Variable pitch

The geometric pitch angle is the angle that the blade is set relative to the direc­tion of rotation, as shown in Fig. 6.4. If we run the engine at a high rotational speed, and set the geometric pitch angle to around 45 degrees near the tips, for

Separated flow

Large pitch angle

Relative velocity due to forward motion

Effective angle of attack of blade

Relative velocity due to rotation

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Fig. 6.7 Propeller blade section at large (coarse) pitch angle and low forward speed

The effective angle of attack is so large that the blade section has stalled

efficient cruising, then at low flight speeds, the blade angle of attack will be high, as shown in Fig. 6.7. The blade lift to drag ratio will be poor, and if the angle of attack is too large, the blade may even stall. It is advantageous, there­fore, to fit a mechanism which allows the pitch angle of the blade to be altered. Fine pitch is necessary for climbing and accelerating at low speed. Coarse pitch is required for high speed fight. The pitch-change mechanism serves a similar function to the gearbox on a car, but has the advantage of allowing continuous rather than step-variation.

To maintain the best pitch at all positions along the blade, it would really be necessary to be able to alter the twist as well, but as most of the thrust comes from the outer portion of a blade, the loss of efficiency due to non-optimum twist is small in practice.

Early variable pitch propellers were operated directly by the pilot, but the number of pitch settings was limited to two or three to avoid giving him an excessive workload. An alternative, and currently preferred method, is to use the automatic so-called constant-speed propeller mechanism described below.