Gyroscopic precession

one_raven

one_raven

God is a Chinese Whisper
Valued Senior Member
We don't have a mechanical engineering forum, so this seemed like the best place to ask this.

I am looking for information about gyroscopes, and although I can find a great deal of web pages, I am finding limited information that is helpful for me.

Basically, I am wondering what would be the results of mounting two gyroscopes 180° opposed (same axial plane, rotating in opposite direction).

Would the effects of precession cancel each other out?
Would it increase, decrease or not affect the stability effects?
What happens if you vary the RPM of each gyro?
What other effects would it have?
What other properties would change in this set up (as opposed to a single mounted gyro)?

If you can point me to a decent page with results of gyroscope experiments such as this one, I would be grateful.
 
Sounds like a tricky question. Perhaps you can figure it out by yourself by using Newton's second law, modified for angular momenta and torques ?

dL/dt = M

Using this in combination with F = ma might give you an insight on how the gyroscopes react on eachother. But personally I always found gyroscopes tricky things to calculate on :).

Bye!

Crisp
 
Tricky

Gyroscopes are tricky indeed. The force that holds a gyro up against gravity and ultimately decays inducing precesss, if stopped (i.e. - put something in the precessional path) and the counter force induced by the stop becomes reflected 90 degrees and canels the lift force and the gyro will drop like a rock.


Knowing to believe only half of
what you hear is a sign of
intelligence. Knowing which
half to believe will make you a
genius.
 
Can you explain what you mean by mounting?

Here's how I can picture it:
You have two gyros end to end. This end to end point is fastened to eliminate any relative motion of the cages of the gyros. This same point is pivoted about a "fixed" point in the lab.

Alternatively, you could pivot one of the extreme endpoints.
 
I mean, two contra-rotating flywheels sharing the same axis.

Picture a long bolt with two bicycle wheels on that one bolt and that bolt attached at either end to a frame.

Wheel one rotates clockwise.
Wheel two rotates counter-clockwise.
 
After I posted, I sat and thought about either case. They both seem, as has been said before, quite tricky. I will work on this and get back to you.

As far as mounting the entire assembly, I would think that a mechanical engineer would be better at this than a physicist.:) I have two suggestions:
1) Tie a string around one end of the bolt and hang it from the ceiling. This will introduce two more degrees of freedom.
2) Construct a ball joint on one end of the bolt that is housed in a tall, and as narrow as possible, stand, which is securely fastened to the ground. This will not introduce any more degrees of freedom, but it will be considerably more construction intensive (and more expensive). You will also have to worry about adequate lubrication of the joint.
 
one_raven,

Picture a long bolt with two bicycle wheels on that one bolt and that bolt attached at either end to a frame.

Wheel one rotates clockwise.
Wheel two rotates counter-clockwise.
Click to expand...

Are you trying to create anti-gravity? :)
 
Originally posted by Prosoothus
Are you trying to create anti-gravity? :)
Click to expand...

LOL.

No.
Do I come off as that much of a crackpot??

Everyone knows that the only way to achieve anti gravity is through a negative ion force field created and suspended around a titanitum hull, anyways! DUH! :rolleyes:
;)


Actually I was just wondering about it after thinking back to a documentary I saw on helicopters.
There are some models that have dual contra-rotating rotors to reduce the effects fo torque and increase stability.
I was just pondering where this stabilization technique may also be useful, and wondering about the mechanics behind it.
 
Last edited:
one_raven,

LOL.

No. Do I come off as that much of a crackpot??
Click to expand...

If you were to assume that the gravitational field has a sister field (just like the electric field has a magnetic field), and if the gravitational field's sister field was created at right angles to both the gravitational field and the motion of the gravitational field, then the essembly that you described could easily be converted into an anti-gravity engine.

All that you would have to do is spin the two wheels in opposite directions, and spin the whole essembly around its axis.

Now back to Earth, I can say with certainty that your essembly would lack any precession force if it is tilted, since the precession force of one wheel would balance out the precession force of the other (that is, if both wheels are of the same weight, and are spinning at the same speed).

Tom
 
Originally posted by one_raven
There are some models that have dual contra-rotating rotors to reduce the effects fo torque and increase stability.
I was just pondering where this stabilization technique may also be useful, and wondering about the mechanics behind it.
Click to expand...
After giving the matter some more thought, I am going to agree with Pros statement above. To that point, I don't think that the technique is employed for stabilization, but more for reducing unwanted effects that make the helicopter <i>too</i> stable.
 
errandir,

I haven't thought this all the way through but if you restrain the precessional force of the cage, do you not then also lose the gyroscopic affect?



