I have just read this, I don't know if it will help in a general context?
As the rotor blades rotate they generate what is called rotational relative wind. This airflow is characterised as flowing parallel and opposite the rotor’s plane of rotation and striking perpendicular to the rotor blades leading edge. This rotational relative wind is used to generate lift. As rotor blades produce lift, air is accelerated over the foil and projected downward. Anytime a helicopter is producing lift, it moves large masses of air downward through the rotor system. This downwash, referred to as induced flow can significantly change the efficiency of the rotor system. Rotational relative wind combines with induced flow to form the resultant relative wind. As induced flow increases when transitioning to forward flight, resultant relative wind becomes less horizontal. Since angle of attack is determined by measuring the difference between the chord line and the resultant relative wind, as the resultant relative wind becomes less horizontal, angle of attack decreases.
