# Precession - the cause. Gyros.

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## Main Question or Discussion Point

I have a question about precession.

I have seen a few videos on youtube. A bicycle wheel that is attached to a thread on one side. The wheel is aligned upright, and spun by hand. The "experimentist" release the wheel, and precession starts right away due to the vertical torque applied by gravity. I also see that the wheel is slowly starting to turn over into horizontal position. The "rigidity" of this gyro is not infinite, but the faster the wheel spin, the longer time it takes for the wheel to align horizontally.
Do precession happens only because of the applied torque, or also because the wheel turns over from upright into horizontal position?

If we prevent precession, will it take the same time for the wheel to align from upright into horizontal position?

I have tried to google this last question several times, but there is nothing to find about this except a few very short (and locked) threads at this forum (That is not exactly answering this question). Everyone just wants to demonstrate precession, precession, precession. Why have no one experimented with preventing precession, and see what's happens when a torque is applied, AND forced the gyro in the SAME direction as the applied torque, and analyze it?

Vidar

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A.T.
AND forced the gyro in the SAME direction as the applied torque
You can't. Forcing means applying a torque, which becomes part of the total applied torque. So the total applied torque doesn't match the change of the rotation axis.

Gold Member
You can't. Forcing means applying a torque, which becomes part of the total applied torque. So the total applied torque doesn't match the change of the rotation axis.
Maybe I was not clear. Sorry for that.

The gyro will precess, but also turn (reluctantly) in the same direction as the applied torque (Seen demonstrated on several videos on youtube). The spin (That is initially spinning about the horizontal axis) will finally be found resting about the vertical axis after a while.

I assume the spin will finally rest about the vertical axis also if precession is stopped by guides. Thus, I assume the gyro will (reluctantly) turn over in the same direction as the applied torque without actually precess, because it has no other options but follow the applied torque. However, I ask if the gyro (That is initially spinning about a horizontal axis) use the same time to turn over into spinning about the vertical axis, as if it also was allowed to precess?

I will try to analyze this further. Gyros really bugs me. Specially if my assumptions are not right

Vidar

A.T.
I assume the spin will finally rest about the vertical axis also if precession is stopped by guides.
Those guides also apply a torque

Thus, I assume the gyro will (reluctantly) turn over in the same direction as the applied torque
No, because the total applied torque now also includes the torque from the guides, and not just the one you apply with your hand. So the gyro in not following the total applied torque.

Gold Member
Ah! Thanks ;-) That probably answered the first question too.

Gold Member
What happens to the "rigidity" when a gyro is inside a guide like this, or inside any kind of mechanism which prevent precession? Does it lose it completlely?

Is it considered as torque when the guides does not move at all?
Wouldn't the "precession torque" be exacly the same but opposite as the guides?
Wouldn't the applied torque be greater than the rigidity (countertorque) of the gyro?

If your explanation is correct, would the gyro act like a normal pendulum - if it is so that the torque from the guide will make precession the same way as I want to apply the torque?

EDIT: Let's take away the guides, and instead put on a second gyro which spinns the oposite direction. Now there would be no precession at all. If we now apply torque the same way as before. Will we feel any form of rigidty now?

Vidar

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Gold Member
Found some information about counter rotating gyros that was used to stabilize the Gyro Cycle:
The Gyro Hawk was a gyro-stabilized vehicle which used a two-gyro counter-rotating system." So, I got an answer to this question. Counter rotating gyros provides rigidity regardless of which perpendicular direction the torque is applied. No precession, just rigid (to a certain extent).

Vidar

Cleonis
Gold Member
I also see that the wheel is slowly starting to turn over into horizontal position. The "rigidity" of this gyro is not infinite, but the faster the wheel spin, the longer time it takes for the wheel to align horizontally.

When the bicycle wheel has settled into a fairly stable precessing motion you observe that it is gradually giving in to gravity; the wheel is sagging.

What is happening:
Because of air friction the spin rate of the wheel is decreasing all the time. Decrease of the spin rate allows gravity to pull the center-of-mass of the wheel down. The motion of giving in to gravity has the effect of increasing the precession rate. The increased precession rate opposes the tendency to sag down further.

Conversely, if you have a driving mechanism in place that sustains the wheel's spin rate then you can sustain the height of the wheel's center-of-mass. If you set the drive to increase the spin rate then the wheel will climb up.

