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Aviation History
1946
1946 - 0725.PDF
APRIL IITH, 1946 FLIGHT 371 Power Units for Future Aircraft Piston Engines : Fuel Choice : Plain and Airscrew Turbines By AIR COMMODORE F. R. BANKS, C.B., O.B.E. ON Monday, April 8th, at the Societe des Anciens i.leves des tcoles d'Arts et Metiers, a lecture by Air Commodore F. R. Banks, Director of Aero Engine Research and Development, M.A.P., on the subject of Power Units for Future Aircraft, was delivered before I'Association Francaise des Inginieurs et Techniciens de I'Aeronautique (AFITA). We here reproduce part of his paper, omitting those sections which deal with historical or current details likely to be familiar to readers of "Flight." HE piston engine has served us well for some forty years,but its place is about to be taken by the gas turbine. The transition time will be about fifteen years. Thereis, however, one requirement which the turbine cannot yet meet, and this concerns fuel economy. For moderate cruisingspeeds around 300 m.p.h. and at altitudes of 25,000 ft. and below, the piston engine must still be considered where longrange is wanted. Aircraft of extreme range, say 10,000 miles, or having aradius of action of 3,000-5,000 miles, are necessary for certain military purposes, and I cannot see the gas turbine filling thisneed for some seven to ten years. Even the piston engine, for a range of 10,000 miles, must be developed to give con-siderably better fuel economy than it now does. I am thinking of the present cruising fuel consumption of 0.45 lb/b.h.p./hrand, in some cases, 0.41 lb/b.h.p./hr. To obtain a reason- able payload and to avoid oversize aircraft, built mainly toaccommodate the fuel, a cruising fuel consumption of not greater than0.35 lb/b.h.p./hr should be the target. To achieve a fuel economy of thisorder some form of "compounding" is necessary to give a higher expansionratio to the working cycle of the engine. The obvious method of attack is toutilize more fully the energy of the exhaust gases. We can, by using Of the three methods, f favour (a). But if relatively high-altitude operation is a requirement, then there is little alter- native to (b). When considering methods (a) and (c), theformer should be considerably lighter than the latter; although engine weight is not a primary factor for extreme range, andthe main considerations must be reliability and economy. There should not be any particular difficulty in designing and develop-ing a reliable turbine and gearing for scheme (a). In the cast- of (c), the mechanical, or frictional losses would be higherthan those of a turbine arrangement. Air Comdre. Banks then stated that a new design of pistonengine might come too late in view oi gas turbine advances, but he visualized two lines of development had this not been the case. f Fig. 1. (Right) On the Theseus I air- screw/turbine, a two-stage turbinedrives both axial and centrifugal com- pressors, while a separate turbinedrives the airscrew. A heat exchanger is incorporated and the weight is2,5001b. At 9,000 r.p.m., 1,950 shaft h.p. and 5001b static thrust are produced. ejector-exhaust systems, obtain measurable propulsive effort, and so help to reduce the effects of drag; but this would not, in my opinion, give the overall improvement in fuel economy required for large aircraft of extreme range. Compounding can be done by three methods: — (a) An exhaust-driven turbine geared directly to the engine crankshaft by double-reduction gearing, and incorporating an " overrun " device. (b) An exhaust turbo-supercharger.(c) By exhausting pairs of working cylinders, alternately, into a low-pressure cylinder which is directly connected tothe main crankshaft or to a separate crankshaft geared to the main shaft. A similar scheme to the latter has already beensuggested by Prescott at Wright Field, U.S.A. Fig. 2. (Lett) The R.-R. Clyde. The forwardturbine drives the centrifugal compressor, and the separate rear turbine, the axial com-pressor and counter-rotating airscrews. Power figures are 3,000 shafth.p. plus i,2oolb static jet thrust at 6,000 r.p.m. The dryweight without airscrew is 2,5001b. Fig. 3. (Left) The Armstrong-Siddeley Pythonhas a two-stage turbine driving an axial-flow compressor and counter-rotating airscrews. Theweight without airscrews is 3,1401b and the power at8,000 r.p.m., 3,600 shaft h.p. ^ |' plus 1,1501b static jet thrus'. These were a compound engine with a long stroke an.I a bore oi,say 8in, and a two-stroke power unit. There is a -case for the two-stroke engine, but only if com-pounding is employed to make full use of the exhaust-ga.s energy. By this means, a high degree of economy can t«-achieved, and a cruising fuel consumption of, say, o.y/ lb/b.h.p./hr is possible with a gasoline two-stroke/turbimcombination and of about 0.32 Ib/b.h.p./hr tor a similar combination arranged for compression-ignition heavy-oiloperation. There is also the free-piston engine, of which the Pescai:<is the classic example; but I consider that the principal vahi'- of this type is for industrial and marine purposes. To developa gas generator such as the Pescara and rombini- it with 1
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