22343822 Aircraft Design

Aerospace Propulsion (MEC 4280/4740)Dr. Raed Kafafy

Aircraft DesignMEC 4200

Semester II2008/2009

Lecture (1 – 2)

Aircraft Design (MEC 4200)Dr. Raed Kafafy

2

Course OutlineInstructor

Dr. Raed KafafyOffice hours: Mon - Wed (10:00 AM – 11:00 AM)Office: E1-2-16.5

Required TextbookDaniel Raymer, AIRCRAFT DESIGN: A Conceptual Approach, 4th edition

Recommended TextbooksMichael Kroes and Thomas Wild, Aircraft Powerplants, McGraw-Hill International Edition 1995Douglas Archer and Maido Saarlas, An Introduction to Aerospace Propulsion, Prentice Hall, 1996


Aircraft Propulsion Options


4

OutlineAircraft Propulsion

Thrust EquationPropulsive EfficiencyGeneration of ThrustUninstalled Engine Thrust

Aircraft Propulsion OptionsPiston-PropTurbojet EnginesRamjet and ScramjetPropulsion Selection


5

OutlineJet Engine Performance

ThrustInstalled ThrustThrust-Drag Book KeepingInstalled Engine Thrust CorrectionsInstalled Net Propulsive Force CorrectionsPart Power Operation

Piston Engine PerformanceEngine CyclePropeller PerformancePiston-Prop Thrust Corrections

Turboprop Performance


6

What is …Propulsion?

In Latin, pro means forward and pellere means to drive.So, in English, propulsion means to push or drive forward.

Aerospace Propulsion System?A device which is used to produce thrust to drive an aerospace vehicle (aircraft, missile, launch vehicle, or a spacecraft) in a preferred direction.

Thrust?The propulsive force which is generated as a reaction to the change in the momentum of a working fluid (propellant).

Thrust


7

The Principle of Propulsion

A practical application of the third law of motion (Sir Isaac Newton)“For every force acting on a body there is an opposite and equal reaction.”


8

Aircraft PropulsionThrust Equation

All forms of aircraft propulsion produce thrust by pushing air (or hot gases) backward, so thrust force will be generated in reaction according to Newton’s third law.

( )

)/()()(

100

0

0

−=−=

−+−= ∞

VVVmVVm

ppAVVm eee

T

EngineV0 V

VeT

drag Ram thrustGrossNet thrust −=−= 0VmVm T


9

Aircraft PropulsionUninstalled Engine Thrust

Engine thrust when operated independently with no interference with aircraftUninstalled engine data are provided by manufacturer

Specific Fuel Consumption (SFC)

Thrust Specific Fuel Consumption (TSFC)

)kN/kW.s(shaftPW

SFC f

=

)kN/kN.s(T

fWTSFC

=

Piston-prop, turboprop & turboshaft

turbojet & turbofan


10

Aircraft PropulsionGeneration of Thrust

Propeller AircraftMost of propulsive force is exerted directly on A/C by the pull (or push) of the propeller through engine mounts.

Jet AircraftForce exerted through engine mounts may be only 1/3 of total propulsive force.Detailed calculation of thrust distribution is quite complicated.


11


Operation of Aircraft Propulsion SystemCompression of ambient airMixing air with fuel, then burning air-fuel mixtureExtracting energy from high-pressure, high-temperature combustion products Exhausting combustion products into the atmosphere


12


Classification of Aircraft PropulsionPiston Engines

Piston-propGas Turbine Engines

TurbojetTurbofanTurboprop Turboshaft

RamjetScramjet


13


Piston-Prop Engines Turbo-Engines

Intermittent CheapLowest fuel consumptionHeavyGreater noise and vibrationLimited to light airplanes and some

agricultural aircraft

ContinuousExpensiveHigher fuel consumptionLighterQuieterWide range of applications:

Military (fighters, bombers, cargo, …)Civil (transport, cargo, …)


14

Aircraft Propulsion OptionsMain Components of a Turbo-Engine

Inlet: adjusts condition of intake air to be suitable for engine operation (reduces air speed to about 0.4 – 0.5 of sonic speed)Compressor: compresses air from intake up to many times of atmospheric pressureBurner: mixes fuel with compressed air, then burns the fuel-air mixtureTurbine: extracts mechanical power required to drive the compressorNozzle: converts thermal energy (enthalpy) of hot gases into kinetic energy


