I'm starting to comment more on helicopter stuff and less on physics stuff. I feel I understand the helicopter stuff, but I'll still try restrict my comments to areas where I have the appropriate background.
Lu said, "If gyroscopic precession plays no part in the tilting of the disc either as a single entity or as individual blades then please explain this."
I guess it gets down to semantics. As best as I can tell, the definition of gyroscopic precession is limited to rotating bodies whose geometry allows the 90 degree apparent lag. Rotor systems whose axis of rotation does not pass through the flapping hinges are one example of a geometry that won't allow a 90 degree lag. Answering your question, since the definition of gyroscopic precession doesn't really apply, it is fair to say it plays no part.
For a teetering rotor, it seems that gyroscopic precession is a valid description.
Since the "blade flying to position" derivation can be used for a general derivation of gyroscopic precession, I think that in special cases the two theories are the same. One (fly to pos.) requiring more basics, the other (GP) accepting more terminology.
Lu also said, "In the light of the above I ask why the 90-degrees if gyroscopic precession is not involved. Why couldn’t they just put the control inputs anywhere and just let the blades seek their own position."
Read through my previous long post where I talk about the pendulum. The natural frequency of the blade acting as a pendulum is exactly one per revolution when the axis of rotation passes through the flapping hinge. It is less than 90 degrees when the flapping hinge is offset.
helmetfire, I believe that flapping to equality applies only with translational movement of the helicopter. In a zero wind hover, there is no flapping to equality.
Matthew.
[ 10 August 2001: Message edited by: heedm ]