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Aviation History
1932
1932 - 1231.PDF
December 1, 1932 Supplement to FLIGHT Edited by C. M. POULSEN December 1, 1932 CONTENTS By W. It. Andrews, Lijjht Aero Engine-Airscrew Combinations. A.F.R.Ae.S Notes on the Use of Stainless Steel in Aircraft Structures. Pollard, Wh. Ex., A.F.R.Ae.Soc Technical Literature— Summaries of A.R.C. Reports and Memoranda Page 81 By H. J. x.-> LIGHT AERO ENGINE-AIRSCREW COMBINATIONS By W. R. ANDREWS, A.F.R.Ae.S.* SINCE the Lympne competitions of 1923 and 1924, the super light aeroplane has died a natural death. Look ing back at the difficult conditions under which the aeroplane designers laboured, it is remarkable that such good performances were obtained. The majority of the engines available, while excellent for motor cycles, were very inefficient as prime movers for aircraft. Tn a few cases gearing was added and the performance was greatly increased—this merely showed what could be done. No one would suggest the continued use of ordinary unlubricated, unprotected motor cycle chain as an ideal reduction gear between engine and airscrew ; yet this was used with marked success. The unreliability of the engines was largely respon sible for killing the type. For this the engines them selves cannot wholly be blamed as in many cases they were running at some 50 per cent, in excess of the maker's ratings. (Ref. 1.) The lessons to be learnt from the Lympne compe titions seem to some extent lost; at any rate, no one has tackled the really light engine problem seriously since that date. The attitude of the engine manufacturers seems to have been that gearing, though helpful, was not invari ably successful, and that the alternative was more power. And so powers were increased 60-80-100-120-150 —until now the process is reversing. (Ref. 2.) Shall we ever get back to the 30-h.p. engine in a single-seater which can be run for about 3s. an hour for fuel and oil? From the evidence of these early experiments the conclusion was formed that it was not a practical possibility to fly a single-seater aeroplane with less than GO h.p. There were tremendous difficulties in the way of * Mr. Andrews is on the Technical Staff of A. V. Roe & Co. Ltd. the designers. The engines available were not designed for the work in hand. The nature of the problem made low airscrew efficiencies inevitable, and as a result the engines were run at higher than rated r.p.m. in an effort to reach the peak of the thrust h.p. curve. The form of engine mounting was not originally intended for aircraft use, and the provision of a rigid mounting was no easy matter. The addition of extraneous gearing made fresh difficulties. The airscrew problem also caused a lot of trouble. The following notes give some of the variations which influence the problems of this much maligned appen dage. The slope of the free air efficiency curve with small pitch ratios is very steep. Small variations in pitch will, therefore, have a marked effect on the free air efficiency. As, however, a change in pitch necessitates a change in airscrew diameter, the losses due to slipstream effect over the body are also changed. As a result the determination of the point of maxi mum net efficiency would become a matter of experi mental trial and error over a large range of Pf/v ratios. The thickness-chord ratio of these small pitch air screws is also of great importance. It is probable that a thin metal airscrew of suitable pitch would improve the performance, even though slightly heavier than a wooden one—providing the tip speed can be kept low enough. As the P//r> ratio increases, the effect of thickness and small variation of P//D ratio become less pronounced, until at a P//D ratio of 1.0 there is comparatively little to choose between any practical airscrews. The air screw which gives slightly the best speed will probably be found worse on climb, but provided the pitch of a wooden airscrew is within +6 per cent, of the value of V/n at top speed, the performance will not be greatly affected. The thinner the airscrew blade the higher must be the face pitch to give the same aerodynamic- properties. It is not possible to generalise too far on what is the optimum engine-airscrew combination for different speeds, as the factors governing the results are too many but they are indicated as under: — (a) V/nD at top speed as large as possible. (b) P//r> ratio to be as high as possible. Pf should not vary more than about ±10 per cent, from the V value of — at top speed. 1152 a c2
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