# Gyroscopic propulsion? Eh?

I have two questions. Not counting the “eh”.

1. How do gyroscopes work? I never really got the explanation in Physics. Maybe now that I can do calculus I could really understand it…

2. Is “gyroscopic propulsion” scientifically possible or is it something like perpetual motion which Art Bell fans adamantly believe in while accusing the “establishment” of supressing the evidence, etc.?

Rav

Well I don’t have a good answer to (2), except that I’m wondering if it’s related to the flywheel effect. That is, you can store energy quite efficiently in a spinning wheel (provided you have some good axle grease), and get it out by gearing it to your car’s wheels. Then when you want to brake, you just put the energy back into the flywheel, also with a complicated gearing system. But I don’t know if that’s ever called gyroscopic propulsion, and of course it isn’t an energy source, but a storage system only.

My answer to (1) is hard to describe. Imagine a globe, sitting on the South Pole. But it’s never going to be exactly on the South Pole, it a little bit off-center. Plus, there is a lump of putty sitting smack on the North Pole.

Since the globe if off-center, the world will start rolling. That should be simple (though I don’t know if I’ve explained it well enough?). If the globe is sitting on the part of the South Pole slightly nearer South America, it will start rolling, and it will across the table with both American continents at touching the table at some point, until the putty hits the table.

Let’s say, however, that our little globe is spinning, just like the Earth does, with the equator fixed. Our globe can’t roll very far, in the direction of the Americas, because by the time it’s gotten anywhere, the Americas have spun around to the wrong side, and they retain their downward momentum. The side that “wanted” to be going up, also due to momentum, has now replaced South America, and cancels out the “desire” to go down.

So the lump of putty is stuck at the top. The heavier the putty, the faster the globe has to spin to keep itself from rolling off the table. Try spinning a child’s top upside-down some time - it will usually work if you spin it fast enough, but it’s more unstable (and more fun, if you ask me).

Nothing I write about any person or group should be applied to a larger group.

Oops. Guess I should have read the link before taking a shot at (2). Err, I was color-blind for a second. That’s the ticket.

I also apologize for my third paragraph. It should read:

Since the globe if off-center, the world will start rolling. That should be simple to visualize (though I don’t know if I’ve explained it well enough?). If the globe is sitting on the part of the South Pole slightly nearer South America, it will start rolling, and it will roll across the table with the American continents touching the table at some point, until the putty hits the table.

Better?

How do gyroscopes work? Well, they have a number of properties, and they work primarily because an object in motion tends to remain in motion, until acted upon by an outside force.

Go out into interstellar space. Let me know when you get there.

Ok, so just imagine you have a disk of very strong steel, and five or six very powerful rocket motors on the edge of the disk, all pointed in the same orientation with respect to the periphery of the disk. (All clockwise, for example) We fire off all the rockets, and the disk begins to rotate. This being deep in interstellar space, there is almost no chance that the disk will encounter any friction, and the exhaust of the rockets has a pretty good vector away from the disk as well. So, we get no friction.

Now we have a gyroscope. Force has accelerated the disk, and the material (steel) allows the tensile strength to act as a centripetal force to make our linear acceleration become angular momentum. Now that the gyroscope is spinning, it has certain properties. One of those properties is that the axis of rotation (through the center of the disk, at a right angle to its surface) is more stable in it’s orientation with respect to the rest of the universe than a disk that is not spinning. If a passing molecule of hydrogen hits our disk off center it will have less effect on the orientation of the axis than it would on a non rotating disk, although it would move the disk the same amount through space.

The cause of this is the momentum of the steel in its circular path. To flip a non-rotating disk, you require only the energy necessary to move the mass of the disk. With a rotating disk, the energy of motion is already moving the disk, and to change that motion, you must have enough energy to effect all the current momentum, and then impart a new motion. This resistance to twist in the axis is experienced as “precession” of the axis. What happens is that your new force combines with the energy released by the momentum of the disk, and the resultant vector is at an angle to the force you applied. Any movement which the disk makes that does not alter the orientation of the axis of rotation is unaffected by the angular momentum. A lot of science fiction is based on the impression that gyroscopes have some hold on their position in space, and if that hold could be modified, we could move along magically without any thrust. Sounds so cool, but it just is not so. You can move a gyroscope as easily as any other object; you just can’t turn it around corners as easily.

Before someone brings up the disk rotating faster than light speed let me explain something. First of all, the tensile strength of such a disk is a number of orders of magnitude greater than the forces that bind atoms together. The disk simply flies apart. Secondly, the energy required to accelerate a disk depends on its mass, which would increase as its edges approached light speed. To have even the smallest part of the edges of the disk exceed C would require an infinite power source.

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Wow, thanks. I guess that makes sense. It just feels so weird when you experience it.

I’ve gotta go remove my bicycle tire and experiment with it again. I remember thinking it could precess either way when I moved it while it was spinning, but for some reason it always moved the same way. I’ll see if I can predict it now.

You’d really burst a bunch of these UFO people’s bubbles talking like that.

One of the coolest demonstrations involving gyroscopes that I remember from high school physics involves a freely rotating stool and a bicycle tire.
Sit on the stool and grab the bike tire by handles on the axle. Start the tire spinning. Now if you incline the axis of rotation counterclockwise (as if one were turning a steering wheel to the left), you and the stool will start spinning to the right. Likewise, if you incline the axis clockwise, you and the stool will spin to the right.
The explanation would involve a more in-depth explanation of angular momentum than I have time for right now, but contact me if you want to know more.

Forrest

BTW –

That’s one of the problems with using flywheels for energy storage in moving vehicles. If you have a big flywheel (I mean really big) in a car and you make a turn it generates all sorts of non-intuitive torques that give the driver fits. Or, with the flywheel mounted horizontally, going over a hill could make the car turn left!

## Also – for your demo, wouldn’t the direction the seat spun depend on which way the wheel was spinning too?

“Vandelay!! Say Vandelay!!”

I meant to add that that is not a purely academic problem – a jet engine has a lot of rotating mass. One of my M.E. professors told us that an early model of a jet fighter aircraft, which was essentially a jet engine with wings and a tail, came apart in flight because they hadn’t accounted for the gyroscopic torques generated by the jet rotor. It wasn’t that they didn’t know about them but they had gotten in the habit of neglecting them because they are small in a propellor-driven plane. I don’t know if it’s actually true, but it was a good story.

Ravenous, I took one look at the link you provided for “gyroscopic propulsion” in the OP, and immediately discerned that the author of that webpage is a certified loonball.

He reminds me of Joseph Newman.

http://mars.spaceports.com/~over/movies/ has some videos of such devices in action

Check it out and judge for yourself.

Gyroscopic precession is not just a problem in jet aircraft. In a propeller aircraft, changing pitch will cause the aircraft to yaw left or right. This isn’t that big a deal in flight with most aircraft, but can be a big deal on takeoff, especially with tailwheel aircraft because there is a large pitch change when you raise the tail, and you often aren’t going fast enough for the control surfaces to have a lot of power to correct it.

More than one pilot has put his tailwheel aircraft into the dirt when it slewed off the runway at rotation. This was a prettty big problem for WWII fighters, which had big propellers with a lot of rotating mass. Many Corsairs and P47’s and the like crashed this way.

Another area where precession caused problems in aircraft is in the gyro-stabilized flight instruments like the direction indicator. Friction in the bearings of the gyro will cause it to precess and become more and more inaccurate during the flight. Instruments with worn bearings can become almost unusable because of this.