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Aviation History
1949
1949 - 2063.PDF
FLIGHT, 29 December 1949 831 DESIGN of TURBOPROP TRANSPORTS Mr. A. E. Russell's Lecture to the I.Ae.S. in Washington PART 7: Design-essentials Imposed by the Turboprop THE 13th Wright Brothers Lecture was this year givenbefore the Institute of Aeronautical Sciences in Wash-ington on December 17th. Under the title "Some Factors Affecting Large Transport Aeroplanes with Turbo- prop Engines," it was presented by Mr. A. E. Russell, B.Sc., A.F.R.Ae.S., chief designer (Aircraft Division), Bristol Aeroplane Co., Ltd. Mr. Russell opened his —— address by stating that, dur- ing the last few years, the Bristol Company had de- voted a considerable pro- portion of its effort towards the design and development of large transport land- planes with turboprop power units. With the adoption of this engine and the con- sequent rise in cruising speed and operating alti- tude, together with the in- creasing size of aircraft, new technical problems had been introduced and old ones had been amplified. An important condition in the effective use of turboprops was that their operating characteristics were intimately linked with the performance characteristics of the aircraft in which they were installed, for the following reasons: (i) the compression ratio of the gas turbine was not con- stant, but increased with reduced temperature; this led to an improvement in efficiency with increase in altitude, (ii) The compression ratio and peak temperature were functions of r.p.m., which resulted in deterioration in effi- ciency at part load. (iii) The ram compression-ratio, besides increasing the air mass flow, enabled the working fluid to expand over a higher ratio with consequent increase of power and improved economy. As was usual in power-plant design, once the fundamental principle had been established, improved economy and per- formance could be bought only at the expense of increased complexity and weight. Thus, a balance between plant weight and specific fuel-consumption had to-be arranged with due consideration of other factors which might affect the overall aircraft performance. A fundamental require- ment for high efficiency was to reduce the rate of energy rejection in the exhaust. It would seem that the immediate pro- gramme of develop- ment for turboprops should concentrate on improvement of com- ponent efficiencies in order fully to exploit the potentialities of the simple cycle when opti- mum values were pre- scribed for the main parameters. This approach would result in a light, simple power unit of low frontal area and attractive fuel eco- nomy. To explore the scope in this direction, COR the past thirteen years, on December 17th, the anniversary of 1 the first successful controlled flight of a heavier-than-air machine, the American Institute of the Aeronautical Sciences has held the Wright Brothers Lecture. It is the complement to the Wilbur Wright Memorial Lecture given each year in England before the Royal Aeronautical Society. Mr. Russell, who presented the Washington lecture this year, is assured of aeronautical fame as the man chiefly responsible for the - Brabazon I design. In the first part of his paper, summarized here, he dealt with the essentials of aircraft design attendant upon the use of turboprops, and in the second part (a digest of which will be included in a subsequent issue) he gave an analysis of structural considerations arising chiefly from accommodating the effects of gusts. 1-6 1-4 1-2 in -8 ACTUAL STRESS IN GUST STRESS but io STATIC I— APPLICATION OF GUST LOAD ZOO RPH(TAS) \ Al iOOO FT IO „ 8 S 3- COMPRESSIO N RATI O L * \ \ s i i El a / / :i 8 —• 10 y 12 15J OHRS. b If: 01 » DURATIO N HR rtki i/i 2 — O 2OO 4OOCOMPRESSOR TEMPERATURE RISE degC) Fig. I. Curves of turboprop performance at a cruising speed of350m.p.h. at 30,000ft I.C.A.N. conditions. it was convenient to separate the thermal cycle from themeans of propulsion and consider the compressor/turbine combination as a gas generator producing "gas horse-power." This procedure was satisfactory for turboprops, as thepropulsive efficiency was very nearly independent of the thermal efficiency; but itwas not so for turbojet types, and might lead toerroneous conclusions. In Fig. 1 was shown a plot ofengine - plus fuel weight against compression ratiofor a range of durations, and it could be seen that,for durations above ten hours, the minimum engine-plus - fuel weight was achieved at a compressionratio of about 8:1. For — durations up to five hours,the importance of engine weight predominated and compression ratio was not critical;low-compression engines were then best from the points of view of simplicity and initial cost. On the subject of aircraft performance, Mr. Russell statedthat, when reciprocating engines were employed, maximum continuous poweravailable at low alti- tude was severelylimited, as was also the take-off power.These conditions would often result intake - off require- ments and airworthi-n e s s performance standards having aninfluence on the choice of wing load-ing and power load- ing ; this necessarycompromise might, in turn, have anadverse effect on the payloa d / range per-formance. A different situa-tion obtained when turbopiops were em-ployed. This arose from the fact that the power output increased continuouslyas the altitude was reduced from the cruising altitude to sea-level, and it was unlikely that aircraft performance atthe lower levels would be important to the extent of deter- mining gross weight or wing design. For turboprop-enginedaircraft, therefore, attention would be directed toward attaining the best possible efficiency under cruising condi-tions. When turboprops were used, it was essential to operate the engines at the maximum permitted continuous r.p.m. and power in order to achieve the minimum specific con- sumption. The only way to reduce the power available so that an economic condition was obtained was to operate at the highest altitude consistent with the maintenance of sufficient reserve climb-pe~formance. This altitude in- creased as weight was reduced by the consumption of fuel and was rather nearer to the ceiling than to the altitude for maximum speed. The lecturer considered the effects of some representative 37OMPH(TAS.) AT 5000 FT ALT O 2 4 6 TIME (sec) Fig. 2. Curves showing the effect of flexibility on wing-root bending stress owing to passage through a gust. 8
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