Range Report Crosswind Effect on Vertical Path of Flight

[ZiaHunter]

I read somewhere and believe it was attributed to David Tubbs that crosswinds not only affect windage but also the bullet's vertical path depending on the direction of the wind and rotational direction of the bullet. Anyone have additional information on this subject?

 

[JLR]

I read somewhere as well that for a right twist barrel, a wind from the right (3:00) will tend to have the bullet impact slightly high and the reverse for a wind from the left (9:00). Can't recall where I read it though.

 

[Lowlight]

Yes, David does a lot with dissimilar wind drift and it's crosswind jump component.

His DTR Reticles has an offset for this built in based off a Right Hand Twist Barrel.

There are some factors that are involved and like most of this stuff, if you are not holding 1/4 MOA At 1000 yards you should probably not worry about it. Train yourself to shoot better first than you can start instituting some of these lesser offsets.

 

[mudpig]

Bryan Litz's book goes into GREAT detail on this subject and many many more regarding in-flight bullet dynamics

 

[_Windrider_]

http://www.davidtubb.com/index.php?route=account/download/free&download_id=30


Posted via Tapatalk HD for iPad

 

[Lowlight]

There is a reason none of these "effects" work out equally for everyone as the science would imply.

The Human Factor trumps all these effects combined.

 

[damoncali]

It has the same causes as deflections when shooting out the side of a forward moving aircraft - wind at the muzzle causing the bullet to "jump" off it's trajectory. Because the bullet is spinning, the horizontal wind causes a vertical jump. At short range, the vertical deflection is a higher percentage of the horizontal deflection, so it matters more. But the numbers are small (you might be talking about a 3/8" vertical deflection at 100 yards in a stiff wind), so only benchresters care about it. (And freaks like David Tubb who shoot better off hand than I do prone - although he didn't discover the idea - I think the first mention of it was in the early 1900's). At long range, the horizontal deflection dominates the vertical, so it's tough to care about, but it can be roughly half a minute or so.

Practically speaking, you will have to pay very close attention and have very confident holds in just the right conditions to observe it. I believe I've witnessed it when shooting F class (and been told I'm full of it for saying so). It's that small of an effect at long range.

 

[Lowlight]

They always forget to tell you the airplanes were going 450MPH when it was noted and recorded.

Left side gunners vs Right Side gunners had different offsets for shooting .50s out of bombers. That is the origin of it, as well, like noted the bench rest crowd when it comes to trying to score micro MOA groups, less than an 1/8th of an inch on paper can determine the winner so they worry about it. Tactical shooters are generally shooting from a variety of positions at 2 MOA Or better targets so it's less of a concern. With factory ammo, you'd be hard pressed to see it. And you better be able to read the wind to better than 1 MPH across the entire course/distance.

 

[jim_k]

It's roughly a 1:10 ratio. If you have to hold 20 in. to the right, you would want to hold 2 in. low, and just the opposite when holding for a left wind (with right twist barrel). It isn't exact comparing left to right, even at that, due to coriolis and spin drift effects. Just plug the shot into your ballistic program on your cellphone, and spin the knobs. Olympic Arms makes their own barrels, and very good ones; some of their rifles have left twist, which is what we should be shooting in the Northern Hemisphere, since the spin drift would almost totally eliminate concern about horizontal coriolis. For vertical coriolis, left or right twist doesn't make any difference, to my knowledge. Again, just plug the shot into your ballistic program, and take note of the effects that are calculated by that. Still, the greatest error will be inability to perfectly assess the average wind along the entire flight of the bullet. Soon, I hope.
Jim

 

[Lowlight]

You're iphone is not calculating anything they put in flat rate numbers that are the same for just about every situation.

You need pretty hefty software and computers to calculate this stuff correctly, none of the phones are doing that.

Change your zero range and the corrections are the same, because the software is NOT calculating anything just plugging in basic data.

 

[ZiaHunter]

Interesting discussion. If my Dad was still alive I would ask him since he was a ball turret gunner in a B24 with the 15th during WWII.

 

[Chanonry]

Just plug the shot into your ballistic program on your cellphone, and spin the knobs.
Jim

and if you do you can watch the drop stay resolutely the same as you increase the crosswind...

 

[damoncali]

I believe Applied Ballistics takes a stab at estimating the effect (roughly). I've not seen any other commercial calculator that does. The data required is the problem - it's just not available for sporting bullets.

 

[jim_k]

If your cellphone app doesn't have it, use the 1:10 ratio, and you'll be close enough. For F-class, if the wind changes from right to left wind, and you don't make a vertical correction, you'll likely think your rifle has lost it's accuracy. This is one of the least worrisome things in long range shooting; just note the hold-off from no-wind zero, and adjust vertical as needed, IAW the ratio above. I see people at matches wasting a shot to "see where it goes." It's going to go the wrong place. Make the change for the very first shot, then adjust off that.
Jim

 

[damoncali]

I think this most often shows up as a mysterious change in zero. You show up for a match in a decent crosswind, and all of a sudden you need to add a quarter or half a minute to your elevation. Most people just chalk it up to unknown variations because you probably aren't going to see a 15 mph cross wind AND dead calm in the same string of fire.

