Rotational velocity and its effect on the Angle of Attack depends on the radius of the blade element in question. A blade element near the blade root (hub) will have a much lower rotational velocity than a blade element near the blade tip. Rotational velocity is usually expressed as NR where R is the position of the element from hub centre and N is the propeller R.P.M.
It will be seen that in order to maintain an efficient angle of attack along the length of the blade, a reducing blade angle is necessary from root to tip. This progressive change in blade angle is called blade twist. It is best understood by considering the requirement for each section of the blade to produce an equal amount of thrust and to advance the same distance in one revolution. Ensuring that the angle of attack remains constant along the total length of the blade is a major factor in achieving this.
For a single revolution of the propeller, the amount of air handled depends on the bit of air a propeller takes. Therefore the blade angle is a means of adjusting the load on the propeller to control the engine R.P.M.