Flap back

Flap back or blowback is a phenomenon affecting the rotor of a helicopter as it overcomes dissymmetry of lift through flapping. In forward flight, rotor blades experience more lift as they rotate forward. This increased lift is a result of an increased relative speed causing the blade to flap up and decrease its angle of attack. As the blade continues to rotate, it achieves its maximum upflapping displacement over the nose of the aircraft and maximum downflapping displacement over the tail. This results in the rotor disk being tilted to the rear and is referred to as flap back, as if the rotor disk had flapped or tilted back, or as if it had been blown back by the relative wind. The effect is more pronounced at higher airspeeds but more easily recognized as the aircraft accelerates to translational lift airspeeds from a hover. Flap back results in a slowing of the aircraft and the pilot counters the effect by applying forward input to the cyclic control.