Originally Posted by
Vessbot
Are phase lag and gyroscopic precession not the same thing? How are the physics different?
They are not. Phase lag is "around 90°" but not exactly, and varies for every helicopter. The exact angle has to do with the relationship between the resonance of the flapping behaviour of the blades compared to the rpm of the rotor system as a whole. Offset flapping hinges (or apparent offset 'hinges' such as in rigid rotor systems) increase the flapping resonance so that it is slightly
higher than 1/revolution meaning that for an applied force, the peak of the flapping oscillation will occur slightly less than 90° later.
To be quite honest, most of the maths is far beyond me, but if you're keen to dig deeper on the fundamental and raw physics of rotating blade motion then Leishman, Principles of Helicopter Aerodynamics, 2nd Ed. 2006, p171-188 will be of interest to you. The book is quite expensive, so maybe
this fragmented preview will be enough.