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Aviation History
1945
1945 - 1263.PDF
JUNE 28TH, 1945 FLIGHT 685 das Turbines for Aircraft Propulsion The Meaning of Thrust Power : Comparative Performances: Support for Turbine-driven Airscrews COMBUSTION CHAMBER ATOMIZINCBURNER AIRSCREW NOZZLE R1NC COOLINC AIR HOLES A diagram of gas turbine unit arranged for combined air- screw and jet propulsion LtI made sol w t * °f **%* f **% ^ Aeronaut'«' Society In April last, Dr. S. G Hooker, of Rolls-com£°nsons °f the performance and consumption of gas turbine compressor units ^ J/C- gener°' inrUoductorY ««*"«« •"fluted descriptions of the operation of i J1 7 '>r^u/s'on un'ts °f s™P'e jet and combined jet ond airscrew types and mcessonfy Fr ^'i y/Oml''^i0 *"' re°derS ^ fea$On °f £te """V °rt/deS « M P^/s/on Which hove Fhght pages and in book form in " Gas Turbines and Jet Propulsion for Aircraft* THE performance of a jet engine is expressed in lbs.of thrust which the engine develops, and it is worthwhile considering the relationship which exists between the thrust of a jet engine and the normal brake h.p. The mathematical relationship is simple, viz.: „ . . ... Thrust x Aircraft Velocity* Equivalent b.h.p. = r. « _ _. . ±- and leads * Airscrew Efficiency to the result that at 300 m.p.h. forward speed 1 lb. of thrust is equal to 1 b.h.p. if the airscrew efficiency is 80 per cent., which is a reasonable figure for this condition. On the other hand, at 600 m.p.h. 1 lb. of thrust is equal to 2 b.h.p. for the same 80 per cent, airscrew efficiency, and, due to compressibility effects upon the blade tips, the airscrew efficiency will probably be not more than 53 per cent., in which case 1 lb. of thrust is equal to 3 b.h.p. Let me elaborate these figures to show the great advan- tage of jet propulsion at high speeds. Let us consider a fighter aircraft fitted with a 1,000 h.p. piston engine. Such a machine, if of the Spitfire size and drag, will have a sea level speed of 300 m.p.h., and consequently, since at this speed 1 b.h.p. equals 1 lb. of thrust, the thrust on the ~* It will be understood that the denominator must also include the factor 550 if aircraft velocity is expressed in feet per second or 375 if the velocity is in miles per hour.—EDITOR eo JO 10 fli PROPELLER Tn i1 1 y / / > no z 0 JO *- \ \ , \ MRCRAfT 5PfXD UPH. Fig. 1 (Above). Comparative efficiencies of conventional propeller and simple jet unit at various aircraft speeds. . Fig. 2 (Right). Equivalent performances of reciprocating engine and jet unit. aircraft will be 1,000 lb., and the fuel consumption will be about 0.5 lb. per b.h.p., or per lb. of thrust per hour. A jet-propulsion engine of 1,000 lb. thrust will also drive this aircraft at 300 m.p.h., but at this condition its esti- mated fuel consumption will be about 1.3 lb. per 1b. of thrust per hour, i.e., more than double that of the piston engine. Cube Law Now let us consider what we must do in order to give this "aircraft'a sea level speed of 600 m.p.h. Since the power required varies as the cube of the speed, we shall require 8,000 h.p., even if the airscrew efficiency still stays at 80 per cent. If it falls to 53 per cent., which is more prob- able, the power required will be 12,000 h.p., and such an engine will weigh at least 12 times the weight of the original 1,000 h.p. engine. On the other hand, the thrust required to double the speed of the machine is only four times as great, so that the jet-propulsion engine will only weigh four times as much as the original 1,000 lb. thrust engine. Piston engines tend to have a constant specific weight per b.h.p., while for jet engines the weight per unit thrust should remain fairly constant. Comparing fuel consump- " tions, at 0.5 lb. /b.h.p. /hr., a 12,000 h.p. engine will use 6,000 lb. of fuel per hour in order to produce 4,000 lb. of thrust, so its fuel consumption will now be 1.5 lb. of fuel per lb. of thrust, whereas the jet- propulsion engine should be approximately 1.4. In other words, there is now very little difference in the fuel consumption of the two engines, and there is a tremendous ad- vantage in size and weight with the jet-propulsion engine. In fact, whereas the 12,000 h.p. engine will probably weigh at least 20,000 lb. when complete with airscrew and radiators (and would, of course, be prohibi- tively large for a Spitfire type machine), the jet engine will probably weigh not more than 2,000 lb., and could be accotn-
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