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Aviation History
1932
1932 - 0404.PDF
u SUPPLEMENT TO FLIGHT APRIL 29, 1932 THE AIRCRAFT ENGINEER and to arrive at the required result we shall have to assume various gear ratios and find out at what speed each permits the engine to develop normal r.p.m. This is done as follows: — The characteristic curves of the airscrew, i.e., KQ and ij(l — s), are drawn in Fig. 8. The b.h.p. of the engine is 440 at 1,775 r.p.m., and the diameter of the airscrew is 13.51 ft. If G is the gear ratio, we have, from equation (i) 0-001394 K« = —QT- Evaluating this expression for various values of G, and reading from the curve of KQ the values of V jnD corresponding with the values obtained for KQ we arrive at the following results: — G ... 0.422 0.424 KQ ... 0.01856 0.01829 0.85 169.6 144 V/nD ... ral> V ft./sec. 0.8 168.8 135 0.426 01805 0.885 170.4 151 0.428 0.01779 0.92 171.2 157.5 0.43 0.01754 0.945 172 162.5 •75 / \ \ \ \ FIG.8 0-025 o -2 -4- -e -e u/ io i-2 14 i« i-a VnD Fig. 8.—Torque coefficient and efficiency curves of airscrews investigated for engine B., gear ratio 0.5/1. If the values of V ft./sec. are plotted against G we find that V is 147 ft./sec. when G = 0.425. The value of V/mD is 0.865, and from the curve of TJ(1 — s) we see that the propulsive efficiency is 0.728. This gives 320 thrust horsepower at climbing speed at ground level, showing an increase of 55 h.p. over the case of the fixed gear ratio. Unfortunately, the mechanical difficulties to be over come in devising such a variable gear prevent us from increasing the thrust horsepower available at climbing speed in this way; so we cannot vary the engine charac teristics by varying the ratio between airscrew and crankshaft r.p.m. The engine characteristics might be varied by varying the b.h.p. while keeping the r.p.m. constant. As we are working up to the maximum b.h.p. available at any r.p.m. such variation can be made in one direction only, that is to say, by reducing the b.h.p. at any r.p.m. to less than the maximum possible. Since this is precisely what we are trying to find means to avoid we need not explore its possibilities any further. The only course left open now is to vary the airscrew characteristics, and the choice of variable is limited. Two airscrews may differ in diameter, blade area, plan form, section and pitch; but once an airscrew is made it is impossible in flight to vary any of these except the last. By doing this we arrive at the variable pitch air screw. The performance of a variable pitch airscrew is not affected by the method of operation; that is to say, the pitch may be altered automatically or by hand-operated gearing to obtain the required r.p.m. In practice the automatically-operated variable pitch airscrew has the great advantage that once the control is set to any desired r.p.m. the governor will cause the pitch to be adjusted to suit fluctuations in air speed or engine power far more rapidly than is possible with any hand- operated gear, and the r.p.m. will remain to all intents constant. The ideal variable pitch airscrew would be one in which the blades not only would be turned through the necessary angle to vary the pitch, but also would be twisted along their length to preserve the helix. This is impossible, and therefore, for values of the pitch other than that for which the blade is designed, the airscrew is not normal, and the torque coefficient curves as drawn are not strictly applicable. It is found in practice, however, that the error intro duced by assuming these curves to be applicable is not serious, and the air performance of the variable pitch airscrew agrees very closely with estimates based on this assumption. We will use, therefore, the standard curves given in Fig. 2 to calculate the advantage given on climb by the variable pitch airscrew over the fixed pitch fixed gear airscrew with engine B, both at super charged height and at ground level. The blades of both types of airscrew having been designed to give maximum efficiency at top speed and maximum r.p.m. at the supercharged height, the thrust horsepower at this condition will not vary. At normal r.p.m., which can be maintained with the variable pitch airscrew over the whole speed range pos sible with the available power, the value of KQ remains constant. Since Vci, n and D are known, we know both KQ and (V/»D)ci, and only have to find the value of P/D, which gives the fixed value of KQ at the known value of (V/flD)C]. This is done by interpolation in Fig. 2. The maximum efficiency at the value of P/D is found from Fig. 5, and the value of V/nD at which it occurs from Fig. 1. The value of »/ is multiplied by 1 — s to find the maximum propulsive efficiency, and this in turn is multiplied by the appropriate value obtained from Fig. 6 to give the propulsive efficiency at (V/»D)ci. Adopting this procedure in the case of engine B we obtain the following results : — Gear ratio ... Altitude *** wDCi P D V -— at 7) max wD !QM(1 ~ «) •»)cl(l - s) H.PT Fixed pitch H.P.T. Gain 0-5/1 1/1 0 0-01116 0-74 1145 0-84 0-778 0-761 334 265 69 10.000 ft. 0-01584 0-86 1-4 1-085 0-802 0-75 345 318 27 0 0-0079 10,000 ft. 0-0112 0-524 0-609 0-885 0-62 0-7 0-676 297 230 67 1085 0-79 0-735 0-686 315 296 19 376 Taking the extra weight of the variable pitch air screw into account, the gain in rate of climb with this engine at supercharged height is not very great, though appreciable. At ground level, however, the gain is considerable, the effect of the variable pitch being to give a nearly constant rate of climb from ground level to the supercharged height. An advantage not to be overlooked is that, with the variable pitch airscrew, it is possible to develop maxi mum r.p.m. at all speeds in case of emergency, such as, for instance, a limited available run for take off. Actual measurements made with a machine of the single-seater fighter class fitted with an engine similar to that which we have been considering showed that the length of run required to take off with a fixed pitch airscrew could be reduced by over 30 per cent, by the use of a variable pitch airscrew. Tt has been said already that the slope of the power 4
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