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!
A rotor, being part of the real universe, obeys all of its laws of physics, including the conservation of angular momentum.
A rotor at constant RPM is in a state of zero net torque: the forward torque applied by the engine equals the reverse torque applied by the drag of the air, the friction of the mountings/mechanisms, etc. An increase or decrease of one of these opposite torques, unbalances the sum from zero, and a change in RPM (or, rotational acceleration) occurs.
If RPM changes due to the above things, momentum is not conserved (within the rotor system), it's being taken from or given to the outside world (in from fuel energy, out to turbulence due to lift/drag, etc.) If RPM changes due to the inward mass shift due to coning, (figure skater effect) it's due to the conservation of angular momentum. You just have to be meticulous with your accounting when multiple effects may be happening at the same time.