Definition
A condition in helicopter flight in which the rotor blade moving away from the direction of flight (the retreating blade) experiences an angle of attack high enough to cause it to stall. As forward airspeed increases, the retreating blade sees progressively lower relative airflow over it, and the pitch must be increased to maintain lift balance with the advancing blade. At a critical point, the angle of attack on the retreating blade exceeds its stalling angle and lift is lost over that side of the rotor disc. This typically produces a pitch-up, a roll toward the retreating side, and vibration, and it sets the never-exceed speed (VNE) for the helicopter.
Plain English
On a helicopter flying forward, the blade moving backward (against the direction of flight) has less air flowing over it. To make up for this, it has to be tilted more steeply to keep producing lift. If the helicopter flies too fast, that blade gets tilted so steeply it stops producing lift altogether, and the helicopter pitches up, rolls to one side, and shakes.
Context Anchor
Encountered in helicopter aerodynamics, especially when discussing high forward airspeed limits and rotor performance.
Derivation
Retreating' comes from the Latin retrahere, 'to draw back.' The retreating blade is literally the one drawing back — moving rearward relative to the helicopter's direction of travel. 'Stall' here is the same aerodynamic stall that fixed-wing pilots know: airflow separating from a lifting surface when its angle of attack gets too high.
Why Pilots Care
Retreating blade stall produces vibration, a rolling moment toward the retreating side, and a hard limit on safe forward speed that helicopter pilots must respect to maintain control.
Grounding Statement
Picture a helicopter flying fast forward: the blade moving backward on one side has less air flowing over it than the blade moving forward on the other side.
Intuition Check
Retreating blade stall does not mean the rotor stops turning or the engine stalls. It means the backward-moving side of the rotor can no longer make smooth lift because the blade angle is too steep for the airflow.
Example Sentence 1
The instructor explained that flying near VNE in turbulence raises the risk of retreating blade stall, so they kept the cruise speed well below the red line.
Example Sentence 2
Never-exceed speed is set to keep the helicopter below the onset of retreating blade stall in level flight.