In the aerodynamics of rotorcraft like 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.
Phase lag may vary depending on 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.
Because of phase-lag, rolling a rotorcraft to the left or right would theoretically require a forward or backward cyclic if there were no mechanical correction.
The rotor control system is angularly shifted as much as necessary to compensate for phase-lag and provide helicopter response that matches movement of the cyclic stick.
The amount of phase lag depends on the distance of the flapping hinge from the rotor hub.