# semi-technical gyroscope idea/question

so a gyroscope as far as i understand, has this huge angular momentum (because of high angular speed), and thus it acts kinda like it was a huge mass, even thought its not heavy. ( of course i am talking about the “original” gyroscope, not electronic one ).

so lets say we wanted to stabilize something, the gyroscope would provide impedance to deflection, yet ( as far as i uderstand ) under constant force it would gradually deflect. but lets say we want a system that can resist deflection for indefinite amount of time.

i am saying, that all you have to do, is connect this gyroscope to an actuator, that as you apply the torque to this system - the actuator will turn the axis of gyroscope in the opposite direction. thus internally the gyroscope will still be slowly succumbing to the force, but externally the system will remain perfectly motionless.

now is more like a math point. to keep resisting a constant force will the speed of rotation of the gyro’s axis ( not gyro itself, just its axis ) have to be increasing linearly or just stay at constant speed ? i think it will just have to stay at constant speed, and this means we will not run into any kind of problems with infinite speeds or anything like that, even if we let time run to infinity.

did i at least get my point across ?

you dopers are too quick to slap useless links into your posts.

to make you happy, i read the pages at your link, but the problem is that i KNOW what a gyroscope is already, i did not ask what it is, why did you sent me off to read about it again ?

i know why, everybody is so afraid to say something that might potentially be not 100% true, or something that one will not find a source for later … that its much simpler to file away useless links, no responsibility in that. BUT HELLO, what is the point ? i can use google too. if i am asking a question then it already means that google can’t answer it, and maybe you’re not reading the question right ( which you admitted to though ).

anyway, if my question is not clear, why dont you ask me to clarify it.

my question is quite simply, if the axis is allowed to rotate around only one axis ( not to deal with precession or whatever ) and a constant force is applied … will the axis turn at constant speed or will it accelerate ?

if the answer is constant speed, then we can do nifty things, namely generate torque for indefinite amount of time without any real point of support, simply by turning the gyro’s axis with a motor.

I’m still not getting it. It takes energy to force the gyro to turn, and friction in the gyro bearings will cause it to precess.

So no, you can’t generate torque for an ‘indefinite period of time’. You can spin up the gyroscope with power, then use it to turn your spacecraft or whatever, until the gyro slows down. Then you have to spin it up again. There are no free lunches.

If you are asking whether a gyroscope can be used as a fixed point so that say, a satellite could be rotated around it, then sure. We do that all the time.

I agree. Why use a motor to torque a gyro to torque something else? The motor itself can do this directly and the only limit to how long the motor can supply torque is the lifetime of the bearings and winding, and brushes if any.

Of course, I could be misunderstanding what the OP is saying because it is in a form that is really easy to misunderstand.

Do you mean like the front wheel of a bicycle, where a wheel spins around the axis and that axis can also swivel in one dimension? And which way do you want to apply force? If support the bike mid-air (frame attached to a repair stand, for example), spin up the front wheel and try to turn the handlebar, I think the handlebar will behave just like a large solid mass:

(torque) = (moment of inertia) * (angular acceleration)

Actually I’m not too sure about the answer, but why don’t you tell me first if I understood the question correctly.

As soon as you put the word infinite into a real world situation, it’s game over. You cannot have infinite speeds or let time run to infinity.

I think you’re confusing infinite with indefinite, which is not helping understanding, but Sam Stone and others have already handled that case as well. Frictional effects can be minimized but not eliminated. Constant force has to come from somewhere; it’s not free for the taking. If you’re constraining the system to avoid precession you are introducing new forces.

I think we’re all agreed that gyroscopes are amazingly useful devices. What’s not coming across is how what you’re trying to do is possible or is different from what already exists.

So, the best that I can guess, you want to use the gyroscope as leverage to apply torque to the system. The ammount of ‘staying power’, or in this case the potential torque, is dependant on the speed of the gyroscope’s angular velocity and will not diminish as force is applied to it. You can force the gyroscope to rotate 1000 times and it will still resist rotation with the same force that it did initially.

I hope that I answered the right question .

For torque perpendicular to the gyro’s axis, the rotation of the gyro’s axis would be at a constant rate, since the angular momentum you are adding to the gyro is perpendicular to the gyro’s angular momentum, so the differential change in the magnitude of the angular momentum is zero. For a component of torque parallel to the gyro’s axis of rotation, of course, the gyro would have to speed up or slow down to keep the platform steady. I think they usually use more than one gyro, so they don’t have to change rotation speeds much, but I don’t work with gyroscopes.

For the general case (neither perpendicular or parallel) with a single gyro, the gyro will speed up or slow down at least a little, so the rate of the gyro’s axis rotation would have to change accordingly.

The friction in the bearings just means you have to expend energy to keep the gyros rotating, but doesn’t affect the system’s angular momentum.

