flap back

blow back

Side view of flapback due to change in wind from the front.The changes in the helicopter disc attitude that occur without any control movement by the pilot. In forward flight, the rotor disc has a natural tendency to tilt back (longitudinally) because of the dissymmetry of lift that would be produced if the advancing blade was not allowed to reduce its angle of attack and the retreating blade increased its angle of attack. Also called blow back and flap back.