Lu,
Why not contact you "friend" Ray Prouty and ask him to explain phase lag. While you're on the phone why not ask himw to explain why it is that one particularly well known book "Helicopter Performance Stability and Control" ISBN 0-89464-929-9 in the chapter "Rotor flapping characteristics" discusses phase lags less than 90 degrees for rotor with hinge offset. Better still contact the writer - Ray Prouty.
