Propeller Sense

PROPELLER SENSE Extreme conditions of flight It is, of course, at the extremes of speed that the variable-pitch propeller is most needed. Modern propellers work at very coarse pitches in level flight. The blade angle is then so large that if this angle were used for take-off the blade would be very thoroughly stalled the propeller w r ould then be putting avery large part of the power of the engine into churning up the air into violent and useless eddies, and imparting a great, useless spin to the slipsteam and it would not allow the engine to getup to anywhere near its maximum r.p.m. and power. At the other extreme, in the dive, there would bethe tendency for BC to overshoot BD and for the angle of attack to reverse. Then the propeller would drive the engine instead of being driven by it, ‘windmilling’ it up to umpty hundred r.p.m. Suppose that the blade angle of a variable-pitch propeller at 3 feet 6 inches out from the axis is 20 degrees in fully-fine pitch at the start of the take-off. Air is being sucked in towards the pro­peller, we shall suppose, at 50 m.p.h.—or 4,400 feet per minute—and the propeller is turning at 1,400 r.p.m. (It is geared down to run at a little under half the r.p.m. shown on the indicator, which is crankshaft speed.) The advance per revolution BC then will be 4,400/1,400 or 22/7 feet. The angle CAB will be about 8 degrees. So the angle of attack will be 20 less 8, that is 12 degrees, which is just about stalling angle. Tryout this calculation yourself on apiece of paper then it will really register in your mind. Now consider the aircraft inflight at 350 m.p.h. true airspeed. BC is 7 times as big it is 22 feet in fact, and the angle CAB will be 45 degrees. It follows that we shall require to have the blades turned through 25 degrees plus whatever small angle of attack we shall need to absorb the power the engine is giving. We should need a pitch-rangc of around 30 degrees. To have any angle of attack at all in a steep dive reaching 450 m.p.h. true speed, we should need more blade angle. This calculation indicates the need for pitch ranges— measured in the angle through which the blade can be turned—of 35 to 45 degrees, and shows why 20 degree range propellers overspeed in a dive and underspeed in the initial stages of the take-off run. Convince yourself once again, with pencil and paper, that you have followed this close but profitable argument. Blade angles at various heights So far we have looked at the problem only from the aspect of changes of airspeed we should also consider flight at different altitudes. So long as the engine was ‘normally aspirated’—i.e. not super­ charged—engine and propeller breathed the same air. The engine developed power, and the propeller absorbed—or transmitted—power, on the same declining scale as the aircraft ascended. But supercharging upset this relationship. The supercharged engine actually gives a slightly rising power as the aircraft climbs, up to the full-throttle height. The power that the propeller will absorb, at given r.p.m. and forward speed at constant angle of attack, falls off indirect proportion to the fall in density of the atmosphere. So we need to be continually increasing the angle of attack on the climb. In addition, climbing as we do at constant I.A.S., the true speed is continually rising, which means that the angle of advance is increasing also. Both effects call for continually increasing pitch to maintain constant r.p.m. What all this means And so we see that to maintain any desired boost and r.p.m. with either varying airspeed or varying altitude we need to be able to make continual re­adjustments to the pitch of the propeller.
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