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Aviation History
1952
1952 - 0927.PDF
FLIGHT, 4 April 1952 425 BOGIE UNDERCARRIAGES A Necessity for Heavy Aircraft: Current Practice Reviewed in R.Ae.S. Lecture IN a Section Lecture to the Royal Aeronautical Society on March 4th, R. C. Cussons, D.F.C., M.A., A.F.R.Ae.S., assistant chief designer of Dowty Equipment, Ltd., gave one of the first public assessments to be made of the bogie undercarriage. He confined his paper to a discussion of the use of bogies in general, together with an appraisal of a num ber of patterns currently in use. A digest of his remarks follows. One of the chief advantages cf the bogie undercarriage, said Mr. Cussons, was that it enabled a given aircraft to impose a much reduced maximum bending moment upon a concrete slab runway. With aircraft weights progressively increasing, the only alternative to the use of bogies appeared to be to increase the con crete thickness—a much more costly undertaking. The lecturer gave examples of aircraft which, originally de signed with single main wheels, had later appeared with bogies, adding that even the 85,000 lb Lincoln was on the verge of exceeding the permissible runway loading and, if designed from scratch today, would have at least two main wheels per under carriage, if not four. The runway loading index, in fact, forced the designer to adopt the bogie undercarriage for any aircraft of over about forty tons gross weight. With the adoption of such an undercarriage second ary advantages usually accrued; in particular, though the gear itself would weigh more, an overall weight-saving was effected by the lower aggregate weight of wheel, tyre and brake assemblies. The prototype Comet's single-wheel main undercarriages weighed together, 3,815 lb; those of the production aircraft, complete with the eight wheels and brakes, weighed only 3,598 lb. Further advantages were, in certain installations, the possibility of easier stowage of a fore aft retracting bogie (with small-diameter tyres) by rotating the bogie during retraction so as to keep the wheel axles in a near-horizontal plane, and the benefit of "safety in numbers" where burst tyres were concerned. Turning to the debit side of the picture, Mr. Cussons cited the difficulty of ground manoeuvre as the bogie's greatest drawback. If a bogie were turned about its vertical centre, the direction of movement of the tyres was almost parallel to the wheel axles, so that they scrubbed sideways with practically no rotation (Fig. 1). Handling notes for bogie-equipped aircraft generally recommended Fig. 2 (left). The first Comet bogie (port unit with the inner wheels removed) with fittings machined from solid. The production bogies, with parts made from drop- stampings, look even neater. Fig. 3 (right). Brabazon 2 bogie on the drop-test rig; again, the inner wheels (four of them) have been removed to reveal the bogie construction. Fig. 1. Careless taxying can cause damage to bogie tyres; this shows the effect of turning a Brabazon 2 unit at 12ft radius in a test rig. a minimum turning radius, together with the corresponding nose- wheel steering angle. Another disadvantage was the difficulty of performing drop- tests; with a single wheel, drag and side loads could be simulated by wedges, but space limitations often prevented this with bogies. The drag case could only be obtained by dropping the bogie with backward-spinning wheels, giving the same effect as occurred at touch-down In the bogie an increase in complication had to be accepted while there had been rumours—from America—of a tendency to pitch, or porpoise, while taxying. This trouble had not, however, been a bother in this country. Dealing first with the "beam-engine" type of bogie, Mr. Cussons said that a good example was the undercarriage fitted to the Convair B-36. This was really a double-levered-suspension unit, with the levers pointing both fore and aft. The main shock- absorber connected the middle of the front lever to the front end of an upper pivoted beam which then transmitted the deflection to the middle of the rear lever. This had the effect of translating the upward movement of the rear wheels (which struck the ground first) into a downward movement of the front wheels, without compressing the shock absorber. Thus the front wheels hit the ground at about twice the aircraft descent-rate. In the de Havilland Comet, a very similar principle was em ployed, although in this undercarriage (Fig. 2) brakeloads were transmitted by links to the main leg instead of to the bogie. These links were so arranged as to give no tipping moment to the bogie at the static position; at other positions of the wheel levers there was a slight out-of-balance, either backwards or forwards, depend ing upon whether the wheel axles were above or below the normal static position. The next unit dealt with by the lecturer [that for the Short
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