On Thu, 19 Sep 2002, John Carmack wrote:
> I'm not sure about that. If you have a shaft with two wheels on it,
> spinning either wheel either way will provide a gyroscopic
> "stiffness". Two spinning wheels, even going opposite directions, should
> have twice the stiffness, not none.
Nope. Gyroscopic stiffness is simply conservation of angular momentum.
When you have a spinning wheel, and you apply a torque to it for a period
of time -- that is, you impart some new angular momentum to it -- that new
angular momentum adds to the existing angular momentum, using vector
addition. If the existing angular-momentum vector is large, adding a
small new angular-momentum vector at right angles to it won't move the
final angular-momentum vector much. That's gyroscopic stiffness.
The net angular momentum of two counter-rotating wheels is zero, so
there is no gyroscopic stiffness to be had.
(Adding angular-momentum vectors is also a simple way to see how
precession of a gyro works.)
> ...I don't think additional rotating masses can't help you do a
> "plane change" of a rotating mass...
There may be large forces induced in the structure that connects the
two rotating masses, but the external force is zero.
Henry Spencer
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