Brief Review

Before moving onto the next section, we will briefly review the

differences between the slip, advance per rev, experimental

pitch and the geometric pitch

• Slip is a variable and is the difference between the advance per rev and the geometric pitch.

• The advance per rev (effective pitch) is less than the geometric pitch and experimental pitch under normal operating conditions. The advance per rev is also a variable.

• When the advance per rev is equal to the experimental pitch, the angle of attack is slightly negative with zero slip and thrust.

• When the advance per rev is equal to the geometric pitch, the blade’s angle of attack is zero with a small amount of slip and thrust still present.

• The experimental pitch is a constant.

• The geometric pitch, which is also a constant, is usually less

than the experimental pitch

• On diagrams 2, 3 and 4, note the location of the relative air

flow (RAF) vector.

In conclusion and referring back to Diagram 2, Propeller Terminology, the following points should now be obvious. All sections of the propeller blade have an advance per rev equal to the aircraft’s forward speed. The vector A-B, represents the rotational velocity of a given blade element. The vector A-C represents the resultant direction of motion of the chosen blade element. Because the length of the vector A-B varies with each individual blade element, it follows each blade element travels on a different helical flight path with its rotational velocity increasing from the hub to the tip.

The vector A-B increases with increasing RPM and therefore the effective pitch (advance per rev) will also increase vector B-C. Conversely, the effective pitch will also increase if the aircraft’s forward speed is increased. Therefore, the effective pitch increases along with the helix angle, which in turn increases the prop efficiency and thrust up to a certain point. As the advance per rev increases, the blade’s angle of attack decreases and reduces the thrust and efficiency. This is where a variable-pitch or constant speed propeller becomes a necessity in order to increase the blade angle and geometric pitch to maintain the required angle of attack, for the increasing combination of prop RPM and the aircraft’s forward speed, known as the speed ratio.