The Navy uses a double gyro for fire control. It consists of one gyro spinning in one direction mounted on top of another gyro spinning in the opposite direction. Not only does this gyro supply true direction to North (one plane on a theoritical flat Earth.) But it supplies information concerning pitch and yaw of the ship. Pitch is side to side roll and yaw is bow into the water movement.
The gyro stays perfectly still (except of course in position) while the ship turns, pitches, and rolls around it.
This information is fed to motors on the fire control turrets which keep them as stable as the gyro. (Sperry makes these.)

Another application is in dead reckoning. This application I am unfamiliar with so the information will be incomplete. A gyro is used to measure inertia of movement. When the ship moves in a direction the gyro measures the acceleration. How all this is translated into dead reckoning I can’t reckon. (Litton makes these.)

Try your next Internet adding the words “fire control”. You’ll see other possible search criteria such as “feedback”. Also try “inertia.”

sam stone : friction is irrelevant

q.e.d. : yes, you don’t get it

david simmons : what if you have nothing to lean against ? such as i want to place an object off-ballance and make sure it stays motionless instead of falling, and i want it to stay that way for LONG time, not just some time.

scr4 : “(torque) = (moment of inertia) * (angular acceleration)” that is if we’re talking about spinning the wheel up or down, but i am saying i think it does not hold true for turning the wheel. instead i think for turning the wheel it will be angular speed, not acceleration.

mapcase : when we talk math we have to deal with infinity, when we talk real life then … you should not even talk about real life in the first place, you can only test it, otherwise its bullshit anyway.

mirage : thats not quite what i asked. rather i can re-phrase the question like this : if i turn the axis of gyro at certain speed by applying torque, and then let go - will the axis keep turning or stop instantly ?

zenbeam : i think you answered my question, and i think you have basically agreed with my assumption, except that you were not very clear WHY.

acidkid : like q.e.d. you have answered the question “what is a gyro” which was not the question

I doubt that the Navy actually uses such a system, since they could achieve exactly the same result using no gyro at all. If they’re spinning in opposite directions (and at the same speed, and they’re the same shape and size), then the system will have zero angular momentum, and will respond to external torques just like any other system with zero angular momentum. There will be some internal stresses on whatever framework is holding the two gyros, but for external effects, they’d cancel out.

Yeah, but you know the military. Why go with a simple solution when you can go with a much more complex one at ten times the cost?

I suspect the gyro’s axes are perpendicular, not anti-parallel.

Well you didn’t ASK for that.

Actually, I’m not sure which part you’re asking the “why” for. For the case where the torque is perpendicular. I did explain why. It’s analogous to a particle moving in a circle; the acceleration is perpendicular to the velocity, so the speed remains constant. When the torque is parallel to the angular momentum, it’s like accelerating a particle in the direction it’s travelling. The particle speeds up or slows down, and the same happens to the gyroscope speed of rotation.

For the general case, the faster the gyo spins, the more angular momentum it has, the less angle the axis of the angular momentum will change for a given change in angular momentum (caused by the torque). The component of torque parallel to the angular momentum will change the angular momentum, and so will change the rate the gyro’s axis changes in response to the component of torque perpendicular to the gyroscope axis. This has analogies to linear momentum as well, but I’m not sure that would make it any clearer.

Yes, but only for torque always perpendicular to the gyroscope axis.

“There will be some internal stresses on whatever framework is holding the two gyros, but for external effects, they’d cancel out.”

i disagree, only the precession would cancel out ( and its an unwanted side effect, we want it to cancel out ). the actual impedance to tilting of axis would remain, since for both gyros it would be in the same direction - the opposite to that in which force is applied.

It was the Mark 19, North information only, and Mark 23, direction, pitch, and yaw, gyroscopes. Both by Sperry. The Mark 23 was internally two Mark 19’s placed back to back. Can’t find anything on the web to show you.

Did find that the ineartia navigation gyros do use two gyros at right angles.

This is incorrect. If both gyroscopes are spinning about the Z axis, but in opposite directions, and you apply torque about the Y axis, they will try to rotate about the X axis, but in opposite directions. There will be no resistance to tilting, unless the internal stresses Chronos mentioned bend or break whatever is holding them together.

why no resistance to tilting ? so they would try to move in opposite directions so what ?

if you looked at them separately, it would take effort to move each of them. so if they are now together, it would take same effort twice.

no ?

No.

With one gyro, you have to supply enough torque over a period of time to account for the difference in angular momentum between the start and finish. To roatate a gyro’s axis from the Z axis to the X axis, you’d have to supply sqrt(2) times as much angular momentum as it took to start the gyroscope spinning in the first place.

With two anti-parallel gyros, the total angular momentum is zero, both before and after you turn the two gyros. This makes it possible for the gyros to “trade” angular momentum, while keeping to total zero. Think of it as a two step process. The gyros are parallel to the Z axis to start, and you apply torque to one gyro about the Y axis. It then tries to rotate about the X axis, applying torque about that axis to the other gyro, which then tries to rotate about the Y axis in the direction you were trying to rotate the first gyro.