. NON-CONSTANT PITCH PROPELLERS

It is possible, by modifying the diagram of Figure 14.6, to lay out a propeller which will not have constant pitch from root to tip of the blade. Instead, the angle of attack will vary in some consistent manner. The nominal or rated pitch, measured at the 7 5 % point, would not necessarily reflect this variation, which is another reason why commercial propellers

often differ from one another and perform differently even though rated the same. The advantages of such radial pitch changes are chiefly that although the propeller is never quite so efficient as the constant pitch variety at its designed operation point, there is a broader band of operating conditions under which the propeller will be reasonably effective. For sport flying models and for aerobatics, this is generally much preferred. To vary the pitch in the way suggested in Figure 14.10 is equivalent to ‘washout’ of a wing, ■reducing the angle of attack at both ‘tips’, i. e. the inner and outer ends of the blades. Various alternative approaches are possible, as illustrated in Figure 14.11. Since the hub end of the blade is least effective, modifications to pitch here are not likely to make much difference, but changes to the outer third of the blade can be very significant and competitive model fliers frequently do a great deal of work on their propellers to improve their performance in this area.