(For this question use annex 081-015 issue date November 2005).
A rotating propeller blade element produces an aerodynamic force F that may be resolved into two components:
a force T perpendicular to the plane of rotation (thrust).
a force R generating a torque absorbed by engine power.
The diagram representing a rotating propeller blade element during the cruise is:
Refer to figure.
Propeller thrust is a component at right angles to the plane of rotation. The thrust force will vary along the length of each blade, reducing at the tip where the pressures equalize and towards the root where the rotational velocity is low. Thrust will cause a bending moment on each blade, tending to bend the tip forward.
Torque is the equal and opposite reaction to the propeller being rotated, which generates a turning moment about the aircraft longitudinal axis. Propeller torque also gives a bending moment to the blades, but in the opposite direction to the plane of rotation.
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