PITCH

If a small segment of a propeller blade, at some distance from the hub, is viewed in cross section, as in Figure 14.3, the meaning ofpitch may be clarified. Conventionally, when a model propeller is being measured, using a simple pitch gauge, the flat, or flattish, back face of the blade is used as a reference. A radial station of 75% of the distance from the hub to the tip is generally used as the point for such a measurement, which yields the ‘rated’ pitch. This is usually given as a length (cm. or ins.), the reason for which appears below.

If the blade section is undercambered, a line tangential to the back face of the blade is used. However, most aerofoils on real propellers are not truly flat on one side. There is a true chord line which runs from the trailing edge to the leading edge. If this is used as the reference line for pitch calculations or measurements, as the diagram shows, the true geometric pitch will be greater than that based on the rated pitch. This may be part of the explanation for the very noticeable variations in pitch measured with the model flier’s gauge and the advertised rating of many commercially produced propellers. It should also be remembered that blade sections invariably change from root to tip, for structural reasons. The segments near the tip are usually thin and those nearer the hub considerably thicker. The geometric chord line may stand in a different relationship, at each place on the blade, to the underside tangential reference.

A further reference line on the drawing is also of importance. Every aerofoil section has an aerodynamic zero angle of attack. Only in the case of a symmetrical profile does this coincide with the geometric chord line. With cambered profiles, the aerodynamic zero line is at a greater angle to the propeller disc than either the conventional or the geometric reference. The aerodynamic zero line represents the angle of attack at which this part of a real propeller blade would produce zero lift and hence no thrust Such a condition can be

reached, for instance in a dive with the engine at low throttle. Beyond this negative angle the propeller becomes an air brake.