Knowing to believe only half of
what you hear is a sign of
intelligence. Knowing which
half to believe will make you a
genius.
 
Originally posted by MacM
I haven't thought this all the way through but if you restrain the precessional force of the cage, do you not then also lose the gyroscopic affect?
Click to expand...
Ya. That's what I was saying in my previous post, though in different words. I don't know that much about the engineering of helicopters, but it looks to me suspiciously like they are trying to eliminate the gyro effect. It may be that the gyro effect is undesired in the props of a helicopter.
 
errandir,
To that point, I don't think that the technique is employed for stabilization, but more for reducing unwanted effects that make the helicopter too stable.
Click to expand...
What do you mean by "too stable"?
As in, if the gyroscopic force was too much the vehicle would not be able to pitch?


MacM and errandir,

I haven't thought this all the way through but if you restrain the precessional force of the cage, do you not then also lose the gyroscopic affect?
Click to expand...

but it looks to me suspiciously like they are trying to eliminate the gyro effect. It may be that the gyro effect is undesired in the props of a helicopter.
Click to expand...

I really think that the addition of a second gyro (regardless of direction of spin) would make the helicopter more stable.
I can't see how the opposite spins would cancel out the gyroscopic force.
The little side mounted propeller in the back (what the hell is it called again? It acts like the rudder of a boat... anyway) is what stops the helicopter from spinning out of control.

What I am uncertain of is if the spin that is involved is due to precession forces, or simply the body of the helicopter reacting to the torque of the engine spinning in the other direction.

Not sure if that is clear...
The engine sits between the blades and the body.
If it takes significantly less torque to spin the blades than the body, the blades will spin.
If it takes significantly less torque to spin the body than the blades, the body will spin.
If the ratio is small, then both may spin.
The little "rudder" keeps it stable by retarding spin.

The problem is that little "rudder" pushing against the force of the spin is part what causes the torque strain on the frame.
The body is trying to spin one way and the tail is being pushed the other way to keep it from spinning out of control.

I distinctly recall them saying that the addition of the contra rotating second set of blades was to retard that spin tendency.

If it is balanced correctly, a helicopter with contra rotating blades wouldn't even need a "rudder" if the blade speeds were individually adjustable.
Slow down the clockwise blade, and you will spin clockwise and vice versa.
Or, you could have a "rudder" that does a lot less work, and exerts a lot less torque stress on the frame because it is free to rotate due to the balanced blades.

What I am unsure of, is where does presssion come into play in this?

If the gyroscopic effect is increased (or at least nor decreased) and the precession effect is nullified, then the hellicopter:
Would be much more stable...
Would not go into that awful wobbling spin that everyone has seen in one action movie or another...
It would not experience torque stress when pitching due to precession...
 
I thought they'd already built helicopters with contra-rotating sets of blades. Perhaps I dreamt it.
 
Originally posted by Canute
I thought they'd already built helicopters with contra-rotating sets of blades. Perhaps I dreamt it.
Click to expand...

They have.

Thinking of those helicopters is what made me wonder about these things.

Actually I was just wondering about it after thinking back to a documentary I saw on helicopters.
There are some models that have dual contra-rotating rotors to reduce the effects fo torque and increase stability.
I was just pondering where this stabilization technique may also be useful, and wondering about the mechanics behind it.
Click to expand...
 
Canute,

You are correct. Counter rotation does not cancel gyroscopic affects. But the point was stopping a gyroscopes "Precessional Freedom" does.

The primary advantage and justification for counter rotating blades is stability in forward flight.

With counter rotating blades the affect cancels but without it any forward motion causes the blade to increase lift one one side more than the other due to the increasd relative velocity of the air over the blade.

To solve that the blade is continuously varied in pitch as it rotates. The magnitude of that pitch change increases with forward speed or wind direction.

PS: That "Thingy" at the back is called "Tail Rotor".:D Counter rotating blades can help reduce the need for it as well.


Knowing to believe only half of
what you hear is a sign of
intelligence. Knowing which
half to believe will make you a
genius.
 
Originally posted by one_raven
What I am uncertain of is if the spin that is involved is due to precession forces, or simply the body of the helicopter reacting to the torque of the engine spinning in the other direction.
Click to expand...
I think you're right. After reading your arguement, I would agree that the counter-rotation is to reduce/eliminate the need for the tail rotor.
 
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