large force to accomplish thedesired work.

Reservoir Double-Acting

Cylinder

Motion

Selector Valve

C.V. C.V.Pump

Hydraulic Fluid Supply

Hydraulic Pressure Return Fluid

System ReliefValve

Figure 5-25. Basic hydraulic system.

Figure 5-26. The landing gear supports the airplane duringthe takeoff run, landing, taxiing, and when parked.

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1. It allows more forceful application of the brakesduring landings at high speeds without resultingin the airplane nosing over.

2. It permits better forward visibility for the pilotduring takeoff, landing, and taxiing.

3. It tends to prevent ground looping (swerving) byproviding more directional stability duringground operation since the airplane’s center ofgravity (CG) is forward of the main wheels. Theforward CG, therefore, tends to keep the airplanemoving forward in a straight line rather thanground looping.

Nosewheels are either steerable or castering. Steerablenosewheels are linked to the rudders by cables or rods,while castering nosewheels are free to swivel. In bothcases, you steer the airplane using the rudder pedals.However, airplanes with a castering nosewheel mayrequire you to combine the use of the rudder pedalswith independent use of the brakes.

TAILWHEEL LANDING GEAR AIRPLANESOn tailwheel airplanes, two main wheels, which areattached to the airframe ahead of its center of gravity,support most of the weight of the structure, while atailwheel at the very back of the fuselage provides athird point of support. This arrangement allowsadequate ground clearance for a larger propeller and ismore desirable for operations on unimproved fields.[Figure 5-27]

The main drawback with the tailwheel landing gear isthat the center of gravity is behind the main gear. Thismakes directional control more difficult while on theground. If you allow the airplane to swerve whilerolling on the ground at a speed below that at which therudder has sufficient control, the center of gravity willattempt to get ahead of the main gear. This may causethe airplane to ground loop.

Another disadvantage for tailwheel airplanes is the lackof good forward visibility when the tailwheel is on ornear the surface. Because of the associated hazards,specific training is required in tailwheel airplanes.

FIXED AND RETRACTABLE LANDING GEARLanding gear can also be classified as either fixed orretractable. A fixed gear always remains extended andhas the advantage of simplicity combined with lowmaintenance. A retractable gear is designed tostreamline the airplane by allowing the landing gear tobe stowed inside the structure during cruising flight.[Figure 5-28]

BRAKESAirplane brakes are located on the main wheels and areapplied by either a hand control or by foot pedals (toeor heel). Foot pedals operate independently and allowfor differential braking. During ground operations,differential braking can supplement nosewheel/tail-wheel steering.

AUTOPILOTAutopilots are designed to control the aircraft and helpreduce the pilot’s workload. The limitations of theautopilot depend on the complexity of the system. The

Figure 5-27.Tailwheel landing gear.

Figure 5-28. Fixed and retractable gear airplanes.

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common features available on an autopilot are altitudeand heading hold. More advanced systems may includea vertical speed and/or indicated airspeed holdmode. Most autopilot systems are coupled tonavigational aids.

An autopilot system consists of servos that actuate theflight controls. The number and location of theseservos depends on the complexity of the system. Forexample, a single-axis autopilot controls the aircraftabout the longitudinal axis and a servo actuates theailerons. A three-axis autopilot controls the aircraftabout the longitudinal, lateral, and vertical axes; andthree different servos actuate the ailerons, the elevator,and the rudder.

The autopilot system also incorporates a disconnectsafety feature to automatically or manually disengagethe system. Autopilots can also be manuallyoverridden. Because autopilot systems differ widely intheir operation, refer to the autopilot operatinginstructions in the AFM or POH.

PRESSURIZED AIRPLANESWhen an airplane is flown at a high altitude, itconsumes less fuel for a given airspeed than it does forthe same speed at a lower altitude. In other words, theairplane is more efficient at a high altitude. In addition,bad weather and turbulence may be avoided by flyingin the relatively smooth air above the storms. Becauseof the advantages of flying at high altitudes,many modern general aviation-type airplanes arebeing designed to operate in that environment. It isimportant that pilots transitioning to such sophisticatedequipment be familiar with at least the basicoperating principles.

A cabin pressurization system accomplishes severalfunctions in providing adequate passenger comfort andsafety. It maintains a cabin pressure altitude ofapproximately 8,000 feet at the maximum designedcruising altitude of the airplane, and prevents rapidchanges of cabin altitude that may be uncomfortable or

cause injury to passengers and crew. In addition, thepressurization system permits a reasonably fastexchange of air from the inside to the outside of thecabin. This is necessary to eliminate odors and toremove stale air. [Figure 5-29]

Pressurization of the airplane cabin is an acceptedmethod of protecting occupants against the effects ofhypoxia. Within a pressurized cabin, occupants can betransported comfortably and safely for long periods oftime, particularly if the cabin altitude is maintained at8,000 feet or below, where the use of oxygenequipment is not required. The flight crew in this typeof airplane must be aware of the danger of accidentalloss of cabin pressure and must be prepared to deal withsuch an emergency whenever it occurs.