15

Axial Compressors Centrifugal Compressors

Long with small frontal areaHigh throughput (air mass flow rate)Low pressure rise per stageSensitive to distortions in inlet

conditions

Short with large frontal areaLow throughput (air mass flow rate)High pressure rise per stageInsensitive to distortions in inlet

conditions

Aircraft Propulsion OptionsTurbomachinery Options


16

Aircraft Propulsion OptionsTurboprop and Turbofan Engines

An additional turbine is used to drive a propeller (in a turboprop) or a ducted fan (in a turbofan) to accelerate a larger mass of air, which increases propulsive efficiency at lower speeds.

core

bypass

mm

BPR

=


17

Aircraft Propulsion OptionsRamjets and Scramjets

At high flight speeds (M > 3), ram compression is enough to combustion. The compressor and driving turbine can then be removed giving a ramjet.At even higher speeds (M > 5 or 6), combustion should be done supersonically giving a scramjet.


18

Aircraft Propulsion OptionsThrust Augmentation (Afterburning) Option

Ideally, air should be just enough to burn all fuel (stoichiometric fuel/air ratio ~ 1/15), which results in highest hot gases temperature, hence highest thrust and propulsive efficiency.Due to temperature limits imposed by turbine blade material (1200 – 1600˚C), excess air should be used (fuel/air ratio ~ 1/60).Turbojets and turbofans may have afterburners to augment thrust during a short period of time.Fuel is injected after the last turbine to burn in uncombusted hot air (~ 75%).Thrust can be doubled, but very inefficiently, since TSFC is typically doubled.


19

Aircraft Propulsion OptionsSelection of Propulsion System

Selection of an aircraft propulsion system is usually obviously determined by mission requirements. Aircraft maximum speed limits the choices.In most cases, the propulsion system with the lowest SFC is chosen at a give flight speed.


20

Aircraft Propulsion OptionsPropulsive Efficiency

)/(1)/(2

)()(

0

02

02

21

000

VVVV

VVmVVVm

KEV

PP

Exp

TP +

=−

−=

∆==η

T

T

V0/V

ηPE

V0/V

power expendedpower thrust useful

=ηP


21

Comparative Performance

At flight speeds below ~ 450 mph (~ 720 km/s), the turbojet is less efficient than propeller-type engineHowever, the propeller efficiency decreases rapidly above 350 mph due to the airflow disturbance caused by the high blade-tip speeds of the propeller. The turbofan and prop-fan engines avoid the turboprop disadvantage.


22

Jet Engine PerformanceVariation of Thrust

Net thrust is roughly proportional to engine throughput.

Modern afterburning turbojets produce 1 – 1.3 kN/(kg/s), while modern turbofans produce 0.1 – 0.3 kN/(kg/s)Net thrust is roughly proportional to air density.

Generally, net thrust increases with flight speed with exceptions of subsonic jet approaching sonic speed or supersonic jet at high Mach numbers.Thrust and propulsive efficiency increase with overall pressure ratio (OPR) and turbine inlet temperature (TIT).

θδ

≅ slsTh TT ,

inlet

exit

ppOPR =

am∝T


23

Jet Engine PerformanceVariation of Propulsive Efficiency

Propulsive efficiency increases and thrust specific fuel consumption (TSFC) decreases with bypass ratio (BPR). Such effect diminishes at higher flight speeds.


24

Effect of Forward SpeedFrom thrust equation, if jet velocity is constant, then as flight speed increases, thrust would decrease in direct proportion. However, due to ram effect, airflow and the jet velocity will increase with aircraft speedThis will offset the extra ram drag


25

Effect of Forward SpeedTurbojet

As forward speed increases, the increased mass airflow must be matched by the fuel flow and the result is an increase in fuel consumption


26

Effect of Forward SpeedTurboprop


27

Effect of Altitude


28

Thrust-Drag BookkeepingThe interactions between thrust and drag are so complex.Only a bookkeeping-like approach can ensure that all forces have been counted once and only once. It is not at all uncommon to discover, during aircraft design project, that some minor drag item has been either included in both drag and thrust calculations or has been ignored by both departments under the assumption that it is being included by the other.Each aircraft company develops its own system for thrust-drag bookkeeping.


