Helicopter Operation and Systems - Lec 8

7/29/2019 Helicopter Operation and Systems - Lec 8

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Resource personAli Khalid

Department of Aviation Management & Technology

Superior University, Lahore.


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By the end of this session , you will be able to:

Understand the basics of helicopters dynamics.

Understand how helicopters fly.

Know about different helicopters systems.


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Helicopters are aircraft whose lift is generated by one ormore engine-driven rotor systems.

Their main advantage over fixed-wing aircraft is theability to vertically lift off, land, to hover, and to flyforward, aft wards and sideways.

These characteristics allow helicopters to reach placesthat other vehicles cannot.


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The helicopter was a late arrival on the aviation scenecompared to more conventional fixed-wing aircraft.

A number of designers experimented with autogiros inthe late 1920s and 1930s but it was not until the 1940sthat serious helicopter developments began.

In general, the helicopter was regarded at the time assomething of an post dated thing and it was not untilthe post-war years that its serious development began,most of it being undertaken in the US.


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The helicopter came of age as a fighting vehicle in thelate 1960s and the US involvement in the Vietnam Warwas probably the first large-scale conflict in which it

played a major part in a variety of roles.

As their roles became more demanding so the

helicopters became more sophisticated and complex.


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Basic helicopters consist of a single main rotor and tailrotor, both powered by a power plant.

Suspended under the main rotor is a hull with a cockpitand cabin, which itself sits upon on a landing gearsystem.

The helicopter can use skids, wheels or floats for

landing, and these systems usually have dampers toabsorb shocks. Various vertical and horizontal stabilizersprovide directional stability during flight.


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HELICOPTERS USE ROTORS with variable pitch angleto provide agile motion, including stationary hovers.

Due to powerful gas-turbine engines which makes therotors to rotate so fast, it simply lifts it self in the air.

The helicopters have proven their usefulness in

many scenarios, either in civil aviation or in military.


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In military operations they are performingnumerous useful task such as:

Troop and material transportation.Reconnaissance. Medical and other evacuations. Airborne command posts. Naval anti-submarine missions. Minesweeping operations.


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They are equally useful in civil world like:

Aerial Crane

Helicopters can be used to carry external loads,connected by long cables, to and from areas that othermeans of transport have difficulty getting to.

Examples of such sling load operations are placingheavy transmission towers, carrying trees duringlogging and even transporting ground vehicles


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Air AmbulanceMEDEVAC helicopters are employed when traditional

ambulances cannot quickly get to the scene of trauma,

or cannot reach it at all. In other cases, aerial transport is simply the safest and

quickest way of transporting patients between medicalfacilities. These aircraft are equipped with medical

equipment to allow caring for patients mid-flight.


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Law Enforcement

Many police departments and other law enforcement

agencies use helicopters in a variety of roles. Theyprovide air support when tracking suspects, and areequipped with specialized cameras and search lights.


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Other ApplicationsOther uses of helicopters include:

Aerial photography Motion picture filming

News gathering and reporting (ENG - Electronic

News Gathering)

Search and Rescue Tourism and recreation

Transport


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Helicopters come in many sizes and shapes, but mostshare the same major components.

These components include a cabin where the payloadand crew are carried, an airframe, which houses thevarious components, or where components areattached; a power plant or engine; and a transmission,

which, among other things, takes the power from theengine and transmits it to the main rotor, whichprovides the aerodynamic forces that make thehelicopter fly.


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Then, to keep the helicopter from turning due totorque, there must be some type of ant torque system.Finally there is the landing gear, which could be skids,

wheels, skis, or floats. This chapter is an introduction tothese components.


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Whereas the lift force for a fixed-wing system isproduced by the passage of air over the wing aerofoil.

However the helicopter rotor blades are aerofoils whichgenerate the lift force to counteract the vehicle weight.


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The fact that the helicopter lift force is generated byrotation of the rotor causes additional complication forthe helicopter.

As the helicopter propulsion system drives the rotorhead in one direction, a Newtonian equal and oppositereaction tends to rotate the fuselage in the other

direction and clearly this would be unacceptable fornormal controlled flight.


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This problem is overcome by using a tail rotor whichapplies a counter-acting force which prevents thehelicopter fuselage from rotating.


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The rotor system found on helicopters can consist of asingle main rotor or dual rotors.

With most dual rotors, the rotors turn in oppositedirections so the torque from one rotor is opposed bythe torque of the other. This cancels the turningtendencies


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In general, a rotor system can be classified as eitherfully articulated, semi rigid, or rigid. There arevariations and combinations of these systems.

FULLY ARTICULATED ROTOR SYSTEM

A fully articulated rotor system usually consists of three

or more rotor blades. The blades are allowed to flap, feather, and lead or lag independently of each

other. feathering means the changing of the pitch angle of the

rotor blades.


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Each rotor blade is attached to the rotor hub by ahorizontal hinge, called the flapping hinge, whichpermits the blades to flap up and down.


