Retreating blade stall

Retreating blade stall is a hazardous flight condition in helicopters and other rotary wing aircraft, where the rotor blade rotating away from the direction of flight stalls. The stall is due to low relative airspeed and/or excessive angle of attack (or AOA). Retreating blade stall is the primary limiting factor of a helicopter's airspeed, and the reason even the fastest helicopters can only fly slightly faster than 200 knots.

Advancing vs. retreating blades

A rotor blade that is moving in the same direction as the aircraft is called the "advancing blade" and the blade moving in the opposite direction is called the "retreating blade."

Balancing lift across the rotor disc is important to a helicopter's stability. The amount of lift generated by an airfoil is proportionate to its airspeed. In a zero airspeed hover the rotor blades, regardless of their position in rotation, have equal airspeeds and therefore equal lift. In forward flight the advancing blade has a higher airspeed than the retreating blade, creating unequal lift across the rotor disc.

A fuller treatment is provided in dissymmetry of lift.

Compensation

Most helicopter designs compensate for this by incorporating a certain degree of "flap" in the blades. Rather than being rigid, the rotor blades are built to have a certain degree of flex. As such, the blade flexes or flaps up during its advance, creating a smaller AOA and lower lift. When the blade retreats, the blade flexes or flaps down, increasing the AOA and generating more lift.

Also, the cyclic pitch control (the "joystick") is used to increase the pitch of the retreating blade and decrease the pitch of the advancing blade. This differential in pitch generates more lift on retreating side of the aircraft, balancing out the effect of the slower airspeed of the retreating blade.

Failure

These compensations can only do so much, and it is possible for a rotary-wing aircraft to move so quickly that the retreating blade no longer moves fast enough relative to the air to provide lift. This situation is called retreating blade stall.All airfoils have a stall speed, defined as the minimum speed at which the airfoil must move through the air to generate lift. Below this speed, slow-moving turbulent air replaces the fast-moving slip air going over the airfoil, disrupting the Bernoulli effect that generates lift. When a fixed-wing aircraft drops below its stall speed, the entire aircraft loses lift and enters a condition called a stall. The usual results of a fixed-wing stall are a sharp drop in aircraft altitude and a dive. Stalls in fixed-wing aircraft are often a recoverable event.In a retreating-blade stall, however, only a portion of the airfoil experiences a stall. The advancing blade continues to generate lift, but the retreating blade enters a stall condition.

Flight performance during a retreating blade stall

As the aircraft approaches the airspeed at which it will encounter retreating blade stall, the aircraft will shudder and the nose will begin to pitch up. The resultant upward pitching of the aircraft's nose will begin to correct the situation as it results in slowing the aircraft. However, if uncorrected, and if the aircraft continues to accelerate, the aircraft may roll in the direction of the retreating blade.

Recovery involves decreasing the angle of attack and allowing the retreating blade to recover from its stalled condition. This is done by lowering the collective pitch.

The stall will not happen due to speed, but a higher pitch on the retreating side. If you are at speed and immediately reduce collective you have potentially put the blade back in a non-stalled state, but you will now begin to slow down, descend and level out.

Causes of retreating blade stall

Retreating blade stall is more likely to occur when the following conditions exist either alone or in combination:
* High gross weight
* High airspeed
* Low rotor RPM
* High density altitude
* Steep or abrupt turns
* Turbulent ambient air

References

"Basic Helicopter Handbook", US Department of Transportation, Federal Aviation Administration