Crosswind effects on Vertical

[Mr.Knowit all]

Copy that.
Thanks

 

[Ned Ludd]

Yep. But I don't think you like mine very much.

I liked your initial response just fine. Both types of information can be useful. One is a working value, the other is a more detailed explanation of the process that underlies it. My point is simply that there is plenty of room for both types of posts here.

 

[M995]

The vertical effect of a cross wind is caused by the gyroscopic effect of the bullet yawing to face the relative air flow produced by the combination of the bullet speed and the wind speed when it leaves the gun barrel. When the bullet yaws slightly left or right the gyroscopic reaction is to make the bullet yaw up or down depending on the wind and spin direction. It is not a Magnus effect since, as the bullet yaws to face the relative air flow, there is no air flow across the bullet.
The percentage of the vertical change compared to the down wind drift will change with range and bullet configuration. This is because the vertical effect is a linear change with range governed by the bullet aerodynamic and inertial properties whereas the down wind drift is closer to exponential, however as both will be affected by the velocity drop, it is not going to be quite that simple.
It is a real effect (some people try to claim it does not exist) and has lead to major challenges in the past for some projects. It is not a function of gyroscopic stability but it is dependent on many similar bullet properties so the two do tend to be related.

Post away. I'm paying attention

 

[dcali]

There’s nothing wrong with simple rules of thumb. But aerodynamic jump is an inherently complicated thing. If you want to understand it more than “it’s not a big deal at 600 yards, just know that it exists”, then it’s worth paying attention when a world class ballistician tells you how it works. If people don’t get it, or don’t want to get it, there is no harm done. They can always go back to “10% or so” and they’re ahead of 95% of the shooting world. As Ned says, there is value in every sort of contribution here.

There are a small handful of people on this forum who are truly expert beyond the level most of us will ever reach. I’d like to keep them around and soak up as much knowledge as possible.

 

[Ballisticboy]

Thank you all for all the kind compliments, I just hope I can live up to it in the future. I try to spread some of the knowledge I was taught as, with the effective closure of many of the ballistic research departments both in the US and over here, there is a very real danger of much of the expertise being lost.
It would be a pity to see the work of the US pioneers in the science such as Murphy, Lieske and McCoy being consigned to archives.

 

[Depart123]

Thank you all for all the kind compliments, I just hope I can live up to it in the future. I try to spread some of the knowledge I was taught as, with the effective closure of many of the ballistic research departments both in the US and over here, there is a very real danger of much of the expertise being lost.
It would be a pity to see the work of the US pioneers in the science such as Murphy, Lieske and McCoy being consigned to archives.

I got a question...

We've all heard of aerodynamic jump. The offset that happens when a bullet leaves the barrel and enters an immediate crosswind. According to Litz, this is a one time effect and is linear in the angular offset all the way to the target.

However, there has been much discussion that I've read concerning subsequent and ongoing vertical effects of crosswind down range. These are usually attributed to either the Magnus effect, or to Bernoulli's principle.

What is your take...in plain english please?

 

[dcali]

Jump is exactly what you said. It’s an angular change in direction due to the bullet tipping relative to the airflow on launch.

Down range, it would take some special circumstances to cause a similar effect, like shooting through a barrier where the wind is very different on the other side.

It’s not Magnus- the magnus force on a bullet is very small. It’s also not Bernoulli, which only applies to incompressible fluids - like water, or very low speeds in air - (among other conditions).

Any vertical deflection such as you describe (if I’m interpreting your question correctly) would require a force acting in the vertical direction for the entire flight of the bullet, such as a vertical wind, or a upwards pointing yaw. I can’t imagine a situation where such a force would be caused by a cross wind. But perhaps there is something I don’t know about.

As a side note, I think a lot of shooters grossly overestimate the effects of head and tail winds, as well as the impact of vertical winds due to terrain. Most of the time, at least in the studdies I’ve seen, vertical wind is confined to small areas, at the crest of a hill for example, within a couple feet of the ground. Maybe it’s different in extreme cases like a ravine or a cliff. I’m just speculating. But the average bullet on the average range isn’t going to see much vertical wind. Have you ever seen a wind flag point up or down? I haven’t.

