In the aerodynamics of rotorcraft such as helicopters, phase lag refers to the angular difference between the point at which a control input to a rotor blade occurs and the point of maximum displacement of the blade in response to that control input, said displacement occurring in the direction of rotor rotation. Phase lag differs from advance angle in that the latter refers to the mechanically fixed angle between the pitch link attachments at the blade and swashplate. Phase lag can vary as a function of rotor tilt rate, ratio of aerodynamic damping to blade inertial forces (Lock number), offset of flapping hinge from axis of rotation (e/R ratio), and coupling of blade flap, drag, and feather motions, and often results in cross-coupling between the aircraft control axes; advance angle is fixed and cannot vary.
As a consequence of the phase lag, to roll a rotorcraft to the left or right would theoretically require a forward or backward cyclic if there was no mechanical correction. The rotor control system is angularly shifted as necessary to compensate for phase lag and provide helicopter response that matches movement of the cyclic stick.