An aerofoil presenting an angle of attack to the relative airflow will experience a single force.
That force is conveniently resolved in 2 mutually perpendicular directions; normal to the relative airflow to give lift and parallel to the relative airflow to give lift dependant drag.
Similarly, a propeller is also an aerofoil. It also experiences a force due to the angle of attack presented to the relative airflow resulting this time from both propeller rotation and forward motion of the aircraft. This force is also conveniently resolved into 2 directions, that normal to the plane of rotation gives thrust, that parallel to the plane of roatation gives 'prop drag' which is the force the engine must work against to maintain the rotational velocity.
Last edited by BEagle; 12th Jul 2002 at 06:50.