In the typical pressurization system, the cabin, flightcompartment, and baggage compartments areincorporated into a sealed unit that is capable ofcontaining air under a pressure higher than outsideatmospheric pressure. On aircraft powered by turbineengines, bleed air from the engine compressor section isused to pressurize the cabin. Superchargers may be usedon older model turbine powered airplanes to pump airinto the sealed fuselage. Piston-powered airplanes mayuse air supplied from each engine turbocharger througha sonic venturi (flow limiter). Air is released from thefuselage by a device called an outflow valve. Theoutflow valve, by regulating the air exit, provides aconstant inflow of air to the pressurized area.[Figure 5-30]

To understand the operating principles ofpressurization and air-conditioning systems, it isnecessary to become familiar with some of the relatedterms and definitions, such as:

• Aircraft altitude—the actual height above sealevel at which the airplane is flying.

• Ambient temperature—the temperature in thearea immediately surrounding the airplane.

Altitude (ft) Pressure (p.s.i.) Altitude (ft) Pressure (p.s.i.)

Sea Level

2,000

4,000

6,000

8,000

10,000

12,000

14,000

14.7

13.7

12.7

11.8

10.9

10.1

9.4

8.6

16,000

18,000

20,000

22,000

24,000

26,000

28,000

30,000

8.0

7.3

6.8

6.2

5.7

5.2

4.8

4.4

STANDARD ATMOSPHERIC PRESSURE

At an altitude of 28,000 feet, standard atmospheric pressure is 4.8 p.s.i. By adding this pressure to the cabin pressure differential of 6.1 p.s.i.d., a total air pressure of 10.9 p.s.i. is obtained.

The altitude where the standard air pressure is equal to 10.9 p.s.i can be found at 8,000 feet.

Figure 5-29. Standard atmospheric pressure chart.

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• Ambient pressure—the pressure in the areaimmediately surrounding the airplane.

• Cabin altitude—used to express cabin pressurein terms of equivalent altitude above sea level.

• Differential pressure—the difference in pressurebetween the pressure acting on one side of a walland the pressure acting on the other side of thewall. In aircraft air-conditioning and pressurizingsystems, it is the difference between cabinpressure and atmospheric pressure.

The cabin pressure control system provides cabinpressure regulation, pressure relief, vacuum relief, andthe means for selecting the desired cabin altitude in theisobaric and differential range. In addition, dumping ofthe cabin pressure is a function of the pressure controlsystem. A cabin pressure regulator, an outflow valve,and a safety valve are used to accomplish thesefunctions.

The cabin pressure regulator controls cabin pressure toa selected value in the isobaric range and limits cabinpressure to a preset differential value in the differentialrange. When the airplane reaches the altitude at whichthe difference between the pressure inside and outsidethe cabin is equal to the highest differential pressure for

which the fuselage structure is designed, a furtherincrease in airplane altitude will result in acorresponding increase in cabin altitude. Differentialcontrol is used to prevent the maximum differentialpressure, for which the fuselage was designed, frombeing exceeded. This differential pressure isdetermined by the structural strength of the cabin andoften by the relationship of the cabin size to theprobable areas of rupture, such as window areasand doors.

The cabin air pressure safety valve is a combinationpressure relief, vacuum relief, and dump valve. Thepressure relief valve prevents cabin pressure fromexceeding a predetermined differential pressure aboveambient pressure. The vacuum relief prevents ambientpressure from exceeding cabin pressure by allowingexternal air to enter the cabin when ambient pressureexceeds cabin pressure. The cockpit control switchactuates the dump valve. When this switch ispositioned to ram, a solenoid valve opens, causing thevalve to dump cabin air to atmosphere.

The degree of pressurization and the operating altitudeof the aircraft are limited by several critical designfactors. Primarily the fuselage is designed to withstanda particular maximum cabin differential pressure.

Several instruments are used in conjunction with thepressurization controller. The cabin differentialpressure gauge indicates the difference between insideand outside pressure. This gauge should be monitoredto assure that the cabin does not exceed the maximumallowable differential pressure. A cabin altimeter is alsoprovided as a check on the performance of the system.In some cases, these two instruments are combined intoone. A third instrument indicates the cabin rate of climbor descent. A cabin rate-of-climb instrument and acabin altimeter are illustrated in Figure 5-31 onpage 5-26.

Decompression is defined as the inability of theairplane’s pressurization system to maintain its designedpressure differential. This can be caused by amalfunction in the pressurization system or structuraldamage to the airplane. Physiologically, decompressionsfall into two categories; they are:

• Explosive Decompression—Explosive decom-pression is defined as a change in cabin pressurefaster than the lungs can decompress; therefore, itis possible that lung damage may occur. Normally,the time required to release air from the lungswithout restrictions, such as masks, is 0.2 seconds.Most authorities consider any decompression thatoccurs in less than 0.5 seconds as explosive andpotentially dangerous.

