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Aviation History
1927
1927 - 0209.PDF
MABOH 31, 1927 23 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT -rrresponds to a tip speed of about 825 ft. per sec. Tip speed is of course proportional to the product of diameter and angular velocity, and hence has a uniform value for airscrews of constant K, varying power, but similar flight speeds. It is noticeable how far the R.A.F. la lies below all modern engines, explaining the high propeller efficiencies obtained with that engine at moderate translational speeds. The abnormally low value of K used by the Wright brothers was, in my opinion, one of two essential features in their design, principally responsible for their success. The two important secondary propeller effects, tip speed losses and slip 0-2 04. 0-6 -4. 0-6 airscrews. Propeller A ( — = 0 • 5 ) shows considerably lower efficiency at low Js than calculated. This general tendency seems to be due to a discrepancy mostly in the calculation of T \thrust, the value of the thrust coefficient ( K T = —^rr-\ pn-U* ; being estimated too high at low J and falling off more steeply The coincidence of calculation and observation is much P nearer for = = 0-5 in R. & M. 892 (see Fig. 22). These model airscrews are running at VI values generally within wind channel range, so that scale effect is practically eliminated. Some of the airscrews of N.A.C.A. 237 have been tested out full scale (N.A.C.A. 1925, Report No. 219). Fig. 23 shows comparison of the model and full scale figures for certain airscrews. Propeller B' D P D P T> = 0- = 0' = 0 6 •7 •8 Tip speed >•> = 830 = 770 ==750 f/s. f/s. f/s. Fig. 22. stream resistance, have the same percentage effect on overall efficiency at different values of B.H.P. if K is constant. To investigate the influence of airscrew design on perform- ance, it is desirable to know how far calculations are sub- stantiated by observations. The propeller designer has two problems, one to make an efficient propeller, two to make the propeller torque resistance so balance the engine torque effort that the greatest possible advantage of the engine's inherent power is taken advantage of at all flight conditions. Firstly, the performance of model airscrews in a wind channel can be compared with calculation. In R. & M. 892 a family of airscrews is compared, calculations against observation. The In this figure Cp a. Kq = Torque coefficient. Or = Kj = Thrust coefficient. It must be realised that the full-scale figures are deduced from flight tests without direct measurement of thrust and torque. So far as efficiency is concerned, one can only consider the agreement remarkable, although both Kg and Ki are higher full scale than model. VThere is of course a marked increase in VZ and also in =- Vc [Vc — speed of sound in air]. No figures for full-scale efficiency corresponding to the R. & M. 892 family are avail- able. The increased full scale values of Kg are, however, in accord with the general experience of propeller designers. There does not appear to be any evidence that the discrepan- cies between full-scale and calculated efficiences are greater than those between model and calculated for tip speeds less than 825 f/s. The differences may be due to the influence of periodicity, the difference between the shape of the experi- .12 06H .10 .05 .08 .04 •06 .03 04 .02 .02 .01 •00 .00 PROPELLER B' JLf / // •f / / /' —-, n Cp •\ PROPELLER D' \J\T / .— _^ ~v A V •""•—. < -—— MODEL TESTS FULL SCALE TESTS •% N1! \ CP kCT PROPELLER t t y/- i / -*•. ^— \ \ vCT D 0.2 0.4 0.6 0.8 .0 '0.2 ,0.4 0.6 0.8 .0 O.Z 0.4 0.6 0.8 '1 7 6 5 A 3 e .1 n Fig. 23. efficiencies generally occuring at a higher value of J i = — than calculated, rising to a higher value, but being below calculation at low values of J. This is more noticeable on airscrews of high pitch/diameter ratio. Mr. T. Whitlock, of the experimental staff of Boulton & Rl Ltd., has brought to my notice some comparisons he has made of a similar nature, based on the experi- mental results of the U.S.A. Advisory Committee, report N.A.C.A. 237. In Fig. 21, the observations of this report are cc npared with Mr. Whitlock's calculations for the same mental and calculated thrust grading curve ("tip loss"), by the mutual effect of body and boss since all experimental mechanism require the use of a body behind the propeller. The last source of difference would more easily be lost between experimental full-scale and model conditions, since similar assumptions are made in both cases in reducing observations. y Increase of t=- up to -75 f/s has been already associated V C with increase in Kq and Kt. Beyond this figure the efficiency also begins to be affected. (To be continued.) 188c
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