# Cause of stability in counter-rotating gyros

## Main Question or Discussion Point

http://en.wikipedia.org/wiki/Gyro_Monorail#Principles_of_operation

It appears that counter-rotating gyros can indeed provide stability without precession.

Is this simply because of the fact that when you accelerate the rotation of the axis of adjacent counter-rotating spinning wheels you are increasing the speed of the edges of both of the wheels with respect to inertial frames (though not in the non-inertial frame of the wheel), thus requiring that one does work on the wheels to accomplish this, requiring energy, without which, would forbid rotation of the wheels' shared axis?

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AlephZero
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However, an identical gyro spinning in the opposite sense will cancel the roll torque which is causing the instability, and if it is forced to precess in the opposite direction to the first gyro will produce a control torque in the same direction.
(My bold-face emphasis).

Two contra-rotating gyros with the same axis will simply cancel each other out. That is the case in designs like twin-rotor helicopters, some aircraft propellors, jet engines with contra-rotating shafts, etc.

The wiki page doesn't elaborate on how to "force" the gyros to behave as required but I would guess the word "force" implies "requires energy to make it work".

Counter-rotating anythings are stable because of conservation of angular momentum. The total angular momentum of the rotating parts stays zero because they cancel each other out, so the base that the rotating parts stays stable. A single helicopter rotor speeding up gains angular momentum. In order for momentum to be conserved, with no second rotor to cancel the torque, the helicopter itself would spin in the opposite direction.