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Aviation History
1939
1939 - 0675.PDF
MARCH 9, 1939 FLIGHT. 239 The last word in small-diameter two- row radials : A sleeve-valve Taurus of over 1,000 h.p. plying their high-compression Jupi ter, the Bristol company fully realised that this was but a temporary pallia tive, and that ultimately the super charged engine was inevitable. There Was at that time no way of telling which of the two types of super charger would be most likely to suc ceed, the exhaust-driven or the gear- driven. With that thoroughness which has always been typical of everything Mr. Fedden does, work was put in hand simultaneously on super chargers of both types. In 1923 a Jupiter engine had been fitted with an exhaust-driven super charger and a special engine, resem bling the Jupiter, but having a smaller diameter and incorporating a super charger, was produced for the Air Ministry. Known as the Orion, it was flight-tested at Farnborough in 1928 in a Gloster Gamecock and main tained a maximum power of 495 h.p. to a height of 20,000ft., which was reached.in less than 13 minutes. Development of a high-speed centri fugal gear-driven type of supercharger was also in hand, and a highly suc cessful version was fitted to a new type of nine-cylinder radial in 1926. This engine was known as the Mer- «, cury, and the stroke being reduced from 7|in. to 6Jin., gave a capacity of 24.9 litres instead of the 28.7 litres of the Jupiter, while overall engine diameter was reduced from 53m. to 47.6m. For special high-performance aircraft the Mercury was supercharged at ground level, and was fitted in the Short Crusader low- wing seaplane for the 1927 Schneider Trophy contest. This specially boosted engine had an output of 808 h.p., though it weighed only 684 lb., giving a specific weight of 0.85 lb./h.p. Unhappily the Crusader was completely wrecked in an accident during practice for the race. The gear-driven supercharger was also fitted to the Jupiter VII engine, which passed its type test in 1928. This engine gave a maximum power of 460 h.p. at 12,000ft., and was fitted as standard equipment in the first batches of Bristol Bulldog fighters to be de- ' livered to the R.A.F. The main reason why the gear- driven type of supercharger was developed and adopted in preference to the exhaust-driven type was that the materials in those days were not sufficiently advanced to with stand the extremely high temperature operating conditions of the exhaust system. One result of the demand for greater power was that crankshaft speeds continued to increase. Thus, between 1920 and 1927 this speed went up from 1,650 r.p.m. to 1,950 r.p.m. The lower speed was admirably suited to the airscrew, but at the n'gher speed the airscrew efficiency °egan to suffer somewhat during take- ofl- The Bristol company therefore acquired the British rights in an air screw reduction gear which had been A complete Hercules installation. The hn if 1S at Present rated at 1,375 «•?•> but much higher powers are °«»ng taken from the latest models. patented by the brothers Farman in France. This gear was of the bevel-epicyclic typs, similar in general prin ciple to the familiar differential of a car. It was first fitted to the Jupiter VIII of 480 b.h.p. in 1927, and after wards to the Jupiter IX, a low-compression unit rated at 525 h.p. at sea-level; the Jupiter X, giving 530 h.p. at 16,oooft. ; and a commercial version, the 500 h.p. Jupiter XI. Crankshaft speeds of these units were 2,000-2,200 r.p.m. and the reduction gear ratio 0.5-1. These engines marked the advent- of airscrews of larger diameter which could be employed with considerably greater thrust effi ciency.
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