Helicopter Dynamics: Gyroscopic Precession
Only for those with a perversion for the technical.
In some not-so-distant thread, an attempt was made to show that gyroscopic precession is not a factor in the activities of a helicopter's rotor. The bicycle wheel, which is normally used to demonstrate gyroscopic precession, was modified. This modified wheel consisted of only four spokes, a hub and the lose connections between them. The wheel (or what's left of it) now more closely resembles a helicopter's rotor. This wheel combined with the notes of Prouty and the remarks by Nick tends to support the idea of no gyroscopic precession. Current textbooks tend to also hold this position.
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Now for the fiendish part;
I believe that a simple teetering rotor, with no delta3, exhibits a 90-degree phase lag, which is totally caused by the blade flying to position. Perhaps aerodynamic precession, but not gyroscopic precession.
As mechanical restrictions are imposed upon a blade's ability to flap, the phase angle will decrease from 90-degrees. Any increasing of the flapping hinge's offset, or an increasing of the dampening of the flapping hinge, or a stiffening of the blade will result in an even smaller phase angle.
Now consider an airplane's propeller. We know that the propeller is subject to gyroscopic precession. Also, the primary difference between a variable pitch propeller and a helicopter's rotor is their rigidity. I would suggest that it is only the out-of-plane (flapping) rigidity that we are concerned with.
If we take this idea of rotor rigidity to the extreme, we will have a totally rigid rotor and a phase angle of zero degrees. If the maximum pitch is applied on the right side, then the maximum rotor thrust (not flap - because the rigid rotor is rigidly attached to the fuselage) will be exhibited on the right side. The helicopter will obviously roll to the left, but, with trepidation, I feel that it will also want to pitch down slightly at the nose. I believe that this small nose down pitch is the result of gyroscopic precession, which in turn is the result of changing the attitude of this very rigid rotor disk (read as gyroscope disk).
Anyone who read this far, want to tear it apart?
[ 03 August 2001: Message edited by: Dave Jackson ]