29

Thrust-Drag BookkeepingFor example, when the nozzles are open wide when the throttle is advanced to the afterburning position, the aerodynamic drag on the outside of the nozzles will change, so the nozzle aerodynamic drag is counted as a reduction in the engine thrust in many thrust-drag bookkeeping systems. In other systems, the drag of a certain nozzle position is considered in aerodynamic drag calculations, then the variation of drag as the nozzle opening is changed is included in the propulsion-installation calculations.Lack of a mutually understood bookkeeping system by both the Aerodynamic and Propulsion departments will cause chaos.


Propulsion System Integration


31

OutlineJet Engine Integration

Engine Sizing (Dimensions)Inlet GeometryInlet LocationCapture Area CalculationBoundary-Layer DiverterNozzle IntegrationEngine Cooling

Propeller Engine IntegrationPropeller SizingPropeller LocationEngine SizingPiston-Engine Installation

Fuel System


32

Jet-Engine IntegrationIntegration of jet engine into aircraft conceptual design is very complicated. Many calculations must be made prior to design layout, especially to determine required thrust (to pick or scale the engine) and inlet duct size. Design layout must depict the engine properly with reasonable allowances for clearance for engine cooling, accessibility and removal. Engine controls, fuel lines and engine-driven accessories must be considered.


33

Jet-Engine IntegrationStrong aircraft structure must be at the locations of the engine mounts to support engine and accessories weight, and transmit thrust.

Commercial: two top mounts; forward and backward. Military: one top forward mount and two side middle mounts.


34

Jet-Engine IntegrationEngine Sizing (Dimensions)

Dimensions of existing off-the-shelf engines are obtained from manufacturers.Estimated data for hypothetical rubber engines are provided by engine manufacturer as nominal engine size and precise scaling laws. Better yet, engine companies sometimes provide a parametric deck (a computer program that provides performance and dimensional data for an arbitrary advanced-technology engine based upon inputs such as BPR, OPR, and TIT).Another method is to assume that a new engine is scaled from existing one, with some improvements due to advancing technology (−20% SFC, −30% weight, −30% length).


35

Engine Scale Factor (SF)Ratio between required thrust and actual thrust of the nominal engine.

The location and size of engine-accessories package beneath the engine (including fuel pumps, oil pumps, power-takeoff gearboxes, and engine control boxes) vary widely for different types of engines. In the absence of a drawing, the accessory package can be assumed to extend below the engine to a radius of about 20—40% greater than the engine radius. On some engines these accessories are located in the compressor spinner or other places.

11

50

40

.actual

.actual

.actual

)(

)(

)(

SFWWSFDD

SFLL

=

=

=


36

Engine Parametric Statistical ApproachIf a parametric deck is unavailable, and no existing engines come close enough to the desired characteristics to be rubberized and updated as described above, then a parametric statistical approach can be used to define the nominal engine.Here, we present two first-order statistical jet-engine models: one for commercial transports with BPR from 1 to 6 (left model), and the other supersonic fighters and bombers (M < 2.5) with BPR from 0 - 1 (right model).


37

Jet-Engine IntegrationInlet Geometry

Slowing down the incoming air to about Mach 0.4—0.5 is the primary purpose of an inlet system to keep the tip speed of compressor blades below sonic.The installed performance of a jet engine greatly depends upon the air-inlet system. The type and geometry of the inlet and inlet duct determine the pressure loss and distortion of the air supplied to the engine, which will affect installed thrust and fuel consumption. Roughly, 1% reduction in inlet pressure recovery πinlet will reduce thrust by ~ 1.3%.

Also, the inlet’s external geometry including the cowl and boundary- layer diverter will greatly influence the aircraft drag. There are basically four types of inlets.

0

1inlet

t

t

p

p=π


38

Jet-Engine IntegrationBasic Types of Inlets

NACA flush inletPitot (or normal shock) inletConical (or spike or round) inlet2-D ramp inlet


39

Jet-Engine IntegrationNACA Flush Inlet

Used by several early jet aircraft but is rarely seen today because of its poor pressure recovery (large losses). However, the NACA inlet tends to reduce aircraft wetted area and weight if the engine is in the fuselage.The NACA inlet is regularly used for applications in which pressure recovery is less important, such as the intakes for cooling air or for auxiliary power units.


