Retreating Blade Stall - Compensation

Compensation

Most helicopter designs compensate for this by incorporating a certain degree of horizontal "flap" movement of the rotor blades. When flapping, a rotor blade will travel upward during its advance, creating a lesser Angle of Attack (AOA) and therefore lesser lift. When the blade retreats, the blade falls downward again, increasing the AOA and therefore generating greater lift.

There are three general designs. The earliest, and by far, least common design today, is the fully rigid rotor system; the blades are rigidly fixed to the rotor hub but made of a flexible material that allows some degree of flap.

Semi-rigid rotor systems have a horizontal hinge at the base of the blades that allow flap as they rotate. By necessity they always have an even number of blades, as each opposing pair is mechanically connected to prevent vibration.

Fully articulated rotor systems use a combination of flapping and a horizontal motion that moves the retreating blades forward slightly and moves them back again on the advancing side, thus creating more relative airflow and lift on the retreating side at the expense of the advancing side.

In all cases, the pilot may compensate the induced roll with left or right cyclic control input (as determined by the rotation of the rotor) up to a degree. However, the rapid rate of change of blade flex and angle of attack causes uncontrolled longitudinal twist and severe vibration in later stages, resulting in total loss of cyclic control if left unchecked.

Assuming no rotor damage, recovery from the condition is possible by using the procedure described below under Flight performance during a retreating blade stall.