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Aviation History
1937
1937 - 1719.PDF
JUNE 24, 1937 • FLIGHT. 643 The parts of a poppet- valve cylinder assembly compared with those of one of the new Bri stol sleeve - valve units. The simple and robust construction of the components of a Bristol sleeve- valve cylinder are strikingly apparent in the view below. ir<^+«••>•;•:• completed their one hundred hours type tests with very gratify ing results, the performance figures being considerably in excess of those promised to aircraft constructois. The Mercury XII will go into service in the R.A.F. as the power plant of the Westland Lysander. The following are the official figures for the Mercury XI and XII: maximum take-ofi power, 830 h.p. at 2,650 r.p.m.; international rating, 790/820 h.p. at 2,400 r.p.m. at 3,500ft.; maxi mum power for all-out level flight (not more than five minutes), 890 h.p. at 2,750 r.p.m. at 6,000ft. The Mercury' has a bore and stroke of 5|in. x6iin., giving a capacity of 1,500 cu. in. (24.9 litres). Its overall diameter is only 5l|in., and its net weight 980 lb. Designated the Pegasus XVIII, the new Bristol engine with the two-speed supercharger has re cently passed its type test. Experimental work on a two-speed blower, with the object of com bining the high take-off power of a medium- supercharged engine with the maximum perform ance at high altitudes by full supercharging, was commenced by the Bristol company early in 1934. After two years of research an hydraulically oper ated clutch was evolved and the supercharger was subjected to the severest possible trials, including 600 hours on test rigs. The following performance figures, using both medium and full supercharge ratios, have been released for publication : — The various periods of maximum cruising, climbing, and high power were run with each supercharge ratio in turn, as well as a high-speed period using full supercharge, and com plete cycles of gear changing were effected twice every hour. One hundred and ten changes were made in both ratios, and the endurance test report comments on the freedom from vibra tion or clutch slip at all speeds. The final power observations exceeded the initial figures in each case. The Perseus and Hercules sleeve-valve engines will, of course, be of special interest, not only on account of their ununie design, but because the Perseus is now undergoing Service tests in squadron use. As long ago as 1926 the Bristol Aeroplane Company foresaw the enormously increased demand for power which is requiied to-day. It was considered that the air-cooled aero engine would maintain its lead in power for weight, but that it would eventually become imperative to surmount the speed and load limitations associated with overhead valves operated by push rods. Several engines with an overhead-camshaft arrangement were designed, and one type was built and tested, but the Bristol company, encouraged by the British Air Ministry. de- Maximum take-ofi power. International rating. Maximum power for all-out level flight (not more than five minutes). Using Medium Supercharge ratio. 940/980 h.p. at 2,475 r.p.m. 820/855 h.p. at 2,250 r.p.m. at 5,000ft. 900/940 h.p. at 2,(500 r.p.m. at 6,500ft. Using Full Supercharge ratio. 765/800 h.p. at 2,250 r.p.m. at 15,500ft. 860/900 h.p. at 2,600 r.p.m. at 17,750ft. Fitted with a two-speed supercharger the new Bristol Pegasus XVIII gives o3o h.p. for take-off, 855 h.p. at 5,000ft. and 900 h.p. at i7,75ott:-
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