Pitch of a propeller

This quantity, p, is called the geometric pitch, since it depends only on the geo­metric dimensions and not on the performance of the propeller. The value of the geometric pitch of a fixed-pitch propeller may vary from about 1 metre for a slow type of aeroplane to the 5 or 6 metres that was used on Schneider Trophy and other racing aircraft (Fig. 41).

Fig 4.11 Geometric pitch

Fig 41 Fixed pitch propeller

(By courtesy of what was the Fairey Aviation Co Ltd)

Two-blader with very large pitch angle, as used in the Schneider Trophy contest, 1931.

The designer of a propeller may find it convenient to consider the pitch from a different viewpoint. When the advance per revolution reaches a certain value, the thrust becomes zero, the reason being that the angle of attack of each part of the blade has become so small that the aerofoil section of the blade provides no thrust. (Notice how this corresponds to the small negative angle at which an aerofoil ceases to give lift.) The experimental mean pitch is defined as the distance the propeller will move forward in one revolution when it is giving no thrust.