Airflow Reversal Airflow Reversal will normally occur before retreating blade stall. You will recall that the airflow velocity is progressively reduced along a blade from being highest at the tip, to lowest at the root end. If the velocity is 300mph at the tip, it is feasible for the velocity to be as low as 100mph at the root. When higher speeds are attempted, the airflow across the root end of the blade can actually reverse and travel from the trailing edge to the leading edge. This is because the airflow velocity produced by the forward speed is greater than that being produced by the rotor blades turning.
Airflow reversal is counter-productive to producing lift and rotor thrust.
To reduce the effects of lift variations from the root to the tip of a blade the manufacturer will either twist the blade along its length, or apply a taper to the blade. Twist is the reduction of angle of attack from the root to the tip. Because the tip is travelling faster than the root, the angle of attack must be reduced toward the tip to maintain the same amount of lift at the tip and the root ends. Taper is the gradual reduction of the width of a blade from the leading edge to the trailing edge. By reducing the chord line from the root to the tip, less surface area is available for the airflow to act on to produce lift.