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Aviation History
1927
1927 - 0208.PDF
SUPPLEMENT TO FLIGHT 22 THE AIRCRAFT ENGINEER MARCH 33 1927 velocity varies inversely as the diameter; i.e., as ;TK- In this respect it is interest- ing to examine Pig. 20, in which curves of equal potential propeller efficiency are given for different values of K = n v/fLP. As K is a large number K x 10~3 is more convenient. n is given as revo- lutions per minute, as the common form of expressing angular velocity of airscrew shafts.* Some objection may be raised to the assumption of constant forward velocity for engines of increasing power, on the ground that with higher powered engines the aeroplane must necessarily travel faster. Such argument is most justified in the case of racing aeroplanes designed for top speed, but does not hold good for that class of machine in which the size is increased to keep constant power loading. Nor does it stand where climb is concerned. Optimum climbing speed is span2propeller efficiency and ——, and there is, naturally, an advantage in having the speed as low as the efficiency losses in the propeller and the induction losses in the planes will allow. High indicated air speed on climb is an indication of inefficiency in one or both of these, and is, of course, not experimental justification for high K. * Except for this case read n = rc\s. per sec. hereafter. 5000 . P.M . a. 4000 2000 1000 TTTT ; ': ; : TIM 0 TTTT t f | 1 TTT <~ R.R \ \ \ \ \ \ t 100 Mil Mir V \ s \ \ \ •P3 800 F n 1 1 1 1 1 1 1 1 \ \ \ \ -RM. \ \ Tirr \ \ \ t J M N 111 < + © • A # N, i J MM —— B • LI 1 1 CIRRUS CHERUB LUCIFER JUPITER ET LIONI CONDOR HI NAPIER TS. (DIRECT TYPHOON R.A.F. IA 500 H • —. 1 1J i 'S1J V^ ^. *1 — 111! >00R F •—. • 1111 /Q3 •^-- •~ •—•—. * , 11 t i (00 200 300 400 500 600 B. H DRIV ; "TTTT " -^ 1 111 f«-4 )o : 111 r l.p. ^9- largely a matter of Froude's " :yt" ftr momentum theory indicates th-1 the added velocity in the race is receiv d, h;i'- in front am; h,t behind the actuator disc. If we examine the forces on an elementary section of the blade, this inflow velocity acts similarly to the in- duced downwash on a wing. Experi- mental aerofi. characteristics mu therefore be correct' to zero inductile, (infinite aspect ratio! before they can be applied to airscrew analysis. The well- known failure of aspect ratio charac- teristics to adapt themselves to an inflow value of ' is easily understood. More accurately. rotation interference must also be taken into account, but it is not so important over the usual sig- nificant flight range. The use of infinite aspect ratio elmrar- teristics, of course. , shows advantage to W- . . high angle of attack. and usually suggest* a " face " pitch as large as the influence of the correspondm? blade width reduction on strength will allow. It will be realised from the above, that the potential propeller efficiency is really decided by the engine designer in his choice of airscrew shaft speed. The curves of Fig. 20 show how little meaning the terms " fast running " propeller and " slow running" propeller have in themselves. For example, the " Typhoon " has a higher value of K than the " Jupiter IV," and a very much higher value than the " Cirrus." A value of K X ICh3 = 30 is fairly representative of modern practice, and for general purpose aeroplanes •008 •006 •004 •002 n •7 •6 •5 •4 •3 : "i i i 1 •a Prop. •/I A — = 0*5 Co leu''"1 *"ni^ IIII lill —y ^~ 1111 ,^- •3 -4- m< - - >^ >> I ' I 1 \ >x lill •? : kqj " \ 1 J_l L •5 6 kq •14- •12 •10 08 06 •8 •7 -6 •5 .inn ; : ': \ -_ \ 1 ih x^ 1111 Prop. F 1 1 1 1 ^^ 1 1 1 1 1 1 1 1 S UD5erv«u mtia<iiCalculal-cd Mil —--, \ \ h \ 3 I.U • •6 -7 -8 -9 1-0 t , —— 1 : * - Fig. 21. 188&
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