ACP 116 UNIT 13Airframes

Chapter 7: Undercarriage

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Learning ObjectivesThe purpose of this chapter is to discuss in more detail thelast of the ‘Four Major Components’ – the Undercarriage(or Landing Gear).

So, by the end of the lesson you should have anunderstanding of the types of undercarriage, it’s functionand the positioning of them.

But first a recap of Chapter 6 with some questions.

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Chapter 6 Revision

A few questions about the previous chapter.

1. What are the two functions of the Reservoir in a ‘Hydraulic System’?

2. How does a ‘Gear Pump’ work?

3. What are the main disadvantages of the ‘Hydraulic System’?

4. What is the main disadvantage of the ‘Pneumatic System’?

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The UndercarriageAn undercarriage (or landing gear) of some sort is neededby all aircraft which operate from land.

Can you think of some of the jobs it is required to do?

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The ‘Tail Sitter’ Undercarriage

Historically, early aircraft had a tail wheel arrangementinstead of the nosewheel.

These aircraft are referred to as ‘Tail Sitters’ due to theattitude they took when on the ground.

Although, some light aircraft have a tail wheel instead of anose wheel, this arrangement is no longer common.

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Tri-cycle Undercarriage

Most modern aircraft are usually supported on the groundby three units - two main wheels and a nose wheel.

This is what is referred to as a ‘Tri-cycle’ undercarriage

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Advantages of Tri-cycle LayoutThe main advantages of employing a tri-cycleundercarriage layout are;

– Ground manoeuvring is easier with a steerable nose wheel.

– The pilot’s view is improved during taxying.

– The aircraft floor is horizontal when it’s on the ground.

– Aerodynamic drag on take-off is reduced, giving much bettertake-off performance.

– Directional stability on the ground is improved, because the Cof G is forward of the main wheels.

– Braking is more straightforward, and brake parachutes can beused.

– There is less tendency to float and bounce on landing, makinglanding easier.

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Disadvantages of Tri-cycle Layout

Despite all the advantages of utilising the tri-cycleundercarriage layout within the airframe design, there aresome disadvantages;

– Nose wheels need to be stronger and therefore heavier thantail wheels.

– More damage is done to the aircraft if the nose wheelcollapses

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Large Aircraft Undercarriage

Large aircraft, such as modern passenger aircraft can have complicated landing gear arrangements.

– The picture shows an Airbus A340 undercarriage just at the point of touchdown.

Main ‘wing’ undercarriage

Main ‘body’ undercarriage

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Landing Forces

– For Transport aircraft, this maybe up to 3 times the weight ofthe aircraft.

– For aircraft landing on the deckof a ship, this can be up to 8times.

To prevent damage to thestructure, and to stop theaircraft bouncing, this shockmust be absorbed anddissipated by theundercarriage.

When an aircraft lands, a large force is applied throughthe undercarriage as it touches the ground.

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Coping with these Forces

On light aircraft, the undercarriage may be just a piece ofspring steel, with perhaps a rubber mounting in the aircraftfuselage.

On heavier aircraft atelescopic shock absorberknown as an oleo leg isalmost always used.

This allows for the force oflanding to be absorbed.

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Types of Oleo Leg

Most service aircraft, as well as most civil transports, arefitted with oleo-pneumatic or oil-compression typeundercarriages.

The operation of both units is very similar.

– An oleo-pneumatic unit compresses air or nitrogen gas.

– An oil-compression unit (often known as liquid spring) worksby compressing oil.

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How an Oleo Works

Compressing the strut reduces the volume inside andcompresses the gas or oil, like operating a bicycle pump.

Any tendency to bounce is prevented by forcing thedamping oil through small holes, so that the strut can onlyextend quite slowly.

The gas or oil will stay slightly compressed when it hasthe weight of the aircraft on it, so it is cushioned whilsttaxying.

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Nose Wheel & Steering

The operation of Nose Wheel shock absorber units issimilar, but their construction differs slightly in that theyare usually designed to allow the Nose Wheel to besteered, by rotating the entire unit, or by steering motorson larger aircraft.

On large aircraft, some of themain body wheels will pivot tohelp prevent the tyres from‘scrubbing’ in tight turns.

Often the Nose Wheelsteering must be capable ofbeing disconnected for towing.

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Undercarriage Considerations

To make sure the aircraft tail does not hit the ground ontake-off or landing, the main wheels must be behind theCentre of Gravity.

If they are too far back, very high loads will be taken onthe nose wheel during landings, which may cause it tocollapse.

Main units are oftenretracted into the wings (orthe body for largeraircraft).

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Wheel Units

All of these factors mean that the undercarriage positionsmust be very carefully designed.

Each main-wheel unit consists of a single, double, tandemor bogie unit, of four or more wheels.

There are even morevariations than this, but theyare not common.

– As aircraft become heavier,the loading on a single wheelincreases, leading to a greatincrease in the damage doneto runways.

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Load DistributionBy having the weight spread over a number of wheels, thecontact pressure of the undercarriage is reduced.

– This leads to reduced undercarriage weight and increasedsafety if a tyre bursts on landing.

The new Airbus A380 has22 wheels

– Four main units, 2 withfour-wheel bogies and 2with 6 wheel-bogies, and adouble nose-wheel unit.

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Military Aircraft ExamplesThis is nose landing gear of theBoeing Goshawk aircraft

– The double wheel layout isnecessary to cope with the landingloads, as this aircraft lands on aCarrier.

This is one of the main landinggear from the EurofighterTyphoon – it just has a singlewheel.

