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It is a common belief that two identical wheels spinning in opposite directions and the same speed have a net zero gyroscopic stability. The basis for this bel

It is a common belief that two identical wheels spinning in opposite directions and the same speed have a net zero gyroscopic stability. The basis for this belief is usually false, but the assertion is true. I've done both the math and the experiment - it's in one of the lectures I give on "Spinning Things". In short, I believe you to be wrong, and this is based on my personal experiments and calculations. I would be interested to know the basis of your claim. When computed in detail, gyroscopic stability is really the effect that torque applied in one plane manifests itself as movement in another. The direction of this movement depends on the direction of spin, and they cancel each other out when you have two wheels spinning in opposite directions. My independent experiments and calculations were done before I read anything formal about it, and it agrees with the work of people who design devices that run on satellites and the ISS. I'd need a lot of convincing that I'm wrong. This is independent of the other effects that help to keep a bike upright, such as the steering geometry. I have experience of that too, having raced bikes with very small rake, and toured on bikes with a larger rake. The difference is unmistakeable, and unrelated to the gyroscopic issues.

hristov 16y ago
Well here is an experiment that I have done. I used a toy double blade helicopter, like these ones: http://www.rc-airplane-world.com/coaxial-rc-helicopters.html http://www.rc-airplane-world.com/coaxial-rc-helicopters.html And it sure seemed to have a lot of rotational inertia. If you were right, then I could touch one side of the helicopter slightly and since it has no rotational inertia it would tilt to the side and completely change direction and fly off somewhere into the wall or the floor. This did not happen however, even when I tried to tilt it the helicopter remained horizontal. And please do not tell me that a $50 toy has an active stability system with internal gyros and all the required electronics and super fast servos. And now that I think of it, I remember that the russians have been making counter rotational helicopters for ages. See for example this one: http://en.wikipedia.org/wiki/Kamov_Ka-25 http://en.wikipedia.org/wiki/Kamov_Ka-25 How does this helicopter stay horizontally stable in the air and does not fly off on a tangent when tilted by the slightest cross wind? Note that this one was designed in the late 50's before any electronic active stability systems could have been invented.
- RiderOfGiraffes 16y ago
  I have limited personal knowledge of RC helicopters, but a great deal of experience with rotational systems. Colleagues tell me that the counter-rotational systemson helicoptors are stable because the CoG is hanging from a point quite a long way above, and cross winds are applied more-or-less at the CoG because of the cross-sectional area distribution. I don't doubt that your experience of a toy double blade helicopter is that it seems to have a lot of rotational inertia. I am certain that it's not due to the gyroscopic stability of exactly matched counter-rotating blades.
  - hristov 16y ago
    If you think about it, this explanation cannot be true. The main force the helicopter exerts on its surroundings is along the axis of rotation. If you tilt the axis of rotation significantly in any direction, the direction of the force will change significantly and the motion of the helicopter will also change significantly. Again, this does not happen.
    - RiderOfGiraffes 16y ago
      It's unclear what you mean by the "this" in the "this does not happen" However ... A helicopter changes direction by using the cyclic setting on its rotors. Then you get more lift from one part of the rotor cycle and less from another. That causes the plane of the rotors to tilt, and then the helicopter goes off in the appropriate direction. For example, having more lift in the rear portion of the cycle and less in the front means the helicopter gets tipped forward, effectively like lifting one part of a plate. The down-draft now has a significant rearwards component, so the rotors get pushed forward, taking the helicopter with it. If you then change the cyclic to "flat" so there is identical lift everywhere, the weight of the helicopter dangling from the rotors causes it come come back upright (after swinging a bit) It's a weird experience sitting in a helicopter when it's doing this, especially if you're mostly accustomed to ordinary aeroplanes. I don't see any contradiction in any of this, and it really doesn't seem to have any bearing on the fact that contra-rotating disks have no net gyroscopic effect.

hristov 16y ago

Well here is an experiment that I have done. I used a toy double blade helicopter, like these ones: http://www.rc-airplane-world.com/coaxial-rc-helicopters.html http://www.rc-airplane-world.com/coaxial-rc-helicopters.html And it sure seemed to have a lot of rotational inertia. If you were right, then I could touch one side of the helicopter slightly and since it has no rotational inertia it would tilt to the side and completely change direction and fly off somewhere into the wall or the floor. This did not happen however, even when I tried to tilt it the helicopter remained horizontal. And please do not tell me that a $50 toy has an active stability system with internal gyros and all the required electronics and super fast servos. And now that I think of it, I remember that the russians have been making counter rotational helicopters for ages. See for example this one: http://en.wikipedia.org/wiki/Kamov_Ka-25 http://en.wikipedia.org/wiki/Kamov_Ka-25 How does this helicopter stay horizontally stable in the air and does not fly off on a tangent when tilted by the slightest cross wind? Note that this one was designed in the late 50's before any electronic active stability systems could have been invented.

RiderOfGiraffes 16y ago
I have limited personal knowledge of RC helicopters, but a great deal of experience with rotational systems. Colleagues tell me that the counter-rotational systemson helicoptors are stable because the CoG is hanging from a point quite a long way above, and cross winds are applied more-or-less at the CoG because of the cross-sectional area distribution. I don't doubt that your experience of a toy double blade helicopter is that it seems to have a lot of rotational inertia. I am certain that it's not due to the gyroscopic stability of exactly matched counter-rotating blades.
- hristov 16y ago
  If you think about it, this explanation cannot be true. The main force the helicopter exerts on its surroundings is along the axis of rotation. If you tilt the axis of rotation significantly in any direction, the direction of the force will change significantly and the motion of the helicopter will also change significantly. Again, this does not happen.
  - RiderOfGiraffes 16y ago
    It's unclear what you mean by the "this" in the "this does not happen" However ... A helicopter changes direction by using the cyclic setting on its rotors. Then you get more lift from one part of the rotor cycle and less from another. That causes the plane of the rotors to tilt, and then the helicopter goes off in the appropriate direction. For example, having more lift in the rear portion of the cycle and less in the front means the helicopter gets tipped forward, effectively like lifting one part of a plate. The down-draft now has a significant rearwards component, so the rotors get pushed forward, taking the helicopter with it. If you then change the cyclic to "flat" so there is identical lift everywhere, the weight of the helicopter dangling from the rotors causes it come come back upright (after swinging a bit) It's a weird experience sitting in a helicopter when it's doing this, especially if you're mostly accustomed to ordinary aeroplanes. I don't see any contradiction in any of this, and it really doesn't seem to have any bearing on the fact that contra-rotating disks have no net gyroscopic effect.