If you prevent the precessing motion from starting at all (by blocking the spin axis from reorienting in that direction) then the wheel will drop down like a brick. The wheel will drop down with the same acceleration as when it would not be spinning at all. (That is, in the friction-free case. In an actual setup it would be very hard to get close to eliminating all friction)

For the initial settling into fairly stable precessing motion the same mechanics applies. The wheel sags a little, the sagging induces precessing motion, the precessing motion opposes further sagging. If the wheel spins very fast the initial sagging is imperceptably small.

There is a paper written by Svilen Kostov and Daniel Hammer titled 'It has to go down a little, in order to go around'

Kostov and Hammer discuss that the gravitational force moves the free end of the wheel axis down a bit. They also created an experimental setup to demonstrate that dip. They found very good agreement between theory and experimental result.

As you asked: the suggestion, by many authors, that the precessing response comes instead of giving in to gravity is not correct. The wheel does sag a little. And after that, as friction dissipates kinetic energy of the spinning motion, the sagging continues along with the decrease in spin rate.

Earlier post
In november of 2010 I posted an answer explaining the mechanics of gyroscopic precession, using images.
(Note that to non-logged-in visitors Physicsforums tends to display old threads without the images.)

More complete discussion
The article gyroscope physics that is on my own website.

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A.T.
Counter rotating gyros provides rigidity regardless of which perpendicular direction the torque is applied. No precession, just rigid (to a certain extent).
No, two identical counter rotating gyros (same speed and axis) have no "rigidity". Changing their axis goes exactly like if they weren’t rotating.

Cleonis
Gold Member
Found some information about counter rotating gyros that was used to stabilize the Gyro Cycle:
The Gyro Hawk was a gyro-stabilized vehicle which used a two-gyro counter-rotating system."
When gyros are used for vehicle stabilization the system is an active system. In an active system control motors manipulate the orientation of the spin axes of the flywheels.

An active system with a single gyroscope wheel would be sensitive to motion imposed on it from the outside. When you have a system with two counter-rotating wheels then for most motions-from-outside the gyroscopic responses of the two wheels will pretty much cancel out.

The active manipulation of the flywheels is set up in such a way that the axis reorienting motion is a counter-motion too, so that the responses of the flywheels to that manipulation reinforce each other.

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A.T.
When counter-rotating wheels are close together (and their spin axes aligned) they will largely cancel each other's gyroscopic effects. The closer the wheels, the more complete the cancellation.
What gyroscopic effects do not cancel completely if the spin axes are aligned but the wheels are not close together?

Cleonis
Gold Member
What gyroscopic effects do not cancel completely if the spin axes are aligned but the wheels are not close together?
Hi A.T.

Good thing you questioned this. I realized I'm not sure about it, so I deleted the statement about cancelation of gyroscopic effects from my message (#10 in this thread).

rcgldr
Homework Helper
A bicycle wheel that is attached to a thread on one side. The wheel is aligned upright, and spun by hand. ... Do precession happens only because of the applied torque, or also because the wheel turns over from upright into horizontal position?
The wheel turns over from upright because the friction and drag (aerodynamic) in the system opposes the precession, which results a component of the wheel's precession about a horizontal axis (perpendicular to it's own axis), rotating "downwards", and becoming horizontal. If there was nothing opposing precession, the rate of precession would increase as the wheel slowed down, and the wheel would remain nearly vertical (see next paragraph).

Precession itself is also part of the angular momentum, so the precession reaction of the wheel won't be exactly 90° perpendicular to the axis of the wheel and the axis of the applied torque, but it will be very close unless the applied torque is large and/or the wheel is spinning slowly.

If we prevent precession, will it take the same time for the wheel to align from upright into horizontal position?
If you reduce precession with a secondary torque with a vertical axis, it results in a component of precession about a horizontal axis (perpendicular to the wheels axis), and the wheel pivots "downwards". If the secondary torque increases rotation in the direction of precession, it results in a component of precession that causes the wheel to pivot "upwards" (assuming there was clearance to avoid rubbing the string, or if the extended wheel axis was supported by a tower).

To prevent all precesssion, the net torque would have to be zero, and the wheel would remain vertical no matter how fast it was spinning.