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Civil Aircraft Examples

The images below show the more robust wheel units asutilised on civil aircraft designs.

In this case, both images are of main wheel units as fittedto the Airbus A380.

– Note the number of wheels required to distribute the heavyload of the aircraft!

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Jockey Wheel Units

A variation of the tandem arrangement is the JockeyUnit, which comprises two or three levered legs in tandemon each side of the fuselage, sharing a commonhorizontal shock absorber.

Amongst the advantages of this design are excellentrough-field performance and the ability to lower the aircraftdown (kneeling) for easier loading.

The units also retract into a small space, withoutpenetrating into the load space.

This makes this arrangement ideal for transport aircraftlike the Hercules.

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Jockey Wheel Unit Example

A Jockey Unit on the Antonov AN-225 Mriya transport aircraft.

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Undercarriage Retraction

An undercarriage causes a lot of drag in flight, so it isretracted into the wings or fuselage in most aircraft,except when needed.

In most cases, a hydraulic jack is used to pull theundercarriage legs, about a pivot at the top.

The doors to the undercarriage well may be attached tothe legs, or may use separate jacks to open and closethem.

In many cases the undercarriage needs to fit into a verysmall space, and the units may be turned, twisted orfolded to enable this to be done.

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Retraction System Components

The components of a simple landing gear and retractionsystem consist of;

1. Retraction Jack

2. Down-lock

3. Oleo Leg

4. Axle

5. Wheel

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Undercarriage Doors

It is important that the doors open before theundercarriage units extend or retract, and closeafterwards.

– This is accomplished by using a sequencer valve to control thesupply of hydraulic fluid.

– The sequencer valveensures that thecorrect hydraulicactuator and/or jack issupplied with hydraulicfluid in the correctorder.

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Undercarriage System Failure

So what happens if the hydraulic system fails – how doesthe undercarriage get lowered?

Airframe designers must consider the potential for failure,so that the aircraft can be landed safely.

– It is common for pressure bottles to be fitted, which storeenough pressure to allow the undercarriage to be extendedonce, if the system fails.

– The undercarriage must then lower to it’s full extension underit’s own weight.

– Nose Wheels are normally retracted forwards and in anemergency, the aerodynamic drag will assist them to reach fullextension.

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Undercarriage Locks

To prevent undercarriagecollapsing on the ground, andto hold it firmly in position inflight, uplocks and downlocksare fitted.

– These are unlocked as part ofthe extension and retractionsequence.

It would be catastrophic if theundercarriage were retractedaccidentally with the aircraft onthe ground, so additional locksare fitted, disabling theretraction mechanism.

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Brake Systems

Modern large aircraft often land at high weights andspeeds.

This means that the braking system must be capable ofabsorbing and dissipating very large amounts of heat, asthe energy of motion is converted into heat.

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Types of Brakes

There are two main types of brake:

– Drum Brakes

– Disc Brakes

The Drum Brake is rarely used, because it suffers frompoor heat dissipation, causing the brakes to overheat andfade.

– Fading is where the brakes lose their braking effectiveness astheir temperature increases.

The Disc Brake is much more effective at dispersing theheat produced, and maintain their effectiveness duringlong periods of heavy braking.

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Disc BrakesThese consist of a disc or series of discs of aluminiumalloy, steel, carbon or other material, gripped betweenpads of friction material.

– These pads are forced against the discs by pistons underhydraulic pressure.

– Control is usually achieved by placing a toe pedal for the brakeon each side on its respective rudder pedal.

– These can then be operated differentially by the pilot, giving theability to steer the aircraft by applying different amounts ofbraking on each main wheel.

– Applying the brakes equally on both main units allows theaircraft to be braked smoothly in a straight line.

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Disc Brakes

Large aircraft may have quite a number of discs in eachwheel, to get the required braking forces and heatdissipation.

Multi-disk brake unit – Airbus A380

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Anti-Skid

An anti-skid unit, called a Maxaret unit, prevents skiddingby detecting when the wheel or wheels on any unit stopturning, and momentarily releases brake pressure on thatunit only.

This gives the aircraft the ability to stop in the shortestpossible distance without loss of control.

Similar units, known as ABS, are fitted to many cars, andwork in the same way.

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Alternative Braking Methods

Another form of braking is air brakes, used in flight, whichconsist of large plates fitted to the fuselage (or wings –Viking and Vigilant) which can be lifted into the airflowwhen required.

They cause a large increase in drag to slow the aircraft.

After touch-down, reverse thrust can be deployed, bymoving doors into the jet exhaust to deflect the flowforwards.

Turbo-prop engines can achieve a similar effect bychanging the pitch of the propeller to reverse the airflow.

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Alternative Undercarriage Types

Over the years, many different designs have been tried.

– An experiment was tried in the 50’s, when an aircraft with noundercarriage was tested – the idea was quickly abandoned.

– Another experiment was with tracked undercarriages for softfield landing on the Convair B-36 – again this idea wasn'tpursued.

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Conclusions

You should now have a basic understanding of the typesof undercarriage, the forces that they must be able tocope with, how they are used to absorb the forcessustained during landing, as well as the types of brakingsystem employed.

Undercarriages are essential components of any airframe.They allow the aircraft to move about on the ground aswell as ensure that the high loads are carried effectively.

Any Questions?

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Questions

Here are some questions for you.

1. What is an oleo leg?

2. What controls the supply of hydraulic fluid in an undercarriage?

3. What are the 2 main type of brakes?