So after all that, I got nothing. I can’t explain that kind of vertical behavior.

 

[Bart B.]

It's a well known fact with 22 rimfire competitors that a right hand twist causes bullet impacts to string to 10 o'clock with winds from the right and to 4 o'clock when from the left.

They strike higher with tail winds, lower with head winds. A 10 mph tail wind has the same effect as a 15 fps increase in muzzle velocity.



Centerfire ammo does the same thing but elevation stringing is much less. About 9:10 to 3:10 oclock and barely discernable so it's seldom a concern.

 

[Depart123]

As a side note, I think a lot of shooters grossly overestimate the effects of head and tail winds, as well as the impact of vertical winds due to terrain.

My experience also.

Most of the time, at least in the studies I’ve seen, vertical wind is confined to small areas, at the crest of a hill for example, within a couple feet of the ground. Maybe it’s different in extreme cases like a ravine or a cliff. I’m just speculating. But the average bullet on the average range isn’t going to see much vertical wind. Have you ever seen a wind flag point up or down? I haven’t.

Vertical winds can be quite prevalent when shooting in the mountains. But as you have said, it is usually within a relatively small area. Basically, if you take the height of obstacle you are shooting in the vicinity of, the affected area will be about a 45 degree angle from that point to the ground. Again, not exact by any means, but a good rule of thumb. You probably can't call wind well enough or hold hard enough to tell the difference.

The real difference concerning updrafts comes in the angle of your trajectory to the target. If you were shooting into a mountain from an opposing ridgeline or from flat ground, the effect would be negligible because the affected area would be so small. Kind of like shooting out of helicopters, the rotor wash of a Blackhawk helicopter is over 140 mph, but the area is so small that you don't worry about it.

Shooting parallel to the same ridgeline however or across a bowl, might expose the bullet to updrafts for almost the entire flight. This is where the voodoo of intuition and experience come in.

 

[Depart123]

Jump is exactly what you said. It’s an angular change in direction due to the bullet tipping relative to the airflow on launch.

Another question...

If AJ happens at the muzzle....and the angle is linear....then why wouldn't that angle be corrected when zeroing at 100 yards in the same wind? Whatever angle was present at the muzzle would be dialed out correct? It would then only present itself when shooting in a different wind than the zero'd wind condition.

Yet...that is not what can be observed. Something is having a vertical effect on the bullet in strong crosswinds when no terrain terrain feature is present to blame. I see this often when shooting cross canyon some 800 ft above the valley floor and the wind damn near perpendicular to the trajectory.

 

[hogpatrol]

 

[chop house]

just a nutty question.... how does all this that has been discussed change if the projectile is a essentially a sphere? i assume it could never be a perfect sphere, since there has to be some form of deformation at the bearing surface (if only engraved) to get it rotating. (no sabots or patches)

 

[dcali]

just a nutty question.... how does all this that has been discussed change if the projectile is a essentially a sphere? i assume it could never be a perfect sphere, since there has to be some form of deformation at the bearing surface (if only engraved) to get it rotating. (no sabots or patches)

A perfect sphere cannot tip, so there can be no jump (or spin drift). If it’s slightly elongated, it works the same as a regular bullet, but the numbers will be different than those we typically see with long range boattails.

 

[Ballisticboy]

I got a question...

We've all heard of aerodynamic jump. The offset that happens when a bullet leaves the barrel and enters an immediate crosswind. According to Litz, this is a one time effect and is linear in the angular offset all the way to the target.

However, there has been much discussion that I've read concerning subsequent and ongoing vertical effects of crosswind down range. These are usually attributed to either the Magnus effect, or to Bernoulli's principle.

What is your take...in plain english please?

As has been said there is no Magnus or Bernoulli effect. However, as the bullet slows down then, assuming the wind stays the same, the bullet will have to adjust its angle and each time it does the jump will change. Also, if the wind speed changes then again the angle of the bullet will have to change. The change will be small due to the loss of bullet velocity but a sharp wind shear can produce a larger angular change. The effect of any changes on the bullet impact point will depend on how far the bullet has to travel.
According to the theoretical equations aerodynamic jump is a linear offset but the equations assume the velocity and the wind speed and direction stay the same. Changes will generally be small but may be significant where there are large changes in wind direction and speed due to terrain etc.

