Originally Posted by
212man
I don't know what the FAA manual says, but in reality a rotor is not demonstrating conservation of angular momentum at all! It can hardly be described as being in a zero torque, frictionless, environment!
Its basically; When a blade flaps up, its center of mass moves inward towards the hub, causing it to accelerate. When the blade flaps down, the center of mass moves outward towards the tip, causing it to decelerate.
Fully-articulated rotors absorb this acceleration and deceleration through the lead/lag hinges. Semi-ridged rotors don't experience it to the extent that fully-articulated rotors do, as the center of mass remains virtually unchanged as the system teeters, due to the underslinging of the blades.