That's why nobody really minds you calling it centrifugal force - it's just easier than explaining it to people like you (Lu).
It's the same when talking about Gyroscpic Precession - It's just easier to explain it like that to people to avoid having to explain what really is going on.
Getting back on topic:
heedm's contribution about a NON-spinning blade in a wind tunnel being commanded to make a sinusoidal flapping motion is a brilliant thought experiment. (maybe wrong - but brilliant)
Also the natural frequency in flap being the same as the rotational frequency. - very nice.
The vacuum is quite good also.(though I think the disc will tilt right if you put the stick forward)
In the NON rotating example at Amplitude zero the upward flapping velocity is max and its accn is zero. If it's Accn is to be zero then neutral pitch would be insuficient implying that the aerodydamic damping (change in relative airflow due to flapping velocity) requires a delay in phase of pitch input. (Heedm:"You stabilize with the blade pitch and flapping position to be 180 degrees out of phase (ie minimum pitch at maximum flap)." So really 160 degrees for example.
-This is why 90deg is not the correct phase delay for a Bell family rotor system
(((Now that's for a blade with Mass but IF it was a blade of Zero(negligable) mass then the phase would be 90deg (neglecting 'damping'))))
If there is a restoring force (spring) of the right magnitude for the mass's oscillation to be in tune with the frequency of the flapping you are demmanding (like there is for a blade) Then you approximately bring the phase around another 90deg..... 160-90=70
Basically a blade tries to flap to the flap position commanded by the pitch and although it is damped in trying to get there. After it has endured 178 degrees of being told to go somewhere it finds it already got there a few degrees before (maybe by 170deg) It is therefore effectively not being commanded to go that way anymore - it's flapped position now being that which one would expect for the pitch.
So -- it's got to be less than 90deg.
And it doesn't really matter that some manufacturers don't incorporate this phenomenon in there designs. Since the pilot sorts out all the disymetries of lift by arranging a disymetry of pitch such that noattitude changes occur - he does this at a higher frequency than Lu grasps these concepts.
Gotta go...