Air Scoops

Heat ShroudCabin

Heat Valve

Heat Exchanger

TurbochargerCompressor Section

Flow Control Venturi

Forward Air Outlet

Floor Level Outlets

To CabinAltitudeController

Outflow ValveSafety/Dump Valve

BLUE – Ambient Air

RED – Compressor Discharge Air

ORANGE –Pressurization Air

CODE

BROWN – Pre-heatedAmbient Air

GREEN – ConditionedPressurization Air

Pressurized Cabin

Figure 5-30. High performance airplane pressurizationsystem.

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• Rapid Decompression—Rapid decompression isdefined as a change in cabin pressure where thelungs can decompress faster than the cabin; there-fore, there is no likelihood of lung damage.

During an explosive decompression, there may benoise, and for a split second, one may feel dazed. Thecabin air will fill with fog, dust, or flying debris. Fogoccurs due to the rapid drop in temperature and thechange of relative humidity. Normally, the ears clearautomatically. Air will rush from the mouth and nosedue to the escape of air from the lungs, and may benoticed by some individuals.

The primary danger of decompression is hypoxia.Unless proper utilization of oxygen equipment isaccomplished quickly, unconsciousness may occur in avery short time. The period of useful consciousness isconsiderably shortened when a person is subjected to arapid decompression. This is due to the rapid reductionof pressure on the body—oxygen in the lungs isexhaled rapidly. This in effect reduces the partialpressure of oxygen in the blood and therefore reducesthe pilot’s effective performance time by one-third toone-fourth its normal time. For this reason, the oxygenmask should be worn when flying at very high altitudes(35,000 feet or higher). It is recommended that thecrewmembers select the 100 percent oxygen setting onthe oxygen regulator at high altitude if the airplane isequipped with a demand or pressure demand oxygensystem.

Another hazard is being tossed or blown out of theairplane if near an opening. For this reason, individualsnear openings should wear safety harnesses or seatbeltsat all times when the airplane is pressurized and theyare seated.

Another potential hazard during high altitudedecompressions is the possibility of evolved gasdecompression sicknesses. Exposure to wind blasts andextremely cold temperatures are other hazards onemight have to face.

Rapid descent from altitude is necessary if theseproblems are to be minimized. Automatic visual andaural warning systems are included in the equipment ofall pressurized airplanes.

OXYGEN SYSTEMSMost high altitude airplanes come equipped with sometype of fixed oxygen installation. If the airplane doesnot have a fixed installation, portable oxygenequipment must be readily accessible during flight. Theportable equipment usually consists of a container,regulator, mask outlet, and pressure gauge. Aircraftoxygen is usually stored in high pressure systemcontainers of 1,800 – 2,200 pounds per square inch(p.s.i.). When the ambient temperature surrounding anoxygen cylinder decreases, pressure within thatcylinder will decrease because pressure varies directlywith temperature if the volume of a gas remainsconstant. If a drop in indicated pressure on asupplemental oxygen cylinder is noted, there is noreason to suspect depletion of the oxygen supply, whichhas simply been compacted due to storage of thecontainers in an unheated area of the aircraft. Highpressure oxygen containers should be marked with thep.s.i. tolerance (i.e., 1,800 p.s.i.) before filling thecontainer to that pressure. The containers should besupplied with aviation oxygen only, which is 100percent pure oxygen. Industrial oxygen is not intendedfor breathing and may contain impurities, and medicaloxygen contains water vapor that can freeze in theregulator when exposed to cold temperatures. To assuresafety, oxygen system periodic inspection andservicing should be done.

An oxygen system consists of a mask and a regulatorthat supplies a flow of oxygen dependent upon cabinaltitude. Regulators approved for use up to 40,000 feetare designed to provide zero percent cylinder oxygenand 100 percent cabin air at cabin altitudes of 8,000feet or less, with the ratio changing to 100 percentoxygen and zero percent cabin air at approximately34,000 feet cabin altitude. Regulators approved up to45,000 feet are designed to provide 40 percent cylinderoxygen and 60 percent cabin air at lower altitudes, with

Cabin Pressure Altitude Indicator (Thousands of Feet)

Cabin Differential Pressure Indicator (Pounds Per Square Inch Differential)

Maximum Cabin Differential Pressure Limit

Cabin Rate-Of-ClimbIndicator

Cabin/DifferentialPressure Indicator

Figure 5-31. Cabin pressurization instruments.

