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Aviation History
1945
1945 - 0128.PDF
FLIGHT JANUARY I8TH, 1945 Undercarriages Some Items of Interest in Landing Qear Operation and Their Effect on Design ALTHOUGH an integral part of an aircraft, the under-carriage more often than not is a subject for col-L laboration by two or more concerns. In the Typhoon, for example, the retraction is by a Dowtyhydraulic jack actuating a Hawker linkage arrangement fitted to a Vickers strut carrying the Dunlop wheel. In the design of the landing gear many factors have tobe considered ; for instance, the disposition of the legs with regard to their fully compressed and fully extended posi-tions, these extremes affecting the airscrew ground clear- ance and the wheel-well centre; then there is the retractiongeometry, which is governed by the wing spar position, wing thickness, wing gun location, the aircraft's centre ofgravity, etc. In addition, there is wheel size, or, rather, tyre size to be considered, and also the degree of springingmovement and shock absorption. When an aircraft touches down in landing it inevitablyhas a certain degree of vertical velocity. Theoretically it is possible for a machine to land with no vertical component,other than that due to the weight gradually imposed on the undercarriage after landing, i.e., when the wings are nolonger supporting the machine; but in practice this is vir- tually beyond the bounds of practicable realisation, andtherefore the vertical velocity of the machine at touch- down must be dissipated in the form of energy. The air- craft designer will know fairly accurately the normal sink-ing speed, and, of course, he will know the weight of the aircraft. With this knowledge, plus the recommendationthat the shock absorber movement should be not more than about four inches, he can calculate the resistance theshock-absorption units must have in order to dissipate the energy of the aircraft's vertical velocity. This resistanceis usually kept within a value equivalent to three times the weight of the machine. Why Undercarriages Break In most landings there is a certain amount of excessenergy which must be cancelled out, and it is for this reason that the shock absorption resistance is arranged tobe within three times the aircraft weight, this value giving a reasonable safety factor without making the undercarriagestructure unduly heavy. But if a pilot drops his machine badly, then the kinetic energy to be dissipated is greaterthan that with which the shock absorbers can deal, witi the consequence that the remaining unexpended energygoes to work on the undercarriage structure and probably breaks it. Retractive geometry is mainly a matter of conveniencewithin the limiting factors of the requirements of the air- craft structure, and sometimes calls for considerable Spitfire : Narrow trackwith splined oleo-legs. Typhoon : Wide track andfully faired when retracted. Kittyhawk: Wheels retract backwards and turn 90 deg.
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