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Aviation History
1948
1948 - 1325.PDF
AUGUST IQ.TH, 1948 FLIGHT 219 LIQUID-SPRING PROGRESS Pioneering Work Now Bearing Fruit DEVELOPED in close secrecy during the war, theprinciple of employing oil as the sole resilientelement in undercarriage shock absorbers was con- ceived in 1938 by Mr. George Dowty, chairman and manag- ing director of Dowty Equipment, Ltd. Hitherto it had been regarded as impossible to make practical use of the compressibility of oil owing to the extremely high pressures required to produce any useful degree of resilience, although the late Dr. F. W. Lanchester had foreseen the theoretical possibilities. Visualizing the many advantages which could be gained if the formidable problems entailed could be solved, Dowty Equipment, Ltd., undertook experiments which culminated in the production of a revolutionary type of shock absorber unit, needing no air inflation but using oil both for cushion- ing and also recoil-damping purposes. The first announcement of the Dowty liquid spring appeared shortly after the war (Flight, June 24th, 1946) and since that date substantial progress has been made by the Dowty company in this field of hydraulic engineering. The progress of the development period has fully satisfied, if not exceeded, the expectations of the designers, and that the majority of new aircraft are equipped with Dowty liquid-spring undercarriages is testimony to the popularity of the product with aircraft designers here and abroad. Mr. Dowty has recently returned from America, where he found lively interest in the liquid spring for appli- cation in both the aircraft and automobile suspension fields. Because of this in- terest, and the dollar-earn- ing possibilities of the oil spring, technical experts are to be sent from the factory in Cheltenham to America in the near future Illustrations of three sizes (not to scale) of Dowty oil spring alighting gear: Below: The levered - suspension steerable nose- wheel of the Fokker Promotor. Centre: The main undercarriage leg of the Brabazon, with its retracting jack and safety locking strut, and (right) a tailwheel as fitted to a modern fighter aircraft. to follow up the many inquiries Mr. Dowty received during his visit there. On the topic of American undercarriage design, Mr. Dowty expressed the view that the '' drag strut'' employed in certain American aircraft to absorb drag reactions when landing is a complicated and cumbersome method of"tatk- ling the problem. Moreover, even with the use of two shock absorbers, the drag-strut is only an attempt to cure the effects of bending; the pivoted trailing link, such as used on British .'' levered suspension '' undercarriages, however, entirely prevents bending loads, with their atten- dant friction, reaching the shock absorber. A number of important factors have contributed to the widespread interest in the liquid spring and to the progress • made during the last two years. The modern trend of air- craft design being towards smaller and faster jet-propelled aircraft, space limitations demand the most compact form of alighting gear. Moreover, the .high landing speed of modern aircraft requires that this compactness should not entail a sacrifice of efficiency. In the field of heavy trans- port aircraft, increases in all-up weight have necessitated a corresponding increase in the undercarriage inflation pressure when conventional oleo-pneumatic struts of the same size are used. Maintenance Advantages The Brabazon undercarriage illustrates this point. The liquid-spring legs designed for this aircraft are iorn in diameter. To have retained this dimension using an air- oil shock absorber would have entailed an initial inflation pressure of 1,300 p.s.i. with the aircraft jacked up (no weight on legs); this would have given rise to formidable service and maintenance problems. However, the use of the liquid spring has enabled the diameter of the under- carriage to be kept comparatively small, and has entirely eliminated the necessity of inflation to very high air pres- sures. The smaller diameter of the legs permits closer spacing of the twin wheels, and therefore smaller openings in the wings to receive the landing gear when retracted. The simplification of ground maintenance has contri- B 27
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