Helicopter Phase Lag

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, said displacement occurring in the direction of rotor rotation. 'Phase lag' differs from 'advance angle' in the latter referring to the mechanically fixed angle between the pitch link attachments at the blade and the swashplate. Phase lag may vary depending on rotor tilt rate, the ratio of aerodynamic damping to blade inertial forces (Lock number), offset of flapping hinge from the 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.

Consequent to phase-lag, rolling 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 much as necessary to compensate for phase lag and provide a helicopter response that matches the movement of the cyclic stick.


The following project demonstrates the phase lag of helicopters with variable CG. The project is interactive and the user can turn Phase lag on or off and change the direction of the rotation of the rotor blades. The following link will redirect you to the project page.
(For the mobile version, please go to the bottom of the page)



Simple Phase Lag Simulation in Helicopters to understand the phenomenon of Phase lag in Helicopters that occur due to the Gyroscopic precession enabled due to Rotor Movement.
Press P to enable or disable Phase Lag
Press C to change the rotation of the rotor blades
Press Left, Right, Down, and Up arrows for movement
The Arrow points towards the direction of the arrow keys pressed

Phase Lag can not be enabled or disabled in real life while the rotor blades are spinning. This is only used as a learning tool for students










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