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Aviation History
1937
1937 - 1095.PDF
APRIL 29, 1937. FLIGHT. 415 ""RELATION OF CRUISING POWER TO AIRPLANE SPEED AND WEIGHT S II II U 1 K X 1 .'< hi O a >NIS I U TOTA L OOO '000 • 000 JOOO A C -AT RUISINC SPEED 40 PPLANE GROSS WEIGHT-THOU S A ja> 7 1 /iy / / SO 200 MOS OF POUNDS WING SECTIONS FOR FOUR-ENGINE TRANSPORT LANDPLANES SECTIONS TAKEN AT OUTBOARD ENGINES -13 4'- A- FOUR ENGINES OF 1000 H P. EACH AIRPLANE GROSS WEIGHT 48000 POUNDS -18.9- B- FOUR ENGINES OF.2000 HP. EACH AIRPLANE GROSS WEIGHT 96000 POUNDS (Left) This graph may be regarded as a forecast for the imme diate future. This cranks, and under-crown and skirt fins had been added, heat flow accounted for the increase in oil-cooler size. Great improvement ,in exhaust-valve cooling had followed a new forging technique allowing chemical filling of both stem and head. Inlet valves were cooled by the induction mixture. Smaller spark plugs with longer reach had helped a little in their cooling problem which had lately been intensified by screening, said Mr. Mead. The increasing difficulty of oiling- up when idling of plugs designed for high-temperature opera tion led him to predict, shortly, a more positive method of cooling. Superchargers, now used for increasing take-off power as much as for high altitudes, were, for compactness and simpli city of mixture distribution, chiefly of the engine-driven centri fugal type located between carburettor and cylinders, but this practically prohibited intercooling. The mixture tem perature, a function of the impeller peripheral speed, limited the amount of boost in the absence of an intercooler, and where two-speed drives had been used to lower the take-off temperatures, improved ground-level and high-altitude per formance had been offset by loss at operational altitudes. Fuel consumption was raised when using the higher gear ratio. There could be little further single-stage development with out intercooling, and the practical advance expected to per mit higher operational altitudes was a two-stage blower with cooler between stages. Ideally, regulated supercharging with infinitely variable impeller speeds was required ; this with inter cooling that did not affect distribution. The exhaust-driven turbo-supercharger, possibly used with direct fuel injection, was attractive for commercial use in essentially meeting these requirements. It also reduced exhaust noise. * Cori'Rod Difficulties Engine operating speeds were limited by valve gear and crank pin loading, 2,700 r.p.rn. being near the limit for a push-rod-operated two-poppet-valve head. Overhead cam shafts, which were more applicable to in-line engines, allowed higher speeds. Connecting-rod bearing loads were also be coming a limiting factor, particularly in radial engines, and connecting-rod systems would have to be made lighter and better bearing metals used. Very low specific fuel consumptions demonstrated on the dynamometer were not necessarily feasible in operation due to cooling problems, now more acute at cruising speeds than at take-off or in climb. Average specific fuel consumption for single-row radials, as used in the United States, was 0.46 lb./b.h.p./hr. at 75 per cent, power on 80-octane fuel with 6:1 compression ratio, and for two-row radials on 87-octane fuel with 6.5:1 compression ratio, 0.44 lb./b.h.p./hr. at a slightly lower power, percentage. The _difficulty of .measur ing power development in flight had almost been surmounted by the new Pratt and Whitney torque meter measuring from the airscrew fixed reduction gear. In view of the proposals to " bury " engines the above diagram is of particular interest. The raising of compression ratios was the best method for lowering specific consumption, but detonation limitations oi the fuels had hindered progress. Ratios of 6£ or 7 : 1 were now standard with 87-octane fuel and 74 or 8 : 1 were to be expected with 100-octane fuel; improvement was made up to at least 9:1 ratio. Sleeve valves were conceded to enable the use of one integer higher in compression ratio than poppet valves opera ting on the same fuel. Each added integer had the advantage of raising power output by 5 per cent., but if power increase only was required the use of greater boost was a better method due to the lower maximum pressures involved. As the leanest cylinder limited performance, equal distribu tion had a definite bearing on consumption. Excellent radial consumptions were possible due partly to the symmetrical intake system, and direct fuel injection held promise of a further improvement. Engine operational and mixture con-> ditions could not be accurately determined and adjusted even with the use of an exhaust-gas analyser, but the Pratt and Whitney automatic mixture control in conjunction with con stant-speed airscrews maintaining fairly low speed and high m.e.p. gave mimimum fuel consumptions. Single-Sleeve Valves The advantages of the single-sleeve valve were the reduc tion of combustion chamber temperatures, due to the elimina tion of exhaust valves, the higher operational speeds and the higher compression ratios permitted. Mr. Mead doubted if volumetric efficiency was .materially bettered because of the limitations of sleeve travel and port flow characteristics. The difficulty of fabricating interchangeable sleeves had slowed development, but splendid work had been done by Mr. Fedden and Mr. Ricardo, encouraged by the Air Ministry. The trend of development was towards smaller cylinders, more in number, with higher boost and crank speeds; the result would be great thrust power in relation to form drag and smoother operation with longer life and lower costs. Consider ably greater engine power was required and a balance needed to be struck between speed, size and revenue. Engine size depended upon the number per aeroplane, but two was thought to be the minimum. Airlines scheduled at 75 to 90 per cent, power had been late quite half the time, and overhaul periods were about inversely proportional to the percentage of power used. A four-engined aircraft scheduled on 65 per cent, power was a good combination, and there was a 15 per cent, power reserve with a dead engine. Load distribution and wing cover by the slip-stream for take-off were favourable with six engines. Land aircraft of 100,000 lb. with 225 m.p.h. cruising speed were to be expected, and the necessary 5,000 b.h.p. for cruising meant engines with a take-off power of 2,300 on a four- engined basis, and even more might be needed for flying boats. Importance of take-off power to pay-load had led to a rating approximately 20 per cent, over the cruising power. In discussing power plant drag Mr. Mead stated that form and cooling drag absorbed about 25 per cent, of the thrust horse power for a 48,000-lb. machine with air-cooled engines and con ventional cowlings at a cruising speed of 225 m.p.h. at 10,000ft. In most installations to date the total power plant drag of
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