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Aviation History
1955
1955 - 0969.PDF
i5july 1955 81 THE ANGLO-AMERICAN CONFERENCE... arranged to give full expansion at a pressure ratio of 2.3 and nosignificant difference had been noted. It was agreed that any large, long-range aircraft must be noisy—buta specially treated Conway could be made quieter than a piston engine. Rolls-Royce had not yet achieved reductions of 18 db but they had gotperhaps 15. Aerodynamic noise was certainly very important. Mr. E. S. Allwright (Vickers-Armstrongs): The turboprop seemedto make a good-neighbour's aircraft. Present jet noise was excessive and reduction was looked for even in engines 50 per cent more powerful. ,,..,, ... His company had investigated the pressure-intensity close to thejet-pipe; on the Valiant—powered by "five-figure thrust" Avons— conical, finger, corrugated (6 to 60 corrugations) and convergent-divergent nozzles had been examined, taking readings chiefly about eight feet behind the nozzle and two feet (about one nozzle diameter)to one side. The results had agreed closely although the convergent- divergent nozzle showed definite advantages and gave the lowestmeasured intensity. A broader Valiant programme had shown read- ing of 150 db at 90ft and 135 db at 150ft behind and at 40 deg tothe longitudinal axis. A photograph was shown of a mobile silencer developed for theValiant, manoeuvred by a tractor which followed a white guiding line and brought the twin intakes of the silencer up to within about onefoot of the bomber's nozzles (two units being needed for ground running of all four engines together). The silencer was a large mild-steel ductcontaining perforated steel splitters enclosing slag wool. The results had shown a reduction from 150 to 110 db and no adverse intake noisehad been traced. Mr. Greatrex, replying, pointed out that there was nothing aboutthe turboprop which made it inherently quiet. DISCUSSION OF MR. LOMBARD'S PAPER THE following discussion took place after Mr. A. A. Lombard(chief designer, aero engine division, Rolls-Royce) had read I his paper entitled Low-consumption Turbine Engines, an abstract; of which was published in our issue of June 24th. i Mr. L. Fischer (G.E.C.) spoke of the design of engines which had[ good supersonic performance and also performed well during the take- loff and subsonic cruise regimes. In general, designers were going to| higher pressure-ratios and temperatures. In supersonic flight the high 1 velocity paid off in specific fuel consumption with increase in turbine- inlet temperature; less afterburning would therefore be used in future,the reheat section being operated nearly dry, with an almost 2:1 gain in s.f.c. In general, aircraft manufacturers wanted engine designersto err on the side of low s.f.c. rather than low weight, because some- thing could be done about high weight. Mr. N. R. Quinn (chief aerodynamicist, Bristol engine division)referred to a report by Illingworth (R.A.E.)—which, it was averred, few could fully comprehend—in which were discussed eleven factorsgoverning s.f.c. and specific weight. The "fully-variable" third formula by Lombard, in which wing loading and take-off thrust wereincreased, was questioned. In the B.E.25 it was cruising thrust which was increased, resulting in higher and faster flight. At present, cautionshould be shown in increasing pressure ratio, on account of handling problems; he would not like values higher than about 12:1. Mr.Lombard had given the by-pass engine about five per cent better range than the simple jet, and also must "better" noise. Pearson's formulawas cited to suggest that, in fact, what was meant was better range or better noise-level.Mr. Lombard, replying, agreed with Mr. Fischer's observations, •* pointed out that the original lecture was confined to non-afterburn-engines. With increase in flight velocity, optimum temperatures ent up, owing to rise in compressor delivery temperature; on the othera, the case of 550 m.p.h. cruise in the stratosphere implied a low le-temperature. Suggesting that the aircraft designer could tolerateeight rather than poor s.f.c. implied volume-limitation; this could These two sectional elevations, illustrating the essential differences in the layout of turbojets of the plain (top) and by-pass types, were among the illustrations accompanying Mr. Lombard's paper. be true of fighters, but the original paper had concerned itself withtransports. If big metallurgical advances were likely to take place with time, then the design should be biased in favour of better s.f.c.The formula questioned by Mr. Quinn was not