In forward flight:

• The relative airflow over the advancing side is higher due to the helicopter’s forward speed, and
• The relative airflow on the retreating side is lower.

This dissymmetry of lift increases as forward speed increases.

To generate the same amount of lift across the rotor disk:

• The advancing blade flaps up and
• The retreating blade flaps down.

This causes the AOA to:

• decrease on the advancing blade, which reduces lift, and
• increase on the retreating blade, which increases lift.

At some point as the forward speed increases, the low blade speed on the retreating blade and its high AOA cause a stall and loss of lift. The stall starts at the tip, then moves inboard because the blade is flapping down to try and compensate for the lost lift, until its stalling angle is reached.

Thus, it is obvious that Retreating blade stall is a factor in limiting a helicopter’s never-exceed speed (VNE).

The factors that may cause Retreating Blade Stall are:

• High All-Up Mass (AUM)
• Low rotor RPM
• High Density Altitude
• Turbulence
• Steep and/or abrupt turns, at high forward air speeds (high "G" conditions)