Definition
On a helicopter rotor system, the rotor blade that is moving in the same direction as the helicopter's flight path during forward flight. Because its motion is aligned with the aircraft's forward speed, the blade's airspeed relative to the oncoming air equals its rotational speed minus the helicopter's forward speed.
Plain English
In forward flight, one rotor blade is sweeping forward into the wind, and the opposite blade is sweeping backward, away from the wind. The one moving backward relative to the air is the retreating blade. Because the helicopter is flying forward while this blade moves backward, the air flowing over it is slower than the air over the blade on the other side.
Context Anchor
Seen in helicopter aerodynamics, especially when discussing lift balance across the rotor disc and limits on forward speed.
Derivation
Retreating comes from Latin retrahere, 'to draw back.' The blade is literally drawing back relative to the helicopter's forward motion through the air.
Why Pilots Care
Uncompensated retreating-blade stall limits forward speed and can produce violent roll if the pilot exceeds the rotor’s design envelope.
Grounding Statement
Picture a helicopter flying forward while the rotor spins: on one side, a blade is moving with the helicopter’s path, and on the other side, the retreating blade is moving against it.
Intuition Check
“Retreating” does not mean the blade is physically backing out of the rotor or slowing down. It means that, during that part of its circular path, the blade is moving opposite the helicopter’s forward travel.
Example Sentence 1
As the helicopter accelerated, the pilot watched for the early signs of retreating blade stall, which can occur at high forward airspeeds.
Example Sentence 2
During preflight the mechanic checks the retreating-blade area for nicks that could worsen stall tendencies in flight.