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Aviation History
1950
1950 - 0913.PDF
FLIGHT, II May 1950 59i THE LIFE OF A TURBOPROP ... methanol injection as a means oi restoring take-off power— I would like to repeat the word '' restoring ''—under tropical conditions. The centrifugal engine has shown itself very responsive to water/methanol injection, both as regards the return we get in power and also the very effective wash Fig. 9. Arrangement of electrically heated shoe for de-icing intake. which the compressor parts get when injection takes place. This washing procedure, incidentally, may become a main- tenance point when, after a certain number of hours, the engine is given a wash of water/methanol in order to restore the compressor performance—the effect of dirt on the centri- fugal is less than on the axial; with no cleaning procedure, we lose about 2 per cent power in 250 hours. A considerable amount of work has been done to establish the correct quantities, for the percentage of water in the mixture is quite critical. The common practice adopted on piston engines of having a mixture available at airports will have to be used with the airscrew turbine, and strict control maintained over the actual mixture proportions. Our final production engines will have an automatic control which, working in conjunction with the torquemeter, caters for varying ambient temperatures up to the full 113 deg F (45 deg C) tropical case, varying the amount of injectant according to the day temperature and also correcting auto- matically the quantities injected at aerodromes above sea level. This unit will ensure that take-off power can be restored to the I.C.A.N. figure appertaining to that particular height (this is not the full 1,400 h.p. sea-level figure). Operation.—We do not believe that the airscrew turbine will differ seriously in its method of operation from estab- lished piston-engine practice. We consider that the operating conditions will have to be determined by a compromise between the best the aircraft can do and the longest overhaul life possible from the engine. It will not be permissible—or possible or desirable—to take from this type of power plant powers up to the limiting type-tested conditions; but we do feel that the method of obtaining the power, and, in par- ticular, flame-temperature control, does mean that the engine can be used at slightly higher pro rata powers than some of the piston engines, but still emphasizing that the extended use at full conditions would not give the overhaul life weenvisage. This view may prick a bubble which has existed for some time that the turbine unit can be operated con-tinuously near its maximum conditions. To obtain the overhaul figure mentioned earlier, it will benecessary for continuous operation to work below limiting conditions, and when an analysis is done on potential aircraftperformance, utilizing the airscrew turbine, we do not think a figure of 50 per cent of the full sea-level maximum powerfor continuous cruise at any height should be exceeded. At 25,000ft this represents 70 per cent of available power at1,890 deg F absolute (1,050 deg K) flame temperature. Conclusion.—A considerable amount of what has been saidregarding the development of the turboprop is equally applic- able to the pure jet, and several items on which I haveelaborated are joint developments between ourselves and Pratt and Whitney on the pure-jet side. A few words regarding the use of the turboprop would notbe out of place. It is difficult technically—I emphasize tech- nically—to make a convincing case for the turboprop aircraftagainst the pure-jet type, but we believe that its introduction is the next logical step in transport development. Apart fromthe increase in speed which is available for cruising, the reaction of the passengers is likely to be most favourable.To sit in an aircraft travelling at 300 m.p.h. with no noise and no vibration is an experience which is almost uncannyto those of us accustomed to the more orthodox machines. Economically it is possible to prove that the turboprop enginewill pay. In addition, the problems which will have to be faced by airport control systems in handling turboprop air-craft are much closer to the piston-engiaed machine than they are to the pure jet. .'. , .• -.. . ., Pullman Fares ? I believe that it would be possible, on the introductionof aircraft powered by gas turbines, to regard them as Pullman coaches of the air. thereby entitling the operator to chargea higher fare for travel in this class of machine than for the third-class travel offered by the piston-engined machines. Generally speaking, I also feel that the time is rapidlyapproaching when it should be possible to convert existing transports of the Constellation, DC-6, DC-4 and Stratocruisertypes to turboprop power plants. Such a conversion would undoubtedly give the aircraft another lease of life and cer-tainly give the passengers the additional peace and quiet which the advance in engine development has brought. In these conversions it may be difficult, initially at any rate,to retain extreme ranges, but the advance in passenger comfort is such that the introduction of modified types on the internalair routes of the world would, we feel, make a great con- tribution towards civil aircraft development and, furthermore,take a step in the right direction to improve the travelling conditions. All the indications up to the present are pointing towardsan engine which will require very little maintenance. I am not so inexperienced as to believe we may not have realproblems to face as soon as these engines are on the routes, but we are confident that basically we have the structureof a very reliable engine which, when coupled to the Viscount, is not overloaded (power loading is less than 10 lb/b.h.p.);and with experience we feel we shall make a real contribution to the future of civil aviation. TURBOJET and TURBOPROP Two Farther S.A.E. Papers : Canadian and U.S. Contributors Among other papers read at the S.A.E. National Aeronautic meeting, and also concerned with gas-turbine development, were the two complementary contributions summarized here- after. In putting the case for the turbojet, Mr. Winnett Boyd had some particularly interesting things to say about overhaul life—he cited details of the 784-hour type-test-continuation run of the Avro Canada Orenda—while Mr. R. M. Hazen discussed, inter alia, the contentious subject of the relative complication of the control systems required for two basic types of gas turbine. TURBOJETS By Winnett Boyd '"THE case for the turbojet was propounded by Mr. Winnett -*- Boyd, assistant chief engineer and chief designer, Gas Tur- bine Engineering Division, A. V. Roe, Canada, Ltd. Mr. Boyd contended that modern (American) cars and premium- fare trains, together with luxury Transatlantic airliners, ex- emplified the public's insistence on speed and luxury of trans- portation, and the willingness to pay for those qualities. Thus, in considering air transportation after 1955, the speed rangesof different types of aircraft had to be taken into consideration. It was doubted whether anyone had seriously considered thepossibility of piston-engined transports flying at cruising speeds of 400 m.p.h. or better, as the two most potent argumentsagainst such aircraft concerned the power-plant and passenger comfort. Turboprop transports could undoubtedly fly econo-mically in the 400 to 500 m.p.h. cruising speed category and would thus be competing with the turbojet transport. Airtransportation after 1955 was the issue at stake, and the dis- cussion thus centred primarily around the advances and de-velopments of the next 5J years. Although it was appreciated that two turboprop transportswere now flying, they were both short- to medium-range air- craft, and had maximum cruising speeds of only 305 to 315m.p.h. This was somewhat better than existing piston-engined aircraft, but was hardly enough really to catch the eye of thepublic. Not only would turboprop transports have to vie with piston-engined transports in the cruising speed category be'ow
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