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Aviation History
1952
1952 - 1520.PDF
6t>6 FLIGHT, 30 May 1952 DOWTY DEEDS . . . Another innovation at this time was an ingenious little electro-mechanical indicator which told the pilot at a glance the position of the undercarriage and tailwheel, and the angle of the flaps. Coloured lights indicated when the components were fully locked home. While the well-established undercarriage systems and components gained ever widening acceptance in this and other countries (the sprung wheels of the Westland Lysander instantly come to mind), new ideas reached fruition in the period 1937-38, the most notable of them being the principle of levered suspension. In one arrangement an intermediate- pressure tyre and wheel unit was supplied complete with brakes and built-in oleo-pneumatic shock-absorber. Four designs were available, covering all sizes of aircraft weighing between 6,000 lb and 24,000 lb, and each size of hub could be provided with a standard brake and shock-absorber to accommodate different weights. Characteristics of the springing mechanism could readily be varied by adjusting the vertical wheel travel and changing the shock-absorber inflation pressure. The wheel was a direct development of the high-pressure internally sprung wheel already mentioned. Springing was oleo- pneumatic and impact loads were no longer dissipated by the conventional telescopic type of shock-absorber strut, but by a pin-jointed oleo-pneumatic shock-absorber free from bend ing and racking loads. Flight testing was carried out on a D.H.89 and the experimental installation is shown in an accompanying photograph. The now-famous levered-suspension principle arose from the company's analysis of the normal telescopic compression- leg, which had the drawback that it was impossible to position so that all combinations of vertical and side loads induced by drift-landings, and drag loads due to braking, gave a resultant load which was truly axial to the shock-absorber. This meant that the strut was subjected to bending, resulting in a deflec tion, from which arose a binding action. Essentially the arrangement consisted of pin-jointing a shock-absorber to a beam or lever which carried the wheel. The legs, as distinct from the telescopic compression-legs, were purely structural members to which were attached the swinging levers forming the mounting for the wheel. The shock-absorbers were attached to the levers, and to the legs, through pin-joints, and controlled the movement of the wheel. Under the action of vertical loads the wheel swung about the pivot bearing, thus compressing the shock-absorber, and drag loads (which also had a moment about the pivot bearing) resulted in the same movement. So the energy absorption characteristics of the undercarriage were utilized for both vertical and drag loads. Among the experimental installations of 1938 was a track- type undercarriage on a Lysander. With this in place the "Lizzie" was taxied at 70 m.p.h., but before take-off and landing tests could be made the work was stopped. The American authorities, however, have proceeded with develop ment work until the present day. The first American machine equipped with the new gear was a Fairchild monoplane; the second, a Douglas Boston. Some hundreds of landings were made with this Dowty gear, in addition to high-speed taxying and the negotiation of obstacles up to 9m square at high speed. Tests were also made on mud and sand, wherein the standard nosewheel buried itself completely, while the tracks remained on the surface. The track gear for the Boston was designed with a low ground-contact pressure but, in an article contributed to Flight in 1943, Mr. Dowty expressed the feeling that future develop ment would be for higher contact pressures, suitable for concrete runways. He believed that a track offered the greatest advantages on aircraft of 100,000 lb gross weight and over, and concluded, "The results of tests so far conducted indicate that improvements can be made on future designs. The successful result obtained with the Boston tracks is un doubtedly a pointer as to the form of the large undercarriage of the future." Here it may be remarked that, even before the establish ment of the Dowty Corporation of New York in 1940, Mr. Dowty's ideas were not unfamiliar to American aircraft constructors. One of our pictures shows an oleo-pneumatic undercarriage installation on the Ryan S.C. cabin monoplane of 1939, during which year Mr. Dowty called on several American manufacturers. Four qf these were later to secure Dowty licences. At home business was continuing to increase as the Royal Air Force expansion scheme gained impetus. Hurri canes, Lysanders, Ansons, Hampdens, Stirlings—all were Dowty-equipped, while on the civil side the Heston Phoenix and several de Havilland types, including the Albatross,Rapide and T.K.4 appeared on the ever-lengthening schedule of types. Much of the experimental work was naturally confidential, but at the 1939 Garden Party of the Royal Aeronautical Society the company was authorized to show, as a working exhibit, "a large retractable undercarriage intended for a medium bomber." At this time the limitations of simple gear-type hydraulic pumps had become obvious, and for high pressures it was necessary to turn from gear-wheels to the solid-displacement principle. The outcome was the "Live-Line" pump, of multi-piston type and made in a variety of patterns, giving various deliveries. By 1939 Dowtys were marketing, in addition to these pumps, the components necessary for all types of aircraft hydraulic systems, including selector and control valves, filters, automatic cut-outs, reservoirs, jacks and hand pumps. The outbreak of war necessitated further large increases in plant and facilities and in October of 1939, the Canadian branch of the company was formed. Production in England soared to yet dizzier heights. Salvage, repair and maintenance departments were set up, and a school was opened, under the aegis of R.A.F. Technical Training Command, for the instruction of R.A.F. personnel in maintaining and ser vicing hydraulic systems and undercarriages. Here the actual Dowty equipment as installed in the Hampden, Hurricane, Typhoon, Lancaster and other Service types was set out. Even the skylights of the classroom—originally opened laboriously by hand—were fitted up with Dowty hydraulic jacks, and proof of the trainees' appreciation of the facilities and comfort afforded was to be found in the visitor's book, wherein one entry read : "A really 'live-line' course with constant 'high delivery.' A pity to have to 'cut out' so soon." The levered-suspension principle, of which we have already taken note, was to find an historic application on the Experimental undercarriage on D.H.89. Experimental ski mounting for Gladiator. Experimental track-type undercarriage.
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