Gold Member
Interesting to read that one person says the prevention of precession gives one result, while another person says it gives another result. Also about rigidity of two contra rotary gyros on the same axle. Other places I got the answer that the rigidity is inforced due to a second spinning mass, and here you say there will not be rigidity at all and the gyro will fall down as a brick. Maybe the question was understood differently.

Actually I cannot find one single full "real" article or experiments that has been done with these particular cases: "What happens when preventing rigidity?", and "How do two contrarotating gyros work?". I will study the links above (Thanks for the links by the way). Maybe I will understand my Powerball one day too :-)

I have however seen examples on single rail trains where two gyros forced the train upright. The gyros was placed side by side, spinning horizontally, but able to "seesaw" in line with the train. These are very old prototypes, so I have a doubt the gyros was electrically manipulated in order to keep the train upright - or were they?

Vidar

Cleonis
Gold Member
I have however seen examples on single rail trains where two gyros forced the train upright. [...] These are very old prototypes, so I have a doubt the gyros was electrically manipulated in order to keep the train upright - or were they?

A purely mechanical system can have good precision tunable feedback capability.
One example is automatic transmission in a car. To my knowledge automatic transmission systems are to this day purely mechanical. (With computer controlled electrically driven gear change systems only in some high end cars.)

A monorail train with active gyroscope stabilization in the form of a purely mechanical system would be challenging, but doable.

Today an example of active gyroscope stabilization is marine gyroscope stabilization. Sea swell will roll a yacht from side to side. An active gyroscope stabilization can reduce that rocking motion. As far as I know those systems on yachts work with two counter-rotating flywheels.

Cleonis
Gold Member
What happens [...] when a gyro is inside a guide like this, or inside any kind of mechanism which prevent precession?
Providing further detail:

If the guide is exclusively preventing precessing motion, then the force from the guide is not doing work. Formulated differently: with the guide only preventing precessing motion the motion of the wheel's center of mass is purely downward. In that downward motion the force-from-the-guide is not doing work.

So if gravitational force is exerting a downward force on the center of mass of a spinning wheel, and a guide just prevents precessing motion, then the cause of the spinning wheel falling down is that gravitational force, and not the force from the precession-preventing guide.

The distinction: for falling gravitational force is both necessary and sufficient. In the particular setup here the torque from the precession-preventing-guide is necessary of course, but it's not sufficient.

Again, onset of precessing motion is a process of converting one motion to another. As experimentally verified by Kostov and Hammer (linked to in post #8), the onset of the precession is that the center of mass gives in to gravity. That downward motion converts to precessing motion. The effect of the precessing motion is that the torque from gravity is opposed. When a guide prevents precessing motion gravity gets its way.

rcgldr
Homework Helper
Interesting to read that one person says the prevention of precession gives one result, while another person says it gives another result.
Take a look at my previous post, there's a difference between reducing (or increasing) rotation about the same axis as precession, and "preventing" any precession reaction (which requires a net torque of zero).

Cleonis
Gold Member
Take a look at my previous post, there's a difference between reducing (or increasing) rotation about the same axis as precession, and "preventing" any precession reaction (which requires a net torque of zero).
Hi rcgldr,

for comparison I take the following case:
A object is sliding frictionless down a helix-shaped tube. The axis of the helix-shaped tube is vertical. Gravity is accelerating the downward component of the motion. The outer wall of that helix-shaped tube is exerting a force upon the sliding object (towards the vertical axis), and that force is not doing work. I take it you agree that the inward force exerted by the tube upon the object is not involved in the downward acceleraration.

Generalizing: for any setup, when there are two forces exerted, with one of the forces doing work and the other not doing work, then the change in kinetic energy that is happening is caused by the force that is doing work.

In post #16 of this thread I argue that a guide that only prevents precessing motion is not doing work.

Gold Member
I have bought a pair of cheap gyros at ebay. I will try different experiments with them. Gyros, as well as power balls, has been a mystery for me to understand fully. For example my arm almost falls off after exersizing with the powerball. Keeping it spinning av 9k to 10k RPM is quite challenging. Even if I am mostly fighting against inertia there is something going on in such gyros which i do not understand. I put quite much work into the powerball, but it is only me that heats up. The power ball keeps cool as allways - like the input work is just bounced back into my hand and arm :-))

Vidar.

A.T.
Gyros, as well as power balls, has been a mystery for me