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Aviation History
1932
1932 - 0401.PDF
April 29, 1932 Supplement to FLIGHT Edited by C. M. POULSEN April 29, 1932 CONTENTS The Mutual Influence of Engine and Airscrew Characteristics. Lt. Col. J. D. Blyth, O.B.E., A.F.R.Ae.S., M.I.Ae.E. Engine Mounting Stresses. By R. Rodger Technical Literature— Summaries of Aeronautical Research Committee Reports .. By Page 25 29 THE MUTUAL INFLUENCE OF ENGINE AND AIRSCREW CHARACTERISTICS By LT. COL. J. D. BLYTH, O.B.E., A.F.R.Ae.S.. M.I.Ae.E. [Concluded from page 23) The value of IJ is affected also by the gear ratio, since this affects the value of P/D. Fig. 5, which is derived from Diehl's general efficiency curve given in N.A.C.A. Report No. 168 shows the values of ijrnax. for varying values of P/D, while Fig. 6, from the same source, shows the variation of n with VjnD. It will be seen that the lower the airscrew r.p.m. the higher pitch diameter ratio becomes, leading in turn to increased maximum efficiency. We can return now to the case of engine B with various gear ratios. Assuming a value of parasitic drag in slipstream which is of the order obtained in*|J?»ctice, and employing Figs. 5 and 6, we can amplify the table as follows: — Gearratio. 1\M 1-S ... UM(1 — «) 7)N(1 — S) H.P.T •" 0-5/1 0-84 0-96 0-805 0-79 363 1/1 0-81 0-92 0-745 0-73 336 1-5/1 0-78 0-88 0-685 0-66 304 2-0/1 0-755 0-84 0-635 0-59 271 2-5/1 0-73 0-8 0-584 0-528 243 Fr' 'ii this table we see that although normal r.p.m. are maintained the propulsive efficiency falls off so rapidly when the airscrew is geared to give higher r.p.m. than the engine that the thrust horsepower lost is considerable. A third important factor which has not been taken into account is the practical limitation to the tip speed of the airscrew. As this approaches the speed of sound the compressibility of the air can be disregarded no longer, and its effect becomes more and more pro nounced; and when the speed of sound i? reached, energy is dissipated in the form of sound waves. In such 10 '9 7 - / r / Fis.5 5 s -6 7 6 9 10 II 1-8 13 V* 15 Fig. 5.—Variation of maximum efficiency with P/D. 10 9 J, 7 •e RGB. i * s •« 7 -e 9 ro ii nD/ (IOM^, 376 a Fig. 6.—Variation of airscrew efficiency with V/nD. D2
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