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Aviation History
1955
1955 - 1392.PDF
504 FLIGHT, 16 September 1955 DERBY TURBOPROPS . . . possibilities as it should enable the engine to be run at full r.p.m.so as to provide the maximum service for anti-icing and flap- blowing and at the same time control the net thrust on the aircraftby adjusting the reversing nozzle. Such an arrangement also opened up the possibility of making a landing approach with allengines running at full power so that, if the aircraft had to go round again, full forward thrust was instantaneously availablemerely by moving the reverse thrust eyelids and without having to wait for the engines to accelerate. The uses of such a systemon military aircraft were obvious, and the lecturer believed that this principle was close to being an essential feature for theoperation of large jet aircraft. Increasing complication, while disliked by the operator, appearedinevitable. In order to get improved consumption, higher com- pression ratios must be used and, to obtain these at reasonablecompressor efficiencies, it had been necessary to desert the rela- tively simple centrifugal compressor for the axial type and thento go to a two-shaft installation to obtain the highest compression ratios. On the turbine side, the demands of increasing efficiencyhad led to multi-stage turbines, while the various sealing arrange- ments through the engine had become increasingly complicated inthe interests of reducing leakage losses and thus improving specific consumption. As a simple example of the unfortunate but inevitable processof complication, the Derwent engine had 9,000 parts, the Dart 12,000, and the RB. 109 a total of 20,000. Basically, this increasedcomplication had been forced on the company by their endeavour to make the engine meet more suitably the requirements of itsenvironment in the aircraft and in particular the requirements of imt>rovd fue1 consumption, less frontal area and less weight. TheRB. 109 would give nearly three times the power of the early Darts, although of the same overall diameter and weighing only50 per cent more. A fundamental difference between piston engine and gas turbine was that the compression ratio of the latter,which was still the basic factor affecting fuel consumption, fell with decreasing engine r.p.m., with the result that the cruisingr.p.m. needed to be as high as possible from an engine efficiency point of view. This unfortunately cut right across the require-ments for low airscrew noise. Strenuous efforts must be made in the design of the engine toobtain the maximum possible spread in r.p.m. between take-off and cruise without sacrificing specific consumption. At presentthe best that could be done in this direction would not give as wide a spread in r.p.m. as normally existed in piston engines,unless a free-power turbine were used, but this had other dis- advantages for high-compression engines. If a reduction gear ratio and airscrew diameter were chosen togive what was considered to be an acceptable helical-tip Mach number from the noise point of view then, compared withpresent-day piston engine practice, the airscrew would be turning too slowly for take-off. Fortunately the requirements of highforward speeds led to engines of a power which, by piston engine standards, gave excess output for take-off. This meant that, inaircraft which cruised at high speeds, there would always be plenty of power available for take-off. The limitation in take-offthrust would then be in the airscrew itself and thus the engine designer would be brought under pressure to overspeed his engineat take-off: pressure which, for obvious reasons, he would resist. For certain long-range aircraft the excess power available attake-off might lead to engines which were virtually throttled at take-off. In fact, as the power of existing turboprops was increasedthere would be a general tendency for this to happen when the important requirement was an increased cruising speed. Theseconsiderations applied essentially to four-engined aircraft. For twin-engined aircraft the excess power available was usuallywelcomed in order to meet the single-engined case for take-off and climb and to enable the aircraft to use shorter runways. On the question of "turboprops versus turbojets," Mr. Pearsonsuggested that the turbojet transport would have greater passenger appeal on the scores of both comfort and speed. The turbojet,when re-established, would increasingly invade the field of the turboprop airliner, in the lecturer's opinion, particularly for first-class travel. This would be so provided the airport noise of turbojet transports could be reduced to an acceptable level. Apart from noise considerations, the limit of the invasion of theturboprop field would be determined by the overall economics, taking into account the effect on load factor of passenger appeal,as well as the seat-mile cost. It thus seemed unlikely that the pure jet would come into the short-range picture within the foreseeablefuture, but some invasion of the medium-range field was probable. In the long-range sphere the turboprop would need to fight hardto retain its place, and might only do so if it could provide very much cheaper seat-mile costs. Turning to the future needs of air transport, the lecturer dealtfirst with the subject of turboprops during the next five years. This period, he said, would see the emergence of the high- SERVICE FAILURES PER 1000 HOURS 7SO Hou«! IO5O HOVM APPROVED LI'E BETWEEN OVERHAUL 2 0 «n JUN JW. *j6 SEP per HOV tic JAN FC> MAR m *AI JUN JO. we str ocr NOY OKJAN fa aut m. 1953 ..: v._--; 19S4 :- . I 19S5 0 6 05 0-4 ENGINES 01 0-2 0-1 FEATHERINGS PER 1000 ( DUE TO ENGINE FAILURE 10URS — APR MKT JUH JUL ADO SCF 1953 ocrTNO* OK VAN — •• -•••• 1 a1 1954 T»u«. sir OCT NOT DSC JAM i 1 MfiIUUM wn«I9J5 Fig. 3. Dart service experience. compression-ratio, two-shaft turboprop which, with its improvedfuel consumption, would tend to make existing single-shaft engines obsolete. The rival virtues of high and low compression ratioswere much the same in turboprops as in reciprocating engines. Civil aircraft would probably use high ratios in only the largersizes of aircraft, for the following reasons: (1) well-established and low-cost small engines already existed; (2) emphasis on fuelconsumption was not so great over shorter distances; and (3) first cost was normally more important in the smaller engine. The aircraft designers during this five-year period would wel-come increased powers from existing engines, as they always had done. The period would see the introduction of medium- andlong-range turboprop aircraft, and also the introduction of twin- engined turboprop aircraft, of which a typical example was theFokker Friendship. Because of the low powerplant weight, coupled with the relatively high powers available for take-off andclimb, the lecturer believed the turboprop to be particularly suitable for fast twin-engined aircraft. These features, he sub-mitted, coupled with the increased reliability, might lead to a revival in popularity of the twin-engined aircraft, where cost ofoperation was paramount. Numbers of turbojet engines at present under development formilitary purposes should be available for civil use during the next five years, and the civil aircraft designer should have a consider-ably wider choice than with turboprops. He would be restricted, however, by the engines available in the particular size required,and at the particular time required, rather than by theoretical considerations of ideal size and type. Turning to the long-term picture, the lecturer concluded:"The path ahead of the propeller turbine is, from a technical point of view, the straightforward one of improving the overallefficiency by increased efficiency of components and development to permit higher flame temperatures, together with a continuingreduction in specific weight by getting more power out of the same size of engine. "How far down the scale of size the two-shaft axial engine canprofitably be carried is a debatable point which will have to be resolved."Far more important from the engine manufacturers' point of view is the point that has been touched on earlier—where will thejet engine take over from the turboprop? As mentioned pre- viously; my own feeling is that by force of public preference thejet engine will invade the turboprop field. "On the turbojet side there is no sign of a slackening in militarydemands for more power at higher speeds, and the pace is likely to be governed by the pace of military developments. "Supersonic flight has arrived and I see no reason why super-sonic transport will not one day follow. "The problems are enormous, but that is only a good reasonfor getting started on them. I can assure the aircraft manufac- turers and the operator that when they are ready for supersoniccivil transports the engine industry will certainly be ready with suitable engines, both to propel them and, if necessary, lift themoff the ground."
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