Originally Posted by
Wide Mouth Frog
I am genuinely interested in the answer. Many instructors have been told over the years that using torque induced precession as a first order explanation for helicopter control phase lag is either plain wrong, or a gross oversimplification. It would be nice to have that confirmed or squashed.
The off axis torque that is induced in a rotor system is the cyclic component of the aeodynamic forces. This creates extra lift on one side of the mast and reduced lift on the other side. Lets say more lift on the left side and less lift on the right side of the helicopter with a rotor turning counterclockwise viewed from above. This does not lead to a sideways tilt of the rotor disk (or rotor cone), but this leads to a forward tilt. This means that the tilt hase 90 degree phase lag. This large scale phenomenon is called the gyroscopic effect.
But the gyroscopic effect is not some magic or extra phenomenon but is an emergent effect from the normal physical behavior of the masses rotating about one center.
When looking at a smaller scale for example at the movement of an individual rotor blade (or a narrow angular section of a gyroscope disk) then it becomes apparent that this delay is only caused by the time it takes to accelerate the blade upwards and downwards. The blade does not instantly reach its highest vertical displacement at the angular position with the highest vertical force. It has its highest vertical acceleration at the point of highest vertical force.
Now this could lead to a highest displacement at any delay angle, not necessarily at 90 degrees. Just looking at the aerodynamic forces one would expect a 180 degree phase lag, because the upwards movement needs to be slowed down first by downward forces.
But the blade is still attached to a center point so the centripetal force has a downward component relative to the original plane of rotation. This downward component leads to the blade decelerating towards its highest point much earlier than from the external forces, and then moving downward. This downward component of the centripetal force is proportional to the rotation speed, the flapping displacement and the mass of the blade.
This leads to a 0 degree phase shift between external accelerating force and vertical speed. Under ideal conditions (one unmoving center point) this leads to a 90 degree phase shift of the displacements.
Real helicopter rotors are not ideal gyroscopes and the rotor blade flapps about a hinge location that is not at the center of rotation. This leads to a phase lag somewhat smaller than 90 degrees.
But this imperfection is small and the phade lag is close to 90 degrees on real helicopters, especially with articulated rotor systems. Therefore I would call a helicopter rotor a gyro.