Gyroscopic drift: Simplified?
11-16-2009 | 07:43 PM
#1
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Nontypical Buck
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Gyroscopic drift: Simplified?
So, I'm trying to understand the behavior of gyroscopes and how they apply to bullets to explain drift. The obsolete "log rolling on water" theory just isn't cutting my curiosity. Now, tell me if I have this right in a simplified manner:
Rotating objects experience a push 90 degrees (in the direction of rotation) from where the force is applied. So, since bullets are dropping, the greatest force they are encountering is underneath them (dropping=hitting the air, or rather a downward acceleration implies the same). A right-twist bullet will then experience a drift as if it were being pushed from the left (which is 90 degrees from bottom of the bullet in the direction of rotation).
Do I have this understood properly in a manner, simple as it may be?
Rotating objects experience a push 90 degrees (in the direction of rotation) from where the force is applied. So, since bullets are dropping, the greatest force they are encountering is underneath them (dropping=hitting the air, or rather a downward acceleration implies the same). A right-twist bullet will then experience a drift as if it were being pushed from the left (which is 90 degrees from bottom of the bullet in the direction of rotation).
Do I have this understood properly in a manner, simple as it may be?
11-17-2009 | 07:19 AM
#2
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Air is a medium, just like water. your example would be valid if the log rolling is under the water, and you take the log buoyancy out of the question. In other words, take a log like IPE (wood denser than water) and put it in a vacuum, and roll it. Reason the log drifts is because of the surface tension delta between the air and water. If this surface tension delta didn't exist, no drift.
Awesome questions, makes me get out my hydrolics and fluids engineering text books from college.
Awesome questions, makes me get out my hydrolics and fluids engineering text books from college.
11-17-2009 | 12:13 PM
#3
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Nontypical Buck
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11-17-2009 | 01:25 PM
#4
Fork Horn
Joined: Mar 2005
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Speaking of gyroscopic effect, a bullet leaves the muzzle at 3000fps. rate of twist 1:12. spin rate on the bullet is 180,000 rpm. At 400 yards, the bullet has slowed to 2000 fps. What is the rate of spin? I can't say this one keeps me up at night, but I have wondered about it. It seems like the surface tension Delta BC refers to would have slowed it down a little, but not as fast as "drag" slows down the bullet.
11-17-2009 | 01:55 PM
#5
Fork Horn
Joined: Jan 2007
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So, I'm trying to understand the behavior of gyroscopes and how they apply to bullets to explain drift. The obsolete "log rolling on water" theory just isn't cutting my curiosity. Now, tell me if I have this right in a simplified manner:
Rotating objects experience a push 90 degrees (in the direction of rotation) from where the force is applied. So, since bullets are dropping, the greatest force they are encountering is underneath them (dropping=hitting the air, or rather a downward acceleration implies the same). A right-twist bullet will then experience a drift as if it were being pushed from the left (which is 90 degrees from bottom of the bullet in the direction of rotation).
Do I have this understood properly in a manner, simple as it may be?
Rotating objects experience a push 90 degrees (in the direction of rotation) from where the force is applied. So, since bullets are dropping, the greatest force they are encountering is underneath them (dropping=hitting the air, or rather a downward acceleration implies the same). A right-twist bullet will then experience a drift as if it were being pushed from the left (which is 90 degrees from bottom of the bullet in the direction of rotation).
Do I have this understood properly in a manner, simple as it may be?
It has to do with the gyroscopic action when the axis of rotation is not parallel with the direction of the center of gravity which then causes a yaw in the flight, which steers the bullet to the side depending on direction of twist. I don't pretend to understand it, the above paraphrases wikipedia.
It is a phenomena of physics with a rotating body tipping as it flies through space. The change in direction is because the as the bullet's nose dives in trajectory, gyroscopic stability turns it to the side, and this yaw steers the bullet.
It is a small effect until the ranges get rather long.
And CZ, the RPM's of the bullet would still be roughly 180,000 rpm. The air resistance to rotation of the bullet is small, so the spin never slows down to much.
I recommend the above book if people are interested in ballistics. It isn't well edited, as there are spelling errors and the pictures aren't good, but it is considered an authoritative book on the technical apects.
It even covers the coriolis effect.
11-17-2009 | 05:40 PM
#6
Nontypical Buck
Joined: Feb 2003
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From: Coralville, IA. USA
The following is a quote from an article by Brian Litz, the chief ballistician and aerodynamics engineer for Berger Bullets from his personal website, www.appliedballisticsllc.com
source: http://www.appliedballisticsllc.com/...iolisDrift.htm
Hope this helps.