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the ratio changing to 100 percent at the higher altitude.Pilots should avoid flying above 10,000 feet withoutoxygen during the day and above 8,000 feet at night.[Figure 5-32]

Pilots should be aware of the danger of fire when usingoxygen. Materials that are nearly fireproof in ordinaryair may be susceptible to burning in oxygen. Oils andgreases may catch fire if exposed to oxygen, andcannot be used for sealing the valves and fittings ofoxygen equipment. Smoking during any kind ofoxygen equipment use is prohibited. Before each flight,the pilot should thoroughly inspect and test all oxygenequipment. The inspection should include a thoroughexamination of the aircraft oxygen equipment,including available supply, an operational check of thesystem, and assurance that the supplemental oxygen isreadily accessible. The inspection should beaccomplished with clean hands and should include avisual inspection of the mask and tubing for tears,cracks, or deterioration; the regulator for valve andlever condition and positions; oxygen quantity; and thelocation and functioning of oxygen pressure gauges,flow indicators and connections. The mask should bedonned and the system should be tested. After anyoxygen use, verify that all components and valves areshut off.

MASKSThere are numerous types of oxygen masks in use thatvary in design detail. It would be impractical to discussall of the types in this handbook. It is important that themasks used be compatible with the particular oxygensystem involved. Crew masks are fitted to the user’sface with a minimum of leakage. Crew masks usuallycontain a microphone. Most masks are the oronasal-type, which covers only the mouth and nose.

Passenger masks may be simple, cup-shaped rubbermoldings sufficiently flexible to obviate individualfitting. They may have a simple elastic head strap or

the passenger may hold them to the face.

All oxygen masks should be kept clean. This reducesthe danger of infection and prolongs the life of themask. To clean the mask, wash it with a mild soap andwater solution and rinse it with clear water. If amicrophone is installed, use a clean swab, instead ofrunning water, to wipe off the soapy solution. The maskshould also be disinfected. A gauze pad that has beensoaked in a water solution of Merthiolate can be usedto swab out the mask. This solution should containone-fifth teaspoon of Merthiolate per quart of water.Wipe the mask with a clean cloth and air dry.

DILUTER DEMAND OXYGEN SYSTEMSDiluter demand oxygen systems supply oxygen onlywhen the user inhales through the mask. An automixlever allows the regulators to automatically mix cabinair and oxygen or supply 100 percent oxygen,depending on the altitude. The demand mask providesa tight seal over the face to prevent dilution withoutside air and can be used safely up to 40,000 feet. Apilot who has a beard or mustache should be sure it istrimmed in a manner that will not interfere with thesealing of the oxygen mask. The fit of the mask aroundthe beard or mustache should be checked on the groundfor proper sealing.

PRESSURE DEMAND OXYGEN SYSTEMSPressure demand oxygen systems are similar to diluterdemand oxygen equipment, except that oxygen issupplied to the mask under pressure at cabin altitudesabove 34,000 feet. Pressure demand regulators alsocreate airtight and oxygen-tight seals, but they alsoprovide a positive pressure application of oxygen to themask face piece that allows the user’s lungs to bepressurized with oxygen. This feature makes pressuredemand regulators safe at altitudes above 40,000 feet.Some systems may have a pressure demand mask withthe regulator attached directly to the mask, rather thanmounted on the instrument panel or other area withinthe flight deck. The mask-mounted regulator eliminatesthe problem of a long hose that must be purged of airbefore 100 percent oxygen begins flowing intothe mask.

CONTINUOUS FLOW OXYGEN SYSTEMContinuous flow oxygen systems are usually providedfor passengers. The passenger mask typically has areservoir bag, which collects oxygen from thecontinuous flow oxygen system during the time whenthe mask user is exhaling. The oxygen collected in thereservoir bag allows a higher aspiratory flow rateduring the inhalation cycle, which reduces the amountof air dilution. Ambient air is added to the supplied

Figure 5-32. Oxygen system regulator.

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oxygen during inhalation after the reservoir bagoxygen supply is depleted. The exhaled air is releasedto the cabin. [Figure 5-33]

SERVICING OF OXYGEN SYSTEMSCertain precautions should be observed wheneveraircraft oxygen systems are to be serviced. Beforeservicing any aircraft with oxygen, consult the specificaircraft service manual to determine the type ofequipment required and procedures to be used. Oxygensystem servicing should be accomplished only whenthe aircraft is located outside of the hangars. Personalcleanliness and good housekeeping are imperativewhen working with oxygen. Oxygen under pressureand petroleum products create spontaneous resultswhen they are brought in contact with each other.Service people should be certain to wash dirt, oil, andgrease (including lip salves and hair oil) from theirhands before working around oxygen equipment. It isalso essential that clothing and tools are free of oil,grease, and dirt. Aircraft with permanently installedoxygen tanks usually require two persons toaccomplish servicing of the system. One should bestationed at the service equipment control valves, andthe other stationed where he or she can observe theaircraft system pressure gauges. Oxygen systemservicing is not recommended during aircraft fuelingoperations or while other work is performed that couldprovide a source of ignition. Oxygen system servicingwhile passengers are on board the aircraft isnot recommended.