intrinsically based upon take-off thrust; this parameter had merely come out in the mathe-matical reduction. Certainly pressure-ratios above 12:1 might today involve handling problems, but the factor to be noted was that increaseof pressures and temperatures in the turboprop indicated "jam tomorrow." Regarding the by-pass engine, if by virtue only of its lower jetvelocity it achieved higher propulsive efficiency it would also be quieter; noise was a function roughly of the ninth power of jet velocity. Mr. G. R. Edwards (managing director, Vickers-Armstrongs aircraftdivision) stressed the importance of Mr. Lombard's plot of turboprop efficiency with increasing pressure and temperature [reproduced inFlight of June 24th]; improvements in efficiency added up with the turboprop and they did not when the prop was taken off. He said thiswhile fully aware that he was as much involved with turbojets as other speakers; and he commented on the fact that, although by-pass engineslent themselves readily to being podded, he had "toothpasted" four into a wing. Certainly jet transports would be built and they wouldfly faster than turboprop aircraft, but he believed that, for the great mass of the travelling public, the high-efficiency propeller-turbine washere to stay. He asked Mr. Lombard whether he believed in high pressure two-spool engines and referred to the early Rolls-Royce Clyde. Mr. Lombard emphasized that he had described the pod as "God'sgift to the by-pass engine" and not as a gift to the by-pass aircraft. There was no doubt in his opinion about high pressure-ratios, and heexpressed regret at cancellation of a paper (which was to have been given by a G.E.C. engineer) on variable-stator engines. One couldonly match compressors when off-design by varying shaft speeds, and the only engine better than a two-spool might be a three-spool. Withfixed stator blading there was but a small range of r.p.m. over which one could match the varions parameters. A large airscrew was neededto achieve high propulsive efficiency; this implied a high tip-speed but, as helical Mach number was of paramount importance to cabinnoise level, cruising had to be done at low r.p.m. A wide range of operating speeds could be achieved only on a two-shaft engine. Alow tip-velocity was also advantageous in that it brought the airscrew operation nearer to its point of peak efficiency. A CRANF1ELD OCCASION AST week we announced the appointment of the new Principal ' of the College of Aeronautics, Cranfield—Professor A. J. lurphy, M.Sc, F.I.M.—and it is now possible to publish a 'ortrait. Prof. Murphy, who is Professor of industrial Metallurgy at Birmingham University, •rtUp *"s appointment on October 1st. The appointment was publicly announced at he College's eighth annual presentation of uplomas, held on July 1st. At this ceremony, he chairman of the Board of Governors, Sir 7 c ck Handley Page, welcomed the Minister )t Supply (who was to present the diplomas) and aid in his opening address that the work of the -ollege had continued to expand during the past ear. Mr. B. W. A. Dickson, Acting Principal, eported that 79 students had completed the two- ear course, 68 of whom were to receive diplomas. Jurmg the year Prof. J. A. J. Bennett and Prof, j A. whitfield had begun duties as heads of the lepartrnents of Aerodynamics and Aircraft Elec- ncai Engineering respectively. A new short ourse in flight-test techniques, Mr. Dickson nnounced, was to be held in September of Professor A. this year, open to University graduates and undergraduates. In his speech following the award of diplomas and prizes, Mr. Reginald Maudling, M.P., Minister of Supply, said that the shortage of highly skilled men in aircraft develop- ment work was a most serious bottleneck at present. In the continued development of aero- nautical science, he suggested, this country was ensuring future prosperity as well as building up her defences. A vote of thanks to the Minister was proposed by Sir Harold Roxbee Cox, who is vice-chairman of the College's Board of Governors. The same evening, a most enjoyable presenta- tion ball again showed that the undoubted abili- ties of Cranfield students are not confined to the science of aeronautics alone. We hope to publish, in an early issue, the names of students who received diplomas and prizes. The theses which were the subject of the majority of the awards totalled over 60, and covered wide fields within the five main subjects of aerodynamics, design, aircraft economics and J. Murphy. production, electrical engineering, and propulsion.
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