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Aviation History
1932
1932 - 1087.PDF
OCTOBER 27, 1932 7:1 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT Starting with equation 6. R Tr = (16) then the addition of an excrescence will increase K;, to Ki, + AKB without altering " a," we hope. To keep the mean value of R/R„ the same when " a " is made unity, " b " is adjusted to give the same value of R/R0 at T,. = 0-25, so that when "a" = 1-0 we may write 2 Id \2 AR = and it follows that b,= AK„ I) (17) 6-K, + AK,,— x from which 2 x K;, + AK6 b2 (Kt + AK,,) - 5K,, AK., (18) For the streamline body without excrescences, b = 13-3 and K6 = 0 • 0258, so that for any other case 2 b„ (0-0258 + AK,,) - 13-3 X 0-0258 > — — x AK, &»K,, S 0-343 AK, The results are shown in the following Table. Data from R.and M. 1040 Kfc a b Adjusted values of "6" when o = l-0 b-K,, AKi _2_ X Annul us at 0 0930 1-0 6-1 61 0-57 0-0562 3-37 8 Large Segments at 1-0 D. 0 123 11 4-64 5-04 0-02 0-0856 3-24 4 Large Segments at 1-0D. 0-0674 10 7-0 7-0 0-471 0-03 3-08 2 Large Segments at 1-0 D. 0 0495 105 81 8-3 0-410 0-012 3 14 2 Small Segments at 1-0 D. 0-0052 105 6-87 7-07 0-461 0-0278 3 25 Annulus at 1-5 D. 0 0806 0-98 6-94 6-86 0-553 0-0432 3-72 The mean value of - for those experiments with the x excrescences at 1-0 D behind the airscrew is 3-21. 71 This suggests an effective disc area of 0 • 79 — D2. If *l 8 7 f- 4 FI6.I VARIATION OF ' r WITH DRAG COEFFICIENT (VALUES OF T FROM R 8, M 1030 TABLE 22) 0^> J O LAW TO CURVE T.32l+^ o 04. 05 -06 07 08 K 09 •13 -14 Unfortunately, the necessary data for the cases where the annulus is at 0-125 and 0-5 D are not included in the report, but it would be expected that as one approaches the 2 airscrew the value of — obtained from the test results as x above would decrease. It is not possible to separate the slipstream effect and the pressure gradient effect on the streamline body alone, but there will be no loss of generality by combining the id ' ' two effects since both are dependent upon I — J . The pressure gradient effect + minimum slipstream effect is then found from K, (b — 3-21), which shows an average value of 0-253, or about \ the pressure gradient effect alone suggested by theoretical considerations for a body extending over the whole length of the pressure gradient. 9 8 6 4- FIG.2 VARIATION OF "V WITH DRAG COEFFICIENT *-, ADJUSTED TO BRING 'Z, - 1-0 C-i -6 + 4- (a-l) FROM R &M 1030 3 . O LAW To CURVE •08 „ -09 The attached curves show the agreement of the above results with the observed and adjusted values of " b." The revised formula for nett efficiency then becomes (when the major drag is about 1 diameter behind the air screw) 7) free D- \,D/ (19) Further data is required to show clearly how the constant of 3-21 is modified by proximity of the major drag to the airscrew, such as in the case of a radial engine in front of a fuselage. It is fairly obvious that an obstruction immediately behind the airscrew at a smaller radius than 0-25 Rro will not be affected by the slipstream. The nearer to the tip the obstruction is placed, the greater will be the loss of thrust due to the slipstream over the part. As an obstruction at some particular radius is moved away from the airscrew, the slipstream effect will be more pronounced. Eventually a point (at probably 1 -5 D) is reached where the effect is a maximum, after which the loss of thrust will fall off with increasing distance. This is partly due to the ineffective centre of the airscrew, and partly to the speed increment at the airscrew being only half that of the maximum slipstream speed increment. It is probable that the loss of thrust due to the slip stream effect over a radial engine fitted to the nose of a fuselage is equivalent to about J of the engine drag placed at the point of maximum slipstream velocity. Until further data is available, the discretion and experience of the operator must be relied upon in deciding such points. REFERENCES (1) R. & M. 1046.—The effects of body interference on airscrew perform ance.—W. G. Jennings, B.Sc. (2) R. & M. 892.—Experiments with a familv of Airscrews. Part III. C. N. H. Lock, M.A., and H. Bateman, B.Sc., A.C.G.I., D.I.C. (3) R. & M. 1030.—Experiments with a family of Airscrews, including effect of tractor and pusher bodies. Part IV. H. Bateman, B.Sc., A.C.G.I., D.I.C., H. C. H. Townend, B.Sc., and T. A. Kirkup. 1008 g
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