ICE CONTROL SYSTEMSIce control systems installed on aircraft consist ofanti-ice and de-ice equipment. Anti-icing equipment isdesigned to prevent the formation of ice, while de-icingequipment is designed to remove ice once it hasformed. Ice control systems protect the leading edge ofwing and tail surfaces, pitot and static port openings,fuel tank vents, stall warning devices, windshields, andpropeller blades. Ice detection lighting may also beinstalled on some airplanes to determine the extent ofstructural icing during night flights. Since manyairplanes are not certified for flight in icing conditions,refer to the AFM or POH for details.

AIRFOIL ICE CONTROLInflatable de-icing boots consist of a rubber sheetbonded to the leading edge of the airfoil. When icebuilds up on the leading edge, an engine-drivenpneumatic pump inflates the rubber boots. Someturboprop aircraft divert engine bleed air to the wing toinflate the rubber boots. Upon inflation, the ice iscracked and should fall off the leading edge of thewing. De-icing boots are controlled from the cockpitby a switch and can be operated in a single cycle orallowed to cycle at automatic, timed intervals. It isimportant that de-icing boots are used in accordancewith the manufacturer’s recommendations. If they areallowed to cycle too often, ice can form over thecontour of the boot and render the boots ineffective.[Figure 5-34]

Many de-icing boot systems use the instrument systemsuction gauge and a pneumatic pressure gauge toindicate proper boot operation. These gauges haverange markings that indicate the operating limits forboot operation. Some systems may also incorporate anannunciator light to indicate proper boot operation.

Proper maintenance and care of de-icing boots isimportant for continued operation of this system. Theyneed to be carefully inspected prior to a flight.

Another type of leading edge protection is the thermalanti-ice system installed on airplanes with turbineengines. This system is designed to prevent the buildupof ice by directing hot air from the compressor sectionof the engine to the leading edge surfaces. The systemis activated prior to entering icing conditions. The hotair heats the leading edge sufficiently to prevent theformation of ice.

Oronasal Mask

Rebreather Bag

Figure 5-33. Continuous flow mask and rebreather bag.

Figure 5-34. De-icing boots on the leading edge of the wing.

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An alternate type of leading edge protection that is notas common as thermal anti-ice and de-icing boots isknown as a weeping wing. The weeping-wing designuses small holes located in the leading edge of thewing. A chemical mixture is pumped to the leadingedge and weeps out through the holes to prevent theformation and buildup of ice.

WINDSCREEN ICE CONTROLThere are two main types of windscreen anti-icesystems. The first system directs a flow of alcohol tothe windscreen. By using it early enough, the alcoholwill prevent ice from building up on the windshield.The rate of alcohol flow can be controlled by a dial inthe cockpit according to procedures recommended bythe airplane manufacturer.

Another effective method of anti-icing equipment isthe electric heating method. Small wires or otherconductive material is imbedded in the windscreen.The heater can be turned on by a switch in the cockpit,at which time electrical current is passed across theshield through the wires to provide sufficient heat toprevent the formation of ice on the windscreen. Theelectrical current can cause compass deviation errors;in some cases, as much as 40°. The heated windscreenshould only be used during flight. Do not leave it onduring ground operations, as it can overheat and causedamage to the windscreen.

PROPELLER ICE CONTROLPropellers are protected from icing by use of alcohol orelectrically heated elements. Some propellers areequipped with a discharge nozzle that is pointed towardthe root of the blade. Alcohol is discharged from thenozzles, and centrifugal force makes the alcohol flowdown the leading edge of the blade. This prevents icefrom forming on the leading edge of the propeller.Propellers can also be fitted with propeller anti-iceboots. The propeller boot is divided into two sections—the inboard and the outboard sections. The boots aregrooved to help direct the flow of alcohol, and they arealso imbedded with electrical wires that carry currentfor heating the propeller. The prop anti-ice system canbe monitored for proper operation by monitoring theprop anti-ice ammeter. During the preflight inspection,check the propeller boots for proper operation. If a bootfails to heat one blade, an unequal blade loading canresult, and may cause severe propeller vibration.[Figure 5-35]

OTHER ICE CONTROL SYSTEMSPitot and static ports, fuel vents, stall-warning sensors,and other optional equipment may be heated by

electrical elements. Operational checks of theelectrically heated systems are to be checked inaccordance with the AFM or POH.

Operation of aircraft anti-icing and de-icing systemsshould be checked prior to encountering icingconditions. Encounters with structural ice requireimmediate remedial action. Anti-icing and de-icingequipment is not intended to sustain long-term flight inicing conditions.

TURBINE ENGINESThe turbine engine produces thrust by increasing thevelocity of the air flowing through the engine. It consistsof an air inlet, compressor, combustion chambers, tur-bine section, and exhaust. [Figure 5-36] The turbine

PROP ANTI-ICE BOOTThe boot is divided into two sections: inboard and outboard. When the anti-ice is operating, the inboard section heats on each blade, and then cycles to the outboard section. If a boot fails to heat properly on one blade, unequal ice loading may result, causing severe vibration.

