To: Dave Jackson
Now lets consider a 'type' of 2-bladed helicopter rotor, which has round 'no lift' blades. We'll make it from a 30-foot long by 2" diameter steel pipe. Drill a hole through both walls at 15-feet from the end and weld in a rod (axle), which extends out both holes. Now spin the horizontal pipe, somehow, on its vertical (rod) axis to about 400 rpm. When released, it will slow and fall over within a second or two.
This is a very poor example. A circular body passing through an air stream or an air stream passing over a circular body will initiate what is known as “vortex shedding”. Vortex shedding at 400 rpm will set up such a severe high frequency vibration as to cause the rotating object either to self-destruct or to break away from its’ driving axis. If you rotated the pipe as described but in a vacuum you would not have vortex shedding but you would have a device that would exhibit gyroscopic rigidity in space and gyroscopic precession if you had a means of perturbing the rotating mass. The vibration will be the result of the vortex shedding frequency but mainly due to the vibratory frequencies being different at each station on the rotating pipe.
A propeller on an aircraft is similar to your pipe example and your totally rigid rotorhead but it is rotating about a horizontal axis as opposed to a vertical axis. When the aircraft is maneuvered in any way the movement of the aircraft causes a perturbing force on the rotating mass of the propeller and the propeller wants to precess. It cannot precess because the propeller drive shaft is rigidized by the bearings contained within the crankcase so the blades will bend within their elastic limits.
On the V-22 there is a similar situation. When the aircraft is maneuvered in the airplane mode the Proprotor, which is not rigid will precess (flap). This precessing is detected and electronically corrected by inputs to the servomechanism. You will note that when in the airplane mode there is no advancing and retreating blade and the air stream passing over the Proprotor is uniform so in this case you have to accept gyroscopic precession over aerodynamic precession. Also, since the blades are independent from each other each blade is a part of a rotating mass and as such each blade will individually respond to the perturbing force and although independent from each other they respond as a solid disc.