Propeller Sense

PRINCIPLE OF THE VARIABL E-PITCH PROPELLER FIXED PITCH Airspeed down Torque (up) Blade angles at various airspeeds But to return to our angles, we should now con­sider what angle of attack we want the blade to have, or at least how we should like the angle to vary with different flight-conditions. We want to use, shall we say, the same boost and r.p.m. at various airspeeds. The same boost means the same mean gas pressure acting on the pistons in the cylinders of the engine and applying the same torque (or turning effort) through the propel­ler shaft to the propeller. We want to have always the angle of attack diat will produce the same torque-reacdon back from the propeller onto the engine. Now we shall suppose—it is near enough except at small attack-angles—that the total reacdon of the air, the total air-force, on our bit of blade is at right angles to it as shown by the arrow in the diagram above. With the fixed blade it will be clear that this reacdon condnues to act in the same direcdon when the angle of attack is changed by change of airspeed, but that it changes in amount. It follows that it cannot have the same torque- component in the plane of rotadon. If the angle of attack goes up, both thrust and torque will go up. But the engine is not giving anymore torque, and so the propeller and the engine slowdown dll equilibrium is reached. VARIABLE PITCH When the propeller slows down, the torque is reduced in two ways. First, the blade is moving more slowly through the air and so is getung less reaction at a given angle, losing lift (like the wing it really is) through loss of airspeed. And secondly, the angle of attack is reduced. Remember that BC is the advance per revolution. So, when die revolu­tions go down, the forward speed remaining the same, the advance per revolution goes up BC is increased, and the angle of attack DAC is dimin­ished. And so, in fact, the torque required to drive the propeller is brought down quite a lot by a comparatively small drop in r.p.m. Conversely, quite a large increase in engine-power is accom­panied by only a comparatively small increase in r.p.m. So you are perhaps beginning to see why a fixed-pitch propeller does as well as it does on the elementary training-types. What blade-angle change does With the variable-pitch propeller we reduce the torque for flying at lower airspeed by turning the blade round in its socket and reducing the blade angle DAB, as in the diagram below. This does two tilings. First, it reduces the angle of attack, and secondly, it swings the direction of the air- reaction on the blade away from the plane of rota­tion. So it both reduces the reaction and it reduces its per-cent contribution to torque. V \\Blade turned o _ 'Reaction reduced and turned o D'.,-I
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