PROP ANTI-ICE AMMETERWhen the system is operating, the prop ammeter will show in the normal operating range. As each boot section cycles, the ammeter will fluctuate.

Inboard Section

Outboard Section

Figure 5-35. Prop ammeter and anti-ice boots.

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engine has the following advantages over areciprocating engine: less vibration, increased aircraftperformance, reliability, and ease of operation.

TYPES OF TURBINE ENGINESTurbine engines are classified according to the type ofcompressors they use. The compressor types fall intothree categories—centrifugal flow, axial flow, andcentrifugal-axial flow. Compression of inlet air isachieved in a centrifugal flow engine by acceleratingair outward perpendicular to the longitudinal axis ofthe machine. The axial-flow engine compresses air bya series of rotating and stationary airfoils moving theair parallel to the longitudinal axis. The centrifugal-axial flow design uses both kinds of compressors toachieve the desired compression.

The path the air takes through the engine and howpower is produced determines the type of engine. Thereare four types of aircraft turbine engines—turbojet,turboprop, turbofan, and turboshaft.

TURBOJETThe turbojet engine contains four sections: compressor,combustion chamber, turbine section, and exhaust. Thecompressor section passes inlet air at a high rate ofspeed to the combustion chamber. The combustionchamber contains the fuel inlet and igniter forcombustion. The expanding air drives a turbine, whichis connected by a shaft to the compressor, sustainingengine operation. The accelerated exhaust gases fromthe engine provide thrust. This is a basic application ofcompressing air, igniting the fuel-air mixture,producing power to self-sustain the engine operation,and exhaust for propulsion.

Turbojet engines are limited on range and endurance.

They are also slow to respond to throttle applications atslow compressor speeds.

TURBOPROPA turboprop engine is a turbine engine that drives apropeller through a reduction gear. The exhaust gasesdrive a power turbine connected by a shaft that drivesthe reduction gear assembly. Reduction gearing isnecessary in turboprop engines because optimumpropeller performance is achieved at much slowerspeeds than the engine’s operating r.p.m. Turbopropengines are a compromise between turbojet enginesand reciprocating powerplants. Turboprop engines aremost efficient at speeds between 250 and 400 m.p.h.and altitudes between 18,000 and 30,000 feet. Theyalso perform well at the slow airspeeds required fortakeoff and landing, and are fuel efficient. The mini-mum specific fuel consumption of the turboprop engineis normally available in the altitude range of 25,000feet to the tropopause.

TURBOFANTurbofans were developed to combine some of the bestfeatures of the turbojet and the turboprop. Turbofanengines are designed to create additional thrust bydiverting a secondary airflow around the combustionchamber. The turbofan bypass air generates increasedthrust, cools the engine, and aids in exhaust noisesuppression. This provides turbojet-type cruise speedand lower fuel consumption.

The inlet air that passes through a turbofan engine isusually divided into two separate streams of air. Onestream passes through the engine core, while a secondstream bypasses the engine core. It is this bypassstream of air that is responsible for the term “bypassengine.” A turbofan’s bypass ratio refers to the ratio ofthe mass airflow that passes through the fan divided bythe mass airflow that passes through the engine core.

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Figure 5-36. Basic components of a turbine engine.

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TURBOSHAFTThe fourth common type of jet engine is the turboshaft.It delivers power to a shaft that drives something otherthan a propeller. The biggest difference between aturbojet and turboshaft engine is that on a turboshaftengine, most of the energy produced by the expandinggases is used to drive a turbine rather than producethrust. Many helicopters use a turboshaft gas turbineengine. In addition, turboshaft engines are widely usedas auxiliary power units on large aircraft.

PERFORMANCE COMPARISONIt is possible to compare the performance of areciprocating powerplant and different types of turbineengines. However, for the comparison to be accurate,thrust horsepower (usable horsepower) for thereciprocating powerplant must be used rather thanbrake horsepower, and net thrust must be used for theturbine-powered engines. In addition, aircraft designconfiguration, and size must be approximatelythe same.

BHP— Brake horsepower is the horsepower actually delivered to the output shaft. Brake horsepower is the actual usable horsepower.

Net Thrust— The thrust produced by a turbojet orturbofan engine.

THP— Thrust horsepower is the horsepowerequivalent of the thrust produced by a turbojet or turbofan engine.

ESHP— Equivalent shaft horsepower, with respect to turboprop engines, is the sum of the shaft horsepower (SHP) delivered to the propeller and the thrust horsepower (THP) produced bythe exhaust gases.

Figure 5-37 shows how four types of engines comparein net thrust as airspeed is increased. This figure is forexplanatory purposes only and is not for specificmodels of engines. The four types of engines are:

• Reciprocating powerplant.

• Turbine, propeller combination (turboprop).