 

[Bart B.]

Study of wind drift and elevation changes with 22 rimfire ammo:

https://apps.dtic.mil/dtic/tr/fulltext/u2/a229713.pdf

Shows shots stringing from about 10 to 4 on the clock.

 

[xswanted]

If you were to ask Bryan, he’d tell you exactly what Ballisticboy, who knows a thing or two, said. 10% is a very rough number, to the point of being misleading in some situations. Jump as a percentage of windage is not a constant. It changes with range.

Knowing that does not require an engineering degree- only that you listen to someone who has figured it out and tested it.

To the op, practically speaking, you don’t have to worry about it at 600 and 1MOA targets unless you’re seeing highly variable, strong cross winds at the firing line, or you’re trying to make a first round hit in a strong cross wind.

Agreed, but I’m most concerned with it as it applies to hitting a coyote standing there so the first round hit is needed.

I’m not that good at making running shots.
And I’m even worse if I’m standing still and the animal is running.

 

[xswanted]

The vertical effect of a cross wind is caused by the gyroscopic effect of the bullet yawing to face the relative air flow produced by the combination of the bullet speed and the wind speed when it leaves the gun barrel. When the bullet yaws slightly left or right the gyroscopic reaction is to make the bullet yaw up or down depending on the wind and spin direction. It is not a Magnus effect since, as the bullet yaws to face the relative air flow, there is no air flow across the bullet.
The percentage of the vertical change compared to the down wind drift will change with range and bullet configuration. This is because the vertical effect is a linear change with range governed by the bullet aerodynamic and inertial properties whereas the down wind drift is closer to exponential, however as both will be affected by the velocity drop, it is not going to be quite that simple.
It is a real effect (some people try to claim it does not exist) and has lead to major challenges in the past for some projects. It is not a function of gyroscopic stability but it is dependent on many similar bullet properties so the two do tend to be related.

This is a delightfully detailed response and is way out of my league if ballistic understanding, but still was very interesting to read.

Thank you

 

[Depart123]

As has been said there is no Magnus or Bernoulli effect. However, as the bullet slows down then, assuming the wind stays the same, the bullet will have to adjust its angle and each time it does the jump will change. Also, if the wind speed changes then again the angle of the bullet will have to change. The change will be small due to the loss of bullet velocity but a sharp wind shear can produce a larger angular change. The effect of any changes on the bullet impact point will depend on how far the bullet has to travel.
According to the theoretical equations aerodynamic jump is a linear offset but the equations assume the velocity and the wind speed and direction stay the same. Changes will generally be small but may be significant where there are large changes in wind direction and speed due to terrain etc.

So you are saying that AJ isn't linear, it is parabolic given a constant wind. You are saying it is not only a result of the initial offset at the muzzle, but changes with velocity decay downrange. Is that correct?

 

[Ballisticboy]

So you are saying that AJ isn't linear, it is parabolic given a constant wind. You are saying it is not only a result of the initial offset at the muzzle, but changes with velocity decay downrange. Is that correct?

The angle the bullet is pointing to is driven partially by the ratio of the wind speed and the bullet forward speed so as the bullet speed changes then that ratio and the angle will change slightly. An additional complication is produced by the aerodynamics also changing as the bullet slows down causing further changes in the bullet angle. The changes in angle are going to be small and gradual and their effects on the POI will get less as the bullet gets nearer to the target.
I am not sure you would call it parabolic, more like a not quite straight line. Of course in assuming a constant percentage of down wind drift then a curved variation in jump with range is assumed.

 

[dcali]

So you are saying that AJ isn't linear, it is parabolic given a constant wind. You are saying it is not only a result of the initial offset at the muzzle, but changes with velocity decay downrange. Is that correct?

More that any change in direction will have an associated jump. If the wind dies down half way to the target, the bullet will re-orient itself into the airfow by tipping a little. That tipping will have an associated “jump” that changes the trajectory, but it’s not a sudden thing like when the bullet leaves the muzzle, so the term can be confusing.

If you assume constant wind, as most ballistic calculators do, this doesn’t happen because the airflow doesn’t change direction.