Propeller Forces & Stress

Prop Forces in Cruise Flight

With reference to Diagram 16, Forces in Cruise Flight, which is an extension of Diagram 2, Propeller Terminology, shows the propeller’s rotational velocity (A-B) in the prop’s plane of rotation and the aircraft’s forward velocity (B-C). The vector (A-C) represents the relative air flow (marked RAF) or the air inflow velocity into the prop disc (from C to A). This is determined by the prop’s speed ratio (the ratio of the aircraft’s forward speed to the prop RPM). The vector A-C also represents the helical flight path of the blade as it travels in the opposite direction, from A to C.

Diagram 16, Forces in Cruise Flight

Due to the prop blade’s motion along the helical path and its angle of attack, an aerodynamic resultant force is produced, the same as found on the wing in flight. The resultant force can
be divided into the components of lift and drag, but of greater importance, into the components of thrust and prop torque. The blade’s lift coefficient, air density, and inflow velocity and blade area govern the strength of the resultant force. This is shown by the familiar lift formula, Lift (or thrust) = CL%pV2S.

The propeller torque acts in the opposite direction and is equal to the engine torque at constant RPM. Ignoring the effects of a constant speed unit for now, the RPM will remain constant as long as these two forces are equal and opposite. Also, the prop’s thrust is equal and opposite the aircraft’s total drag. The forward component of the resultant force (the dotted line under the word ‘lift’) is equal and opposite the rearward component of prop drag (shown by the dotted line above the words ‘prop drag’). These two forces cancel out leaving prop thrust to equal prop drag.

The vectors of relative air flow, thrust and torque, etc, have been drawn as emanating from the trailing edge of the prop blade element. In practice, this is not true because the forces all act from the blade’s centre of pressure. This diagram and the following two have been drawn this way for the purpose of clarity Note here, how the blade element theory differs from the axial momentum theory covered in the section on Propwash-Thrust.