• Turbine engine incorporating a fan (turbofan).

• Turbojet (pure jet).

The comparison is made by plotting the performancecurve for each engine, which shows how maximumaircraft speed varies with the type of engine used. Sincethe graph is only a means of comparison, numericalvalues for net thrust, aircraft speed, and drag arenot included.

Comparison of the four powerplants on the basis of netthrust makes certain performance capabilities evident.In the speed range shown to the left of Line A, thereciprocating powerplant outperforms the other threetypes. The turboprop outperforms the turbofan in therange to the left of Line C. The turbofan engineoutperforms the turbojet in the range to the left of LineF. The turbofan engine outperforms the reciprocatingpowerplant to the right of Line B and the turboprop tothe right of Line C. The turbojet outperforms thereciprocating powerplant to the right of Line D, theturboprop to the right of Line E, and the turbofan to theright of Line F.

The points where the aircraft drag curve intersects thenet thrust curves are the maximum aircraft speeds. Thevertical lines from each of the points to the baseline ofthe graph indicate that the turbojet aircraft can attain ahigher maximum speed than aircraft equipped with theother types of engines. Aircraft equipped with theturbofan engine will attain a higher maximumspeed than aircraft equipped with a turboprop orreciprocating powerplant.

TURBINE ENGINE INSTRUMENTSEngine instruments that indicate oil pressure, oil temper-ature, engine speed, exhaust gas temperature, and fuelflow are common to both turbine and reciprocatingengines. However, there are some instruments that areunique to turbine engines. These instruments provideindications of engine pressure ratio, turbine dischargepressure, and torque. In addition, most gas turbineengines have multiple temperature-sensing instruments,called thermocouples, that provide pilots withtemperature readings in and around the turbine section.

Aircraft Drag

Airspeed

Net

Thr

ust

ACB

D

E F

TurbojetTurbofan

Turboprop

Reciprocating

Figure 5-37. Engine net thrust versus aircraft speed and drag.

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ENGINE PRESSURE RATIOAn engine pressure ratio (EPR) gauge is used toindicate the power output of a turbojet/turbofan engine.EPR is the ratio of turbine discharge to compressorinlet pressure. Pressure measurements are recorded byprobes installed in the engine inlet and at the exhaust.Once collected, the data is sent to a differentialpressure transducer, which is indicated on a cockpitEPR gauge.

EPR system design automatically compensates for theeffects of airspeed and altitude. However, changes inambient temperature do require a correction to beapplied to EPR indications to provide accurate enginepower settings.

EXHAUST GAS TEMPERATUREA limiting factor in a gas turbine engine is thetemperature of the turbine section. The temperature ofa turbine section must be monitored closely to preventoverheating the turbine blades and other exhaustsection components. One common way of monitoringthe temperature of a turbine section is with an exhaustgas temperature (EGT) gauge. EGT is an engineoperating limit used to monitor overall engineoperating conditions.

Variations of EGT systems bear different names basedon the location of the temperature sensors. Common tur-bine temperature sensing gauges include the turbine inlettemperature (TIT) gauge, turbine outlet temperature(TOT) gauge, interstage turbine temperature (ITT)gauge, and turbine gas temperature (TGT) gauge.

TORQUEMETERTurboprop/turboshaft engine power output is measuredby the torquemeter. Torque is a twisting force appliedto a shaft. The torquemeter measures power applied tothe shaft. Turboprop and turboshaft engines aredesigned to produce torque for driving a propeller.Torquemeters are calibrated in percentage units,foot-pounds, or pounds per square inch.

N1 INDICATORN1 represents the rotational speed of the low pressurecompressor and is presented on the indicator as apercentage of design r.p.m. After start the speed of thelow pressure compressor is governed by the N1 turbinewheel. The N1 turbine wheel is connected to the lowpressure compressor through a concentric shaft.

N2 INDICATORN2 represents the rotational speed of the high pressurecompressor and is presented on the indicator as apercentage of design r.p.m. The high pressurecompressor is governed by the N2 turbine wheel. The

N2 turbine wheel is connected to the high pressurecompressor through a concentric shaft. [Figure 5-38]

TURBINE ENGINE OPERATIONALCONSIDERATIONSBecause of the great variety of turbine engines, it isimpractical to cover specific operational procedures.However, there are certain operational considerationsthat are common to all turbine engines. They are enginetemperature limits, foreign object damage, hot start,compressor stall, and flameout.

ENGINE TEMPERATURE LIMITATIONSThe highest temperature in any turbine engine occursat the turbine inlet. Turbine inlet temperature istherefore usually the limiting factor in turbineengine operation.

THRUST VARIATIONSTurbine engine thrust varies directly with air density.As air density decreases, so does thrust. While bothturbine and reciprocating powered engines are affectedto some degree by high relative humidity, turbineengines will experience a negligible loss of thrust,while reciprocating engines a significant loss of brakehorsepower.

