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Aviation History
1945
1945 - 2006.PDF
OCTOBER IITH, 1943 FLIGHT 393 Airscrews A Specialist Examines Their Future Scope and Probable Development in the Next Five Years NOT inappropriately, the first Royal AeronauticalSociety lecture of the season, given last Thursday,was on the subject of aircraft propulsion. We are on the eve of great changes ; in fact, the jet a,nd turbine may be said to mark the opening of a new chapter in flying. Mr. L G. Fairhurst, chief engineer of Rotol, had chosen as his title " The Future Scope of Propellers." We have taken the liberty of substituting the word airscrew in conformity with our usual custom, since this applies logically to all layouts. Mr. Fairhurst has written a most admirable paper, and one which must be studied in detail by technicians. We can but give a general outline of his arguments and con- clusions. Ultra high-speed flight at great altitudes presents the greatest problems, and a considerable part of the paper \ deals with this subject. # By way of background Mr. Fairhurst showed curves of increase in b.h.p./ relative density and in aircraft speeds [ during the years 1939-1945. The foimer ratio rose from a value of about 1,000 to one of approximately 5,000. Air- craft speeds rose from 300 m.p.h. to 460 m.p.h. in the fighter class. In the same period the airscrew designer catered for the increasingly difficult requirements by pro- ducing, step by step, the three-blader, the four-blader, the five-blader, and the six-blader contra-rotating airscrew. The end of the war had taken us up to engine powers of 2,400-2,800 b.h.p. The next stage to be considered was the engine of a basic 3,000 b.h.p. capable of development up to 4,000 b.h.p., and a basic of 4,000 b.h.p. capable of being developed up to 5,000 b.h.p., the former for use in fighters and the latter for bombers and transport aircraft. The airscrews to cater for these types ranged from six- bladers (3 + 3) to contra-rotating ten-bladers (5 + 5). The greatest diameter envisaged was of about 2rft., this being a 4 + 4 for 5,000 h.p. engines working at 40,000ft. Turbine-Airscrew Combinations f When changing from piston engine to jet and turbine/ airscrew combinations the lecturer foresaw two classes: "•O.straight jet with airscrew, and gas turbine engine and airscrew. For the former combination he forecast that an airscrew used as a thrust augmentor and absorbing some 20 per cent, of the power would improve take-off and initial climb by 45-50 per cent. One difficulty might be that of control of the airscrew by a governor responsive to the r.p.m. of the whole unit. The solution might be to have a separate turbine to drive the airscrew. This would ensure satisfactory constant-speeding on take-off and climb, and the neutralisation of the airscrew at top speed could be done by feathering. The combination of gas turbine and airscrew would, the lecturer thought, be the stage immediately following the piston engine in aircraft such as multi-engined bombers and transports, up to a maximum speed of 400 m.p.h. The JUJJUJfUJfUirti^. REDUCTION* GEAR COMPRESSOR COMBUSTION CHAMBER Diagrammatic Jayout of jet engine withgcared-down airscrew as a thrust augmentor. propulsion unit would comprise the usual compressor, com- bustion chamber and turbine, the latter driving the air- screw through a reduction gear, as shown diagrammatically in one of the illustrations we reproduce. Approximately 80 per cent, of the power would be allotted to the airscrew, the remaining 20 per cent, issuing in the exhaust gases as kinetic energy. The lecturer would expect to see turbine/ airscrew units of 1,000 h.p. to 10,000 h.p. over the next five years. Operating altitudes of 10,000ft. and 40,000ft. were considered the most extreme to be met, and curves were shown which depicted the range of airscrews appro- priate to these conditions at 300 and 400 m.p.h. These included four-bladers, five bladers, double-threes, double- 40.000 FT. DOUBLE FOUR 15-22 FT WEK3HT I43O-3OOO LB. II60-2DOOLB. 30,000 FT. WEIGHT IJ50-24O0 LB. DURAL 93O - I60O LB. WOOD 2O.OOO FT. DOUBLE THREE IOJOOO FT, WEIGHT 78O-H7O LB. DURAL 660-B7O LB. WOOO 3.OOO H.P DOUBLE FOUR WEIGHT IOBO-220O LB. 880-I44O LB. DURM. WOOO DOUBLE THREE WEIGHT 78O-II7OLB. 66O- 87OLB. DURAL WOOD •4.,OOO H.P DOUBLE FIVE WEIGHT 2OOO-32OO LB. DURAi. I35O-I85O LB. WOOR DOUBLE FOUR WEIGHT I35O - 26O0 L B. DURAI IOOO-17OO LB. WOOD kg. (1(1 THREE WEIGHT IOOO-I+SO LB. DURAL 75O-95O LB. WOOD 5,000 H.R Pictorial analysis of future airscrew types for engines of 3,000 to 5,000 b.h.p. fours, and double fives. For maximum efficiency, diameters of 30ft. would De necessary at 40,000ft, and these would present serious problems to the aircraft designer. On the subject of ducted fans the lecturer was not very optimistic. The advantages were not considered very great, and the disadvantages considerable, especially the size of fan and ducts required to deal with the large volume of air. The next section of the paper dealt with aerodynamic development needed in connection with high-speed, high- altitude flight. Blade sections would have to be thinner, and with the maximum ordinate much farther back than the ones used previously. Another means oi keeping air- screw helical tip speeds at a reasonable local Mach number was the two-speed gear. This could, at 550 m.p.h. between 25,000 and 40,000ft., reduce the tip Mach number for top- speed flight to something like 0.95— 97, a value commonly used in normal desi?*i. In dealing with contra-rotating airscrews the lecturer also referred to what he termed co-axial airscrews, viz., a type in which each half of the combination is driven by an independent engine. Some years ago the Fairey Aviation 7
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