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Aviation History
1955
1955 - 0885.PDF
24 June 1955 883 size—and therefore in engine casing and shafting weight—should be possible. It seemed unlikely that large gains in overall turbine efficiency would be possible. The most fruitful source of efficiency gains lay in the numerous detail parasitic losses. Leakage, particularly in a small engine where the ratio of tip-clearance :blade-height was large, depended upon good bearing location, rigid shafts, concentric and distortion- free casings, and with strains and expansions well matched under all conditions. The author illustrated a typical Rolls-Royce tur- bine, with one H.P. and three L.P. stages, supported by a roller bearing at both front and rear and with a labyrinth tip seal around the shrouded rotor blades. The paper went on to stress the supreme importance of con- trolling the weight of rotor blades. For adding a known weight to the tip of a blade a growth factor could be applied; in a typical turbojet, 1 lb more on the tip of the compressor rotor involved a total engine-weight penalty of 12 1b, which would then have to be multiplied by the aircraft growth factor. To obtain an idea of the efficiency of converting fuel energy into work, a forward-speed term had to be introduced. In Fig. 4 a high-efficiency turbojet and turboprop were compared over a range of Mach numbers. The turbojet operated at maximum thermal efficiency at around M=2.5, the exact value varying with intake efficiency. The high-pressure turboprop found its maxi- mum efficiency at about M = 0.6, this value largely depending upon the airscrew efficiency. Between these two extremes were the simple turbojets, of increasing pressure ratio as flight Mach number was reduced. For comparison, a high-pressure-ratio by- pass engine was also included. These efficiency curves had to be weighted to allow for the differing powerplant weights; such a weighted curve was shown for a turboprop. It was this weighted curve which, added to the thermal efficiency curve, indicated the optimum form of propulsion for a given flight-speed. A great deal of investigation into the by-pass engine had been made. Such engines had over-size early compressor stages, the excess air being by-passed to rejoin the main stream directly behind the turbines. At the order of speeds considered, the result was a propulsive efficiency better than that for the turbojet, and this factor, in turn, permitted the use of higher cycle temperatures. On the other hand, for a given thrust, the mass flow and engine weight would be higher. A precise comparison could be made between the turbojet and the by-pass engine, in which the high-pressure sections of the two engines were actually scales of each other. The turbojet com- bustion temperature could be reduced until its s.f.c. equalled that of the by-pass engine. To achieve this, it was necessary to depress the temperature to some 120 deg C below that of the by-pass unit, and it was found that the engine weight was then some 16 per cent greater and the installed weight some 20 per cent greater. For high-altitude long-range operation, this would lead to a five per cent gain in range. One could design the turbojet and the by-pass to the same TABLE I: BY-PASS:TURBOJET PERFORMANCE RATIOS 550 m.p Thrust S.f.c. ... Combustion h., 40 temp 000ft (deg K) SameCombustion Temp. 0.925 0 93 1,040 SameThrust 1.0 0.955 995 Same s.f.c. 1.331.0 885 0-5 1O 1-5 20 2-5 FLIGHT MACH N° 3O Fig. 4. The overall efficiency of various types of powerplant. Curves are: A, high-pressure-ratio turbojet; B, low-pressure-ratio turbojet; C, high-pressure-ratio by-pass; D,, turboprop; D>, "weighted" turboprop curve. The random numbers are propulsive efficiencies. take-off thrust at the same combustion temperature. Then, the altitude performance could be compared as shown in Table 1. It could be seen that the by-pass was roughly five per cent better all-round. A further very useful characteristic of the by-pass engine was its low noise level. Other gains in ease of installation were possible, as a result of the reduced surface temperature. In a conventional buried turbojet installation, fire- proof bulkheads weighed 80 lb (three per cent of engine weight), and cooling air flow amounted to 3.5 per cent of the total mass flow, representing a drag equivalent to two per cent of engine thrust at representative flight conditions. Another 50 lb penalty was required by the fire extinguishing system. The by-pass engine normally had a surface temperature of some 100 deg C (150 deg C maximum after take-off in tropical conditions). This did not eliminate the need for fire protection or ventilation, but showed that the problem was greatly eased in comparison with a turbojet. Generalizing, it was the author's opinion that about 425 m.p.h. represented a fair upper limit for the turboprop, and it was stressed that at such speeds absolute dependence had to be placed on airscrew safety devices. Should the airscrew tend, inadver- tently, to "fine off" in flight, enormous drag and overspeeding would result. Overall, however, the future of the turboprop seemed very rosy. The first commercial turboprop machine had proved itself capable of earning a profit, although its engines were some of the earliest turboprops ever designed (in which the component efficiency and design standard achieved in turbojets had not yet been incorporated). For the distant future, however, the turbojet seemed to be an ideal power unit for quite high supersonic speeds. In spite of the low cruising lift/drag ratios, it was clear that aircraft operating at Mach numbers as high as 2.5 could compete with "high- subsonic" types, and could give really worthwhile reductions in long-range block times. For example, two return Atlantic cross- ings could be made in one day. BOULTON PAUL INITIATIVE PI his statement issued in advance of the annual generalmeeting of Boulton Paul Aircraft, Ltd., on June 16th, the chairman, Mr. J. D. North, spoke of the company's particular ability to make specialized contributions, assessed in advance, to changing trends in aircraft development. Such a policy, he said, necessitated investment—not least in intangible assets— much of which was of a long-term character. Conditions of official secrecy made it difficult to be more specific, but it could be said that already a large majority of the heavy bombers for the R.A.F. were using Boulton Paul powered control systems; he thought the work the company had done, and was doing, in this direction was likely to assure a continuance of its lead. The balance sheet presented at the meeting showed a net profit, for the year ended July 31st, 1954, of £270,085 before taxation (previous year, £226,483). A final dividend of 15 per cent, less tax, was recommended. MR. ARTHUR AINSWORTH WE record with regret the death of Mr. Arthur Ainsworth,M.B.E., on June 5th, following a short illness. He was a well- known member of the A. V. Roe organization. Mr. Ainsworth joined Avros in 1915, working in the erecting shop, and later travelled abroad for the company as service engineer. In the inter-war years, as manager of the Woodford Aerodrome, he was responsible for preparing several aircraft which later completed outstanding flights. Among these were Bert Hinkler's Avian, which in 1928 flew to Australia in 15]- days, and Kingsford-Smith's Avro X, which made a similar journey four years later in 12^ days. Perhaps Mr. Ainsworth's best-remembered achievement was the organization of some 2,300 workers, during the peak wartime production period, to build 40 Lancasters per week. In recog- nition of this work he was appointed M.B.E. in 1945. After semi-retirement in 1949, he supervised maintenance matters at Woodford Aerodrome. Mr. Ainsworth leaves a widow and a son. NEW BRITISH STANDARDS RECENTLY issued British Standards for aircraft componentsinclude the following (obtainable at the prices quoted from the sales branch of the British Standards Institution at 2 Park Street, London, W.I): 2 SP 105—Swaged cable-end assemblies (Unified threads) for preformed steel wire rope (W.9) (price 4s); 2 SP 106—ditto, for preformed non-corrodible steel wire rope (W.ll) (4s); 2 G.I 11—Rate-of-climb indicators (2s. 6d); SP. 61 to 64—Wood-screws (4s); G.150—Tachometer indicators, synchronous type (2s 6d); SP 59 and SP 60—Spring catches (2s 6d); SP 114—Eye-bolts (Unified threads) (2s 6d); E.22— Dummy sparking-plugs (2s).
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