Mike
Gyroscopic (spin) Drift
First, a quick fact about spinning objects…
Picture a spinning object like a bullet or a top. The spinning thing has a ‘spin axis’, about which it’s spinning. If you try to disturb the spin axis by applying a force, or a torque to that axis, the spinning object reacts in a strange way. Rather than simply moving in the direction that you pushed it, the spin axis reacts by moving 90 degrees from the applied force, in the direction of rotation. In other words, if you have a top spinning clockwise on the table in front of you, and you push the top of it’s stem straight away from you, the stems first reaction is to jump to the right. After the initial reaction, it will precess into its new equilibrium. Now, on to bullets.
Consider a bullet fired at some angle on a long range trajectory. The bullet starts out with its spin axis aligned with its velocity vector. As the trajectory progresses, gravity accelerates the bullet down, introducing a component of velocity toward the ground. The bullet reacts like a spiraling football on a long pass, by 'weather-vaning' it's nose to follow the velocity vector, which is a nose-down torque. The price you pay for torqueing the axis of rotation is that the nose points slightly to the right as it 'traces' to follow the velocity vector. This slight nose right flight results in a lateral drift known as ‘gyroscopic drift’.
Having a left or right twist will change the direction of gyroscopic drift. Bullets fired from right twist barrels drift to the right, and vise versa by the same amount, typically 8-9 inches at1000 yards for small arms trajectories. Gyroscopic drift is an interaction of the bullets mass and aerodynamics with the atmosphere that it’s flying in. Gyroscopic drift depends on the properties (density) of the atmosphere, but has nothing to do with the earth’s rotation.
First, a quick fact about spinning objects…
Picture a spinning object like a bullet or a top. The spinning thing has a ‘spin axis’, about which it’s spinning. If you try to disturb the spin axis by applying a force, or a torque to that axis, the spinning object reacts in a strange way. Rather than simply moving in the direction that you pushed it, the spin axis reacts by moving 90 degrees from the applied force, in the direction of rotation. In other words, if you have a top spinning clockwise on the table in front of you, and you push the top of it’s stem straight away from you, the stems first reaction is to jump to the right. After the initial reaction, it will precess into its new equilibrium. Now, on to bullets.
Consider a bullet fired at some angle on a long range trajectory. The bullet starts out with its spin axis aligned with its velocity vector. As the trajectory progresses, gravity accelerates the bullet down, introducing a component of velocity toward the ground. The bullet reacts like a spiraling football on a long pass, by 'weather-vaning' it's nose to follow the velocity vector, which is a nose-down torque. The price you pay for torqueing the axis of rotation is that the nose points slightly to the right as it 'traces' to follow the velocity vector. This slight nose right flight results in a lateral drift known as ‘gyroscopic drift’.
Having a left or right twist will change the direction of gyroscopic drift. Bullets fired from right twist barrels drift to the right, and vise versa by the same amount, typically 8-9 inches at1000 yards for small arms trajectories. Gyroscopic drift is an interaction of the bullets mass and aerodynamics with the atmosphere that it’s flying in. Gyroscopic drift depends on the properties (density) of the atmosphere, but has nothing to do with the earth’s rotation.
Hope this helps.
Mike
11-17-2009 | 07:04 PM
#7
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Good article drift. I was doing some studying in my old college vector equations. I finally started understanding. They were talking about a golf ball sliced or hooked, and the reason it slices or hooks is one side of the ball taking more friction (due to spin) than the other. But for it to work you need a force perpendicular to the spin. All I could think of in bullistics is only force is parrellel to the bullet which doens't do anything. But then it dawned on me gravity is the vector force perpendicular to the spin, not as strong as a golf club , but its there. I have shot some past 700 yards, and can't say I see it. But with 10" groups out there, its hard to tell what it is.
12-02-2009 | 09:59 AM
#9
Nontypical Buck
Joined: Nov 2007
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From: Titusville Florida
12-02-2009 | 11:22 AM
#10
Typical Buck
Joined: Dec 2004
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From: Nebraska
Air is a medium, just like water. your example would be valid if the log rolling is under the water, and you take the log buoyancy out of the question. In other words, take a log like IPE (wood denser than water) and put it in a vacuum, and roll it. Reason the log drifts is because of the surface tension delta between the air and water. If this surface tension delta didn't exist, no drift.
Awesome questions, makes me get out my hydrolics and fluids engineering text books from college.
Awesome questions, makes me get out my hydrolics and fluids engineering text books from college.
.
Last edited by LaneNebraska; 12-02-2009 at 11:25 AM.