40

Jet-Engine IntegrationPitot inlet

It is simply a forward-facing hole (also called normal shock inlet in supersonic flight).It works very well subsonically and fairly well at low supersonic speeds. The cowl lip radius has a major influence upon engine performance and aircraft drag. A large lip radius tends to minimize distortion and accommodate additional air required for takeoff , especially at high angles of attack and sideslip. However, it will produce shock-separated flow outside the inlet as speed of sound is approached which greatly increases drag. Hence, supersonic jet cowl lip are nearly sharp.


41

Jet-Engine IntegrationFor subsonic jets, the lip radius ranges from 6—10% of the inlet radius.To minimize distortion the inner lip radius on a subsonic inlet is frequently greater than the outer. Some aircraft have a lower lip radius 50% greater than that the upper lip to reduce the effects of angle of attack during takeoff and landing.Normally the inlet is about perpendicular to local flow direction during cruise.


42

Supersonic IntakesSpike and 2-D Ramp Inlets

Better performance than the normal shock inlet at higher supersonic speeds .Supersonic flow over cone (spike) or wedge (2D-ramp) .Spike inlets are typically lighter and have slightly better pressure recovery but with higher cowl drag and more complicated variable geometry mechanisms. Ramp inlets are used up to Mach 2, while spike inlets are used beyond that.Pressure recovery through a shock depends on the strength of the shock.N-S: (M0 = 2 → M1 = 0.57, p1/p0 = 72%) – (M0 = 1.1 → M1 = 0.91, p1/p0 = 99.9%)An oblique shock does not reduce the air speed all the way to subsonic. Final transition from super to subsonic speed occurs through a normal shock.Speed reduction and pressure recovery depends on the wedge or cone angle.


43

Spike intake


44

2D-ramp intake


45

M0 M1

δ σ

M0M1

M2

Supersonic IntakesExample:1. O-S: (δ = 10º, M0 = 2 → σ = 39º, M1 = 1.66, p1/p0 = 98.6%).2. N-S: (M1 = 1.66 → M2 = 0.65, p2/p1 = 87.2%). Then, p2/p0 = 87.2 × 98.6% = 86%The greater the number of oblique shocks, the better the pressure recovery.

Theoretical optimal is the isentropic ramp inlet (infinite O-S) with 100% pressure recovery, which works properly only at its design Mach number, so rarely used


46

Supersonic IntakesOblique Shock

σδ

M1

M2


47

Supersonic Intakes


48

Supersonic Intakes


49

Supersonic IntakesAs speed approaches Mach 2, the total flow turning is about 40 deg.Turning flow back to freestream direction by outside cowl lip may not be possible.


50

Supersonic IntakesInternal compression inlet: needs starting and unstable.Mixed compression inlet: high efficiency over large Mach number range.


51

Selection of Inlet System


52

Jet-Engine IntegrationInlet Diffuser

the interior portion of an inlet where subsonic flow is further slowed down to engine required speed.The required length of a diffuser depends upon the application. Subsonic aircraft has as short diffuser as possible with an internal angle < 10 deg.Typically, this produces a pitot inlet with a length about equal to its front face diameter.Supersonic aircraft has a diffuser length for max. efficiency about eight times diameter.


53

Jet-Engine IntegrationLonger diffuser will have more frictional losses.Shorter diffusers may produce some internal flow separation, but the weight savings can exceed the engine performance penalty. Diffusers as short as two times the diameter are used with axisymmetric spike inlets.Long diffuser it is important to verify that the cross-sectional area of the flow path is smoothly increasing from the inlet front face back to the engine using volume-distribution plot.


54

Inlet IntegrationInlet Location

Embedded engines


55

Inlet Integration

F-111

F-16 Su-27

F-107

MiG-21

Comet 4C

Hawker-Siddeley TridentL-1011

http://www.suchoj.com/andere/MiG-21/images/MiG-21_03.jpg


56

Inlet Integration

Podded engines


57

Inlet Integration

L-1011

Illyushin II-76

Tu 22

B-727

Hawker 125-400A

Myasishochev M-52


58

Inlet Integration

B-777 A-380


59

Inlet Integration


60

Inlet Integration


61

Inlet IntegrationCapture Area

In a jet propulsion system, the engine is the boss.Excess air provided by the inlet must be spilled out the front.If less air is provided, the engine will try to suck the extra required air.Inlet capture area must be sized to provide sufficient air at all aircraft speeds. A typical subsonic jet inlet is sized for cruise at about Mach 0.8—0.9Diffusion takes place about half within and half outside the inlet duct.The area at the inlet front face is both the capture area and the throat area.