FOREIGN OBJECT DAMAGEDue to the design and function of a turbine engine’s airinlet, the possibility of ingestion of debris alwaysexists. This causes significant damage, particularly tothe compressor and turbine sections. When this occurs,it is called foreign object damage (FOD). Typical FODconsists of small nicks and dents caused by ingestionof small objects from the ramp, taxiway, or runway.However, FOD damage caused by bird strikes or iceingestion can also occur, and may result in totaldestruction of an engine.

Prevention of FOD is a high priority. Some engineinlets have a tendency to form a vortex between theground and the inlet during ground operations. Avortex dissipater may be installed on these engines.Concentric Shaft—One shaft inside another shaft having a common core.

Low PressureCompressor (N1)

High PressureCompressorDrive Shaft

High PressureCompressor (N2)

Low PressureCompressorDrive Shaft

Figure 5-38. Dual-spool axial-flow compressor.

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Other devices, such as screens and/or deflectors, mayalso be utilized. Preflight procedures include a visualinspection for any sign of FOD.

TURBINE ENGINE HOT/HUNG STARTA hot start is when the EGT exceeds the safe limit. Hotstarts are caused by too much fuel entering thecombustion chamber, or insufficient turbine r.p.m. Anytime an engine has a hot start, refer to the AFM, POH,or an appropriate maintenance manual for inspectionrequirements.

If the engine fails to accelerate to the proper speed afterignition or does not accelerate to idle r.p.m., a hungstart has occurred. A hung start, may also be called afalse start. A hung start may be caused by aninsufficient starting power source or fuel controlmalfunction.

COMPRESSOR STALLSCompressor blades are small airfoils and are subject tothe same aerodynamic principles that apply to anyairfoil. A compressor blade has an angle of attack. Theangle of attack is a result of inlet air velocity and thecompressor’s rotational velocity. These two forcescombine to form a vector, which defines the airfoil’sactual angle of attack to the approaching inlet air.

A compressor stall can be described as an imbalancebetween the two vector quantities, inlet velocity andcompressor rotational speed. Compressor stalls occurwhen the compressor blades’ angle of attack exceedsthe critical angle of attack. At this point, smoothairflow is interrupted and turbulence is created withpressure fluctuations. Compressor stalls cause airflowing in the compressor to slow down and stagnate,sometimes reversing direction. [Figure 5-39]

Compressor stalls can be transient andintermittent or steady state and severe. Indications of atransient/intermittent stall are usually an intermittent“bang” as backfire and flow reversal take place. If thestall develops and becomes steady, strong vibration anda loud roar may develop from the continuous flowreversal. Quite often the cockpit gauges will not show amild or transient stall, but will indicate a developedstall. Typical instrument indications includefluctuations in r.p.m., and an increase in exhaust gastemperature. Most transient stalls are not harmful to theengine and often correct themselves after one or twopulsations. The possibility of engine damage, whichmay be severe, from a steady state stall is immediate.Recovery must be accomplished quickly by reducingpower, decreasing the airplane’s angle of attack andincreasing airspeed.

Although all gas turbine engines are subject tocompressor stalls, most models have systems thatinhibit these stalls. One such system uses variable inletguide vane (VIGV) and variable stator vanes, whichdirect the incoming air into the rotor blades at anappropriate angle. The main way to prevent airpressure stalls is to operate the airplane within theparameters established by the manufacturer. If acompressor stall does develop, follow the proceduresrecommended in the AFM or POH.

FLAMEOUTA flameout is a condition in the operation of a gasturbine engine in which the fire in the engineunintentionally goes out. If the rich limit of the fuel/airratio is exceeded in the combustion chamber, the flamewill blow out. This condition is often referred to as arich flameout. It generally results from very fast engineacceleration, where an overly rich mixture causes thefuel temperature to drop below the combustiontemperature. It also may be caused by insufficientairflow to support combustion.

Another, more common flameout occurrence is due tolow fuel pressure and low engine speeds, whichtypically are associated with high-altitude flight. Thissituation also may occur with the engine throttled backduring a descent, which can set up the lean-conditionflameout. A weak mixture can easily cause the flame todie out, even with a normal airflow through the engine.

Any interruption of the fuel supply also can result in aflameout. This may be due to prolonged unusualattitudes, a malfunctioning fuel control system,turbulence, icing or running out of fuel.

Symptoms of a flameout normally are the same asthose following an engine failure. If the flameout is dueto a transitory condition, such as an imbalance betweenfuel flow and engine speed, an airstart may be

Normal InletAirflow

Distorted InletAirflow

Figure 5-39. Comparison of normal and distorted airflow intothe compressor section.

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attempted once the condition is corrected. In any case,pilots must follow the applicable emergencyprocedures outlined in the AFM or POH. Generally,

these procedures contain recommendations concerningaltitude and airspeed where the airstart is most likely tobe successful.

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