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Each blade can move up and down independently of theothers.

Each rotor blade is also attached to the hub by a verticalhinge, called a drag or lag hinge, that permits eachblade, independently of the others, to move back andforth in the plane of the rotor disc


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A semi rigid rotor system allows for two differentmovements, flapping and feathering.

This system is normally comprised of two blades, whichare rigidly attached to the rotor hub.

As one blade flaps down, the other flaps up. Feathering

is accomplished by the feathering hinge, which changesthe pitch angle of the blade.


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The rigid rotor system is mechanically simple, butstructurally complex because operating loads must beabsorbed in bending rather than through hinges. In this

system, the blades cannot flap or lead and lag, but theycan be feathered.


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TAIL ROTOR Most helicopters with a single, main rotor system

require a separate rotor to overcome torque.

This is accomplished through a variable pitch,antitorque rotor or tail rotor.


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Another form of antitorque rotor is the fenestron orfan-in-tail design.

This system uses a series of rotating blades shroudedwithin a vertical tail.

Because the blades are located within a circular duct,

they are less likely to come into contact with people orobjects


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The NOTAR system is an alternative to the antitorquerotor. The system uses low-pressure air that is forcedinto the tailboom by a fan mounted within the

helicopter. The air is then fed through horizontal slots, located on

the right side of the tailboom, and to a controllablerotating nozzle to provide antitorque and directional

control.


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A typical small helicopter has a reciprocating engine,which is mounted on the airframe. The engine can bemounted horizontally or vertically with the transmission

supplying the power to the vertical main rotor shaft.

Another engine type is the gas turbine. This engine isused in most medium to heavy lift helicopters due to its

large horsepower output. The engine drives the maintransmission, which then transfers power directly to themain rotor system, as well as the tail rotor.


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When you begin flying a helicopter, you

will use four basic flight controls. They

are the cyclic pitch control; thecollective pitch control; the throttle

and the antitorque pedals.


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Tilting the rotor head provides the longitudinal (fore-and-aft) and lateral (side-to-side) forces necessary togive the helicopter horizontal movement.

This is achieved by varying the cyclic pitch of the rotorhead. Picture on the next slide will show you how itworks.

Moving the pilots stick forward alters the cyclic pitchsuch that the rotor tilts forward, thereby adding aforward component to the lift force and enabling thehelicopter to move forwards.


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In general the helicopter is more unstable than itsconventional fixed-wing counterpart.

It follows that flying a helicopter is generally much moredifficult than flying a fixed-wing aircraft.

For this reason, some sophisticated helicopters possess

auto-stabilization and multimode autopilot systems tominimize the workload of pilots.


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The basic principles of many helicopter systems areidentical to similar systems in fixed-wing aircraft.

Engine and transmission system. Hydraulic system.

Electrical system.

Health monitoring system.

Flight control system.


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Many helicopters today have a number of engines tosupply motive power to the rotor and transmissionsystem.

In fact, all but the smallest helicopters usually have twoengines, and some larger ones have three.

The need for multiple engines is obvious; helicopter liftis wholly dependent upon rotor speed, which in turndepends upon the power provided by the engines


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For helicopters, the hydraulic systems are a majorsource of power for the flying controls as for variousother auxiliary services.

A typical large helicopter, such as the EH 101 Merlin,has three hydraulic systems.


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AC generation is supplied by two main generators eachof 45 kVA capacity driven by the accessory gearbox.

A battery is provided, mainly to start the APU; howeverthis can provide short duration emergency power in the

event of a triple electrical systems failure.


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If the correct critical parameters in an engine andtransmission system are monitored then it is possible toidentify deterioration of components before a critical

failure occurs. Just as in the case of human beings weregularly have medical check ups.

This is done by establishing a time-history of the

parameter during normal operation of the aircraft, andby constantly measuring those parameters.


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The tendency for a parameter to exceed set thresholdson either an occasional or regular basis can be readilyidentified as may a steadily rising trend in a componentvibration measurement.

The expense of overhauling an engine after a majorfailure is considerable.

It therefore makes sound economic sense to monitorkey engine parameters and forestall the problem byremoving the engine for overhaul when certain criticalcondition have been attained.


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Most helicopter flight control systems use conventionalrod and lever mechanical control units.

In the cockpit, dual controls for cyclic (pitch and roll),collective and yaw are provided.

Handling a large helicopter such as the EH 101 Merlin

requires a great deal of effort and concentration bypilots who have other considerable demands placedupon them, for instance by Air Traffic Control or missionrequirements.


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The need for an advanced AFCS is therefore necessary.

The AFCS functions may be split into two main areas:

Auto stabilization functions

Pitch, roll and yaw auto stabilization

Pitch and roll attitude hold Heading hold

Turn co-ordination


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Autopilot functions Radar altitude hold Air speed hold

Hover hold Vertical speed acquire


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Helicopter Operation and Systems - Lec 8