Retreating Blade Stall
Helicopters experience unequal lift across the rotor system since there is an advancing blade and a retreating blade. This is called dissymmetry of lift. Helicopters equalize this dissymmetry of lift with blade flapping. The advancing blade flaps up to decrease the angle of attack and the retreating blade flaps down to increase the angle of attack.
When the critical angle of attack on the retreating blade is reached (15 degrees), the helicopter will vibrate and the nose pitches up. This is called retreating blade stall and can be corrected by lowering the collective some and slowing down with aft cyclic. Other things besides speed beyond VNE can cause this. Other contributors are 1. Low RPM 2. Steep or abrupt maneuvers 3. Turbulent air 4. High Gross weight/ Density altitude.
Remember the VNE (never exceed speed) is partially due to retreating blade stall but is also due to the actual structural limitations of the aircraft.
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