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Aviation History
1953
1953 - 0201.PDF
FLIGHT, 13 February 1953 MODERN BOGIES a manner suitable to the airframe designer, who must be allowed to lay down the best structure without ref erence to the undercarriage designer. In the Vulcan, the undercarriage lies alongside the outer engine ribs; nothing is yet permitted to be said of the undercarriage stowage of the Handley Page Victor. The landing gear of the Vulcan is unusually long. This permits advantage to be taken of the high angle of attack possible with a napless delta both at take-off and landing. This nose-up condi tion can, to some extent, be offset by streaming the tail parachute, but one cannot rely upon this always being the case with a bomber in squadron service. In any case, the tail parachute, while raising the tail considerably before touch-down, also increases the angle of glide, and therefore the rate of descent. Another peculiarity of tricycle undercarriages occurs in the take-off condition. Back ward movement of the control column raises the nose and also causes the centre of gravity of the aircraft to move rearwards; this, in turn, results in an increase in the nose-up moment, which must be counteracted by returning the control column to neutral or even by moving it farther forward. This is more marked on a delta because of the short elevation moment-arm when in the static position. The Vulcan also has a lengthy nose leg; this unit experiences somewhat severe loading during braking, but the design is carried out to cater for the even more severe three-point landing case. One of the most difficult problems in the design of a tandem- wheel bogie unit is the prevention of excessive loading on the front wheels. Two factors can cause such overloading : "slam ming" of the front wheels on to the runway at touch-down, due to the bogie-beam geometry, and overloading caused by brake torque reaction. The former can cause immediate bursting of tyres as a result of excessive dynamic overloading, and this trouble was first manifest with the Convair B-36 which, in its early life, suffered from front-tyre bursts to a serious degree. The anti-pitch dampers on the B-36 bogie have since been greatly enlarged with beneficial results; in this country thousands of Comet hours and experience witfi several other types of air craft have been amassed without this trouble occurring. The Electro-Hydraulics undercarriage of the Handley Page Victor has a marked trail angle, with the rear axles hanging con siderably lower than the front; a high-capacity damper is employed to reduce the consequent rotational speed of the bogie beam, and so relieve the overloading of the front wheels. This damper also absorbs some of the energy of landing which would otherwise go to the main leg, and helps to ensure a gentle touch-down. The Dowty undercarriage of the Vulcan completely removes this slamming characteristic by its basic configuration. The bogie beam is hinged at the front axle to the main leg. This leg con tains no shock-absorber and is free to move inwards, but cannot be pulled out beyond a fixed stop. The main shock-absorber is attached between the rear of the main leg casting and the mid- 199 The main bogie of the Avro Vulcan is a Dowty unit. It employs but one shock-absorber and conceals the brake piping within the massive, hollow main leg. Both drawings on this page show the free-trail wheel position as a broken line. Three per cent of the all-up-weight of the H.P.Victor is the claim made by Electro-Hydraulics, Ltd., for two of these eight-tyre bogies together with the double-wheel nose unit. point of the bogie beam. The rear wheels touch the runway first and rotate the bogie beam about the front axle, which is prevented from moving downwards by the aforementioned stop. This rotation of the bogie beam compresses the shock-absorber, and the front wheels strike the runway with no greater vertical velocity than does the whole aircraft. By the use of this system, only one shock-absorber is needed; this reduces the number of working parts to an absolute minimum. Overloading is injurious to the tyres, particularly from the point of view of damage to the walls, although the actual wear on the tread is small. Torque reaction caused by braking is of a quite different nature, being applied for considerably longer periods, and also introducing the risk of severe tyre wear, particularly if the wheels are allowed to lock. Brake torque in the first B-36 bogies was transmitted to the bogie beam, a fundamental error that soon became apparent, as this torque can then be counteracted only by overloading the front tyres. In the Vulcan the torque from the front brakes is carried direct to the sliding member of the leg structure, whilst that from the rear brakes also reacts, through parallel linkage, to the main-leg, so that there is no resultant torque applied to the bogie beam. On the Victor, torque of both front and rear brakes is transmitted by links to the main-leg structure with the same result. Many tyre manufacturers will not permit any brake-appli cation overloading at all; they specify tyres designed for the normal maximum loading. It is an advantage (not, perhaps, readily apparent) to trail the bogie beam at an angle such that one group of wheels touches down in advance of the other. By this means, spin-up drag can be halved, provided that the bogie beam is sufficiently resistant to motion to cause complete spin-up of the rear wheels before the front set touch the runway. Usually it is the rear group of wheels which touch first, but the Short S.A./4, with Messier bogies, shows the opposite pro cedure. In a normal landing, the front bogie wheels of this aircraft touch down first; in this way the rotational moment applied to the bogie beam is reduced because the resultant load line has a shorter moment-arm about the bogie hinge than would be the case if the rear wheels touched first. In this con nection, it may be recalled that there have been many attempts at introducing pre-rotation, both by aero dynamic and mechanical means, but that none of these experiments has shown the procedure to be worthy \^\=/
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