airframes chapter 7 undercarriage

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    ACP 116 UNIT 13Airframes

    Chapter 7: Undercarriage

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    Learning Objectives

    The purpose of this chapter is to discuss in more detail the

    last of the FourMajorComponents the Undercarriage

    (or Landing Gear).

    So, by the end of the lesson you should have anunderstanding of the types of undercarriage, its function

    and 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 aHydraulic System?

    2. How does a Gear Pumpwork?

    3. What are the main disadvantages of the Hydraulic

    System?

    4. What is the main disadvantage of the PneumaticSystem?

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    The Undercarriage

    An undercarriage (or landing gear) of some sort is needed

    by 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 SitterUndercarriage

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

    These aircraft are referred to as TailSittersdue to the

    attitude they took when on the ground.

    Although, some light aircraft have a tail wheel instead of a

    nose 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-cycleundercarriage

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    Advantages of Tri-cycle Layout

    The main advantages of employing a tri-cycle

    undercarriage layout are;

    Ground manoeuvring is easier with a steerable nose wheel.

    The pilots view is improved during taxying.

    The aircraft floor is horizontal when its on the ground.

    Aerodynamic drag on take-off is reduced, giving much better

    take-off performance.

    Directional stability on the ground is improved, because the C

    of G is forward of the main wheels.

    Braking is more straightforward, and brake parachutes can be

    used.

    There is less tendency to float and bounce on landing, making

    landing 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 are

    some disadvantages;

    Nose wheels need to be stronger and therefore heavier than

    tail wheels.

    More damage is done to the aircraft if the nose wheel

    collapses

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    Large Aircraft Undercarriage

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

    The picture shows an Airbus A340 undercarriage just at the

    point of touchdown.

    Main wing

    undercarriageMain 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 the

    undercarriage.

    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 aircraft

    fuselage.

    On heavier aircraft a

    telescopic shock absorber

    known as an oleo leg is

    almost 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 type

    undercarriages.

    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) works

    by 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 the

    damping oil through small holes, so that the strut can only

    extend quite slowly.

    The gas or oil will stay slightly compressed when it has

    the weight of the aircraft on it, so it is cushioned whilst

    taxying.

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    Nose Wheel & Steering

    The operation of Nose Wheel shock absorber units issimilar, but their construction differs slightly in that they

    are usually designed to allow the Nose Wheel to be

    steered, by rotating the entire unit, or by steering motors

    on larger aircraft.

    On large aircraft, some of the

    main body wheels will pivot to

    help prevent the tyres from

    scrubbing in tight turns.

    Often the Nose Wheel

    steering must be capable of

    being 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 the

    Centre o f Gravity.

    If they are too far back, very high loads will be taken on

    the nose wheel during landings, which may cause it to

    collapse.

    Main units are often

    retracted into the wings (orthe body for larger

    aircraft).

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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, tandem

    or bogie unit, of four or more wheels.

    There are even more

    variations than this, but they

    are not common.

    As aircraft become heavier,

    the loading on a single wheel

    increases, leading to a great

    increase in the damage done

    to runways.

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    Load Distribution

    By having the weight spread over a number of wheels, the

    contact pressure of the undercarriage is reduced.

    This leads to reduced undercarriage weight and increased

    safety if a tyre bursts on landing.

    The new Airbus A380 has22 wheels

    Four main units, 2 with

    four-wheel bogies and 2

    with 6 wheel-bogies, and adouble nose-wheel unit.

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    Military Aircraft Examples

    This is nose landing gear of theBoeing Goshawk aircraft

    The double wheel layout is

    necessary to cope with the landing

    loads, as this aircraft lands on a

    Carrier.

    This is one of the main landing

    gear from the EurofighterTyphoon it just has a single

    wheel.

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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 fitted

    to the Airbus A380.

    Note the number of wheels required to distribute the heavy

    load 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 tandem

    on each side of the fuselage, sharing a common

    horizontal shock absorber.

    Amongst the advantages of this design are excellent

    rough-field performance and the ability to lower the aircraft

    down (kneeling) for easier loading.

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

    This makes this arrangement ideal for transport aircraft

    like the Hercules.

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    Jockey Wheel Unit Example

    A Jockey Uni ton the Antonov AN-225 Mriya transportaircraft.

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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 the

    undercarriage legs, about a pivot at the top.

    The doors to the undercarriage well may be attached to

    the legs, or may use separate jacks to open and close

    them.

    In many cases the undercarriage needs to fit into a very

    small space, and the units may be turned, twisted or

    folded to enable this to be done.

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