Cross wind effect

T

tangolima

New member
I have been thinking about this for some time.

Let's say we have 9 o'clock cross wind and right-hand spinning bullet. I think the bullet yaws to the left and downward, so it turns into the cross wind and has a negative angle of attack. The yaw is due to gyroscopic precession. The cross wind exerts a net lateral force on the bullet. It accelerates to the right during its flight to the target. So even with its meplat (nose) pointing to the left, it deflects to the right. When it hits the target, the POI will be to the right and below the POA.

Do I think correctly? Your comments are appreciated.

-TL
 
Right on. Or left, if the wind's from the other side. And the Magnus effect is insignificant.

Rimfire smallbore ammo strings from 10 o'clock to 4 o'clock on targets from cross winds; a well known issue with right hand 1:16" twist barrels. Some folks cant their sight on the rifle so they don't have to come up or down 1 MOA for a 3 MOA windage change.

High power rifles do it, but not nearly as much. It's more like 9:05 to 3:05 on the clock face and is typically ignored as it's not all that visible. After getting a no-wind zero at 800 yards a day before, I've put on a left 28 MOA wind correction and elevation was good with 7.62 NATO 147-gr. ball ammo.

I think the bullet's spin and shape axis is well aligned with its trajectory axis so it's BC stays the same. If it goes through the air at any angle to its trajectory, it's BC will be higher increasing its drag and it'll drop more for a given travel down range.
 
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Thanks for your replies, gentlemen. I'm glad you support my conclusion. But it actually opens up another head scratcher. Sorry I didn't mean to set this one up as a trick question ambush.

I believe the bullet yaws to the left and downward. It is solely due to gyroscopic precession. Magnus effect is not considered.

I had been quite content with this conclusion till I read the external ballistics section in the Sierra reloading manual. There they said the bullet would yaw to the right, following the cross wind, and it would yaw upward, i.e. to have positive angle of attack. They even claim that the bullet deflects to the right NOT because it is being blown by the cross wind. Instead it is because the cross wind makes the bullet yaw to the right, sort of changes its direction of flight. The end result would be a POI to the right and above the POA. Again, I'm afraid I cannot agree to what they profess.

-TL
 
TL, can you post (or PM me) that text from Sierra's manual? Or, if from an online link, forward it. Thanks.
 
Bart B.,

http://www.exteriorballistics.com/ebexplained/index.cfm

I'm not convinced that a boat tail spitzer bullet alligns with trajectory (IE follows the wind as Sierra explains in Exterior Ballistics). I think that Bryan Litz has a better argument that the net drag function on the bullet points the bullet into the wind.

But when Sierra's book and Berger's ballistician are arguing over which way the wind blows a bullet, mere mortals like myself just need to pop some popcorn and enjoy the show.

I do know that 22lr rimfire bullets are round nose flat base heeled projectiles. I expect them to act more like "musket balls" than high BC BTHP projectiles, and most rimfire bullets start out in the transonic range near the muzzle, so that is where I would expect the winds to affect the impact on target.

Jimro
 
Jimro said:
But when Sierra's book and Berger's ballistician are arguing over which way the wind blows a bullet, mere mortals like myself just need to pop some popcorn and enjoy the show.
Click to expand...

I concur. While human knowledge is important, it's not important for me to understand it all.
 
Jimro, if the net drag function on the bullet points the bullet into a 9 o'clock wind, does that mean the bullet's axis is aligned to a point to the right of a down range target while it's trajectory axis is moving it to the left of the line of sight?
 
In a cross wind, the bullet's back half is a larger "sail" catching more wind than the front half. Back end gets pushed further than the front. The applied force is at the base end. The bullet tends to point into the wind.

So thinks me.

I'm not too knowledgable about this gyro stuff.
 
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But the back also has well more mass then the front. That is why the center of mass is always behind the center of pressure, and that is why bullet needs to be gyroscopically stabilized, or it tumbles.

-TL
 
Bart B.

That is what I think as well, that crosswinds change the net drag function to "downwind" on the tail, forcing the nose into the wind. So even while the bullet is turning into the wind (rotating) the net drag function is pulling the bullet downwind.

This "net drag" explanation is consistent with what we know about the bullet "nosing over" on the ballistic arc. Shoot a bullet slightly at the sky and from the bullet's perspective the net drag is to the back and top, pulling the bullet to the horizontal at max ordinate. On the way down the net drag is still on the back top as the wind from the bullets perspective shifts to a bottom to top.

If Sierra were correct that bullets turn downwind, we would see bullets go more and more "nose up" on the second half of the ballistic arc. Since we have known impacts that bullets on a parabolic arc land nose down instead of destabilizing, I'm inclined to believe the "net drag" function keeps the nose pointed into the center of pressure. If it didn't, at some point the bullet would destabilize.

Jimro
 
Tangolima,

Bullets are spin stabilized, arrows are fin stabilized. Different in that the bullet has to be turned by overcoming an "overturning moment" due to gyroscopic stability, and experiences a vertical component to a crosswind due to the Magnus Effect.

But as an analogy, yeah, like an arrow. Or possibly a shuttlecock which is drag stabilized but not spin stabilized.

Jimro
 
Hmmm... I may have different opinion on that. I think bullet is fundamentally different from an arrow, so they require different stabilization mechanisms, although the end result of the stabilization action is similar. The difference lies in location of the center of pressure relative to the center of mass.

An arrow has an empennage, or a fin, at the end and a heavy arrow head in front. That put the center of pressure behind the center of mass, so it is stable in flight. Any perturbation will produce torque in counter direction, just like what you and Bart have described.

An bullet has a heavy base and a pointed tip, so its center of pressure is ahead of its center of mass. It is inherently unstable in flight. Any perturbation will produce tumbling torque. It has to be spin stabilized. It pitches down to follow the trajectory by yaw of repose. It turns into cross wind and pitches up or down. It is all due to gyroscopic precession, and has little to do with Magnus effect. Note that although an arrow does turn into cross wind, it doesn't pitch up nor down.

-TL
 
tangolima,

Like I said, as an analogy it is somewhat useful, but like all analogies doesn't convey all the information. If you drop an arrow it will self stabilize to fall point down, and if you drop a rifle bullet it will fall on its side. All analogies are imperfect comparisons.

Arrows don't experience the Magnus effect because they aren't spinning. Also hunting broadheads are quite heavy compared to target tips, and so you see the "planing" effect which is detrimental to accuracy as the tip interacts more with the airstream.

Of course a round musket ball fired through a smoothbore is less accurate than the same ball fired through a rifled musket. Why is that? The answer is that somehow the spin helps the ball interact with the air in a more consistent and predictable manner. I think the tendency of gyroscopes to resist deflection perpendicular to the angle of rotation is the answer.

Jimro
 
Thanks gentlemen for your inputs. Disagreement on certain details aside, I think we pretty much come to the conclusion on the following, which was the purpose of my original posting.

1. Bullets turn into cross wind.

2. They deflect in the direction of the cross wind. The POI will be above or below the POA depending on the direction of the cross wind and the bullet spin. For a 9 o'clock wind and a right hand spinning bullet, the POI will be to the right and below the POA.

3. The stipulations in the Sierra manual on cross wind are incorrect. This is surprising. They make good, and certainly expensive, bullets. Most people take their words as cannons.

Thanks again for your time.

-TL
 
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