THE ‘P’ EFFECT

Even if, by careful design and trimming, the drive shaft of the propeller is exactly aligned with the flight direction at some airspeed, in every other trim, such as ‘nose-up’just prior to landing, the propeller will not meet the airflow exactly ‘square’ on. The disc of rotation will be inclined at some angle other than 90 degrees to die approaching flow.

This produces a force component acting at right angles to the drive shaft The explanation is sketched in Figure 14.17. Here the aeroplane is in a ‘nose-up’ trim. The propeller blade on the port side experiences a reduction in angle of attack and the blade on the starboard side an increase. This produces more thrust on the starboard side and less on the other so the’ ‘P’ force tends to yaw the aeroplane to port (left, viewed from aft). Similarly, if the propeller disc is aligned nose-down, the ‘P’ force tends to yaw it to the right and side slipping trims have equivalent nose-up or nose-down force effects.

In normal flight, these forces are trimmed out without any difficulty. That is, part of the exercise of trimming a model for level flight involves use of rudder to counteract any yawing imbalances and elevators to balance the total pitching moments, whatever their cause. Model fliers are thus rarely conscious of the ‘P’ force since this is lost in the general balance equation. However, if there is a change during flight, in the alignment of the propeller axis, the ‘P’ force changes and the balance is upset. However, the reaction of the propeller to such an imbalance is not a simple yawing or pitching force. The ‘P’ force is at right angles to the drive shaft, and the propeller’s reaction, due to gyroscopic precession, is at right angles again to the ‘P’ force, in such a manner as to de-stabilise the aircraft, i. e. to exaggerate the change.