62

Inlet Integration


63

Nozzle IntegrationNozzle Integration Problem

Mismatch in desired exit areas at different speeds, altitudes, and thrust settings is the fundamental problem in jet engine nozzle designThe engine can be viewed as a producer of high-pressure subsonic gasesThe nozzle accelerates the gases to the desired exit speed controlled by exit area.The nozzle must converge to accelerate the exhaust gases to a high subsonic exit speedIf the desired exit speed is supersonic, a converging- diverging nozzle is requiredExit area depends upon the engine mass flow. This is a problem with afterburning where desired exit area for supersonic afterburning condition can be three times desired area for subsonic, part-thrust condition.


64

Nozzle IntegrationNozzle Performance

Variable-area nozzle


65

Nozzle IntegrationNozzle Configuration


66

Propeller-Engine IntegrationPropeller Engine Sizing Model

The following tables provide statistical models for sizing of four types of propeller-driving engines


67

Propeller-Engine Integration


68

Propeller-Engine IntegrationPropeller Sizing

The actual details of the propeller design, such as the blade shape and twist, are not required to lay out a propeller-engine aircraft.But the diameter of the propeller, the dimensions of the engine, and the required inlets and exhausts must be determined.Generally speaking, the larger the propeller diameter, the more efficient the propeller will be (Keep it as long as possible, as long as possible) The limitation is the propeller tip speed, which should be kept below sonic speed.The tip of a propeller follows a helical path through the air.


69


At sea level the helical tip speed of a metal propeller should not exceed 290 m/s.A wooden propeller, which must be thicker, should be kept below 260 m/s.If noise is of concern, the upper limit should be about 213 m/s during takeoff.Compressibility associated with near-sonic tip velocity and severe strength requirements due to dominance of high centrifugal loading require thick sections near hub but allow thin sections toward the tip.Highly-swept multi-blade propellers are proposed for high subsonic cruise.Noise is a design restricting factor imposing tip Mach number limit as low as 0.6 and low blade loading. Both imply lower rotational speed and more rotor blades.


70

Propeller-Engine IntegrationPropeller Performance

Swept-backward bladeTypical radial distribution

of a propeller blade


71

Propeller-Engine IntegrationDimensionless Parameters

Thrust Coefficient:

Torque Coefficient:

Speed Coefficient (advance ratio)

Reynolds number (significant for very small propellers at low speed)

Mach number (significant for large propellers at high subsonic speed)

2242 VnDnC C ρ

=ρ

=TTT

T ,

3252 VnDnC C ρ

=ρ

=QQQ

Q ,

nDVJ =


72

Propeller-Engine IntegrationDimensionless Parameters

Power Coefficient:

Efficiency:

Speed-Power Coefficient

Other ParametersPower Loading

Activity Factor

QPQP C

DnDnC π=

ρΩ

=ρ

= 25353

JCCJ

CCVV

P

T

Q

T

QT

PT

=π

=Ω

==η21

51

51

2

5

/

/

)( PP CJ

nVCs =

ρ=

332

53

2 VJC

DDnC

DPL ρ=

ρ== PPP

= ∫ D

rdDr

DcAF

tip

root

3510


73


Propeller Design Using Propeller Charts

(c) η vs. J.

Clark-Y three-bladed NACA 5868-9 propeller test (β at 75% radius).

(a) CT vs. J.

(b) CP vs. J.


74


Propeller Design Using Propeller Charts

η and Cs vs. J and β (at 75% radius).Clark-Y three-bladed NACA 5868-9 propeller test.

2

2

)(

)(

)/1(1

ppp

pp

psp

pp

JCJ

JJ

β−β+=η

+=β

−=

−−=ηη

gfe

db

ba


75

Propeller-Engine IntegrationPropeller Location


76


Home-built aircraft

Jetstream 31

B-36

Cessna Turbo StationAir T206H

Beach Starship

Documents

22343822 Aircraft Design