 

[Ro_Bird]

I am with Lowlight on this one. You've got to be a world class shooter to start seeing these effects, more often than not there are going to be other errors that will shadow these effects so stressing about them is a waste of time for some of us. Same problems present themselves when we start talking about spin drift and such. But if it’s something you want to know about I recommend finding a copy of Applied Ballistic For Long-Range Shooting, it covers the topic a bit plus a lot of other good stuff that will be infinitely more useful.

 

[Analytical Rifleist]

Cross Wind Discussion

Deflections and Drift of a Bullet in a Crosswind

http://www.exteriorballistics.com/ebexplained/articles/article2.pdf

 

[M40_A1]

Deflections and Drift of a Bullet in a Crosswind

http://www.exteriorballistics.com/ebexplained/articles/article2.pdf

That's some pretty funny stuff right there! I guess it takes an engineer.

I don't really think the phenomenon is the rocket science that pdf makes it out to be. Anyone who realizes the principal that causes an airplane wing to create "lift" should maybe have an idea why this happens. This is just my theory (though others may see the same, I don't know), but to me it makes sense.

As a bullet spins, the relative velocity of air around the sides of the bullet is equal....as long as there is no side wind causing a difference in velocity on one side as opposed to the opposite side of the bullet. An aircraft wing creates "lift" when the velocity of air across the top is higher than the velocity along the bottom. The higher velocity along the top in relation to the lower velocity along the bottom creates a vacuum which pulls the wing "up".

I believe it is the same with a spinning bullet. If a bullet has a right spin, and wind comes from the right, the wind causes the velocity across the top of the bullet to increase (against the spin, adding to the velocity), and to decrease across the bottom of the bullet (with the spin, subtracting from the velocity). This relative difference in airspeeds must create a vacuum at the top of the bullet, just like an airfoil (cross section shape of an aircraft's wing). That would be why a wind from the right causes the bullet to rise (to the 10 o'clock position, as some say).

A wind from the left would cause the opposite effect, increasing the velocity across the bottom while decreasing the velocity across the top, "sucking" the bullet down (to the 4 o'clock position, as some say), as the bottom would be where the relative vacuum would be in that case.

Makes perfect sense, I think.

 

[DTubb]

A full value cross wind at any range with a 2.0+ bullet gyroscopic stabilization will move the bullet impact up and or down (depending on direction) 3/8 of a MOA per 10 MPH full value wind.

Example - you can shoot 1/2 MOA at 200 yards with your rifle.
You go to a prairie dog town in the Texas Panhandle where I have been working them over. The only target you will see all day are their heads poking out of the their holes. We will give them a 2" head diameter for this illustration. All of the shots in this example are at 200 yards on a flat level plane.

You CAN kill them all when in a calm condition....
10 AM the wind comes full value from 3 oclock at 10 MPH. (right wind and right hand twist rifle will make you shoot high)
Your rifle's point of impact now shifts upwards 3/8 MOA or .75" at 200 yards. Your rifle's true zero is now 1/4" under the top edge of the Prairie Dog's head and the rifle shoots only 1/2 MOA.
This means your % of hits will fall slightly to slightly over 50%. The Right hand wind will cause you to shoot over the PD's 2" diameter target.

You shoot till lunch with a much lower kill %. You probably will dismiss your misses to Sunlight direction /Temp Sensitivity of you ammo/ change in position ETC...

When you finish lunch the Panhandle wind has now increase to it's afternoon full value of 15 MPH and you POI has shifted again upwards another 3/16 MOA or .375 MOA. Your total move is now .75" and .375" for a move of 1.125" (1 1/8"). You will now shoot the rest of the afternoon and only score a hit when your shot impact is in the lower edge of your 1/2 Moa group. The bottom 3/8" to be specific.

Your hit probability now approaches 25%..
If the wind increases to 20 MPH YOUR HIT PROBABILTY WILL GOTO ZERO.

Let us also imagine that when you sighted your rifle in at the 200 YD range there was a 5MPH wind from the right.. You already went to the dog town with your elevation zero off 3/8" high to start with.
Little stuff does add up..

DTubb

Thought this was worth posting again.

If you use BLITZ's data the error is slightly larger. -- Every 10 mph full value wind is worth .4 MOA..
DTubb

 

[Jaeger308]

A full value cross wind at any range with a 2.0+ bullet gyroscopic stabilization will move the bullet impact up and or down (depending on direction) 3/8 of a MOA per 10 MPH full value wind.DTubb

David,
<O</O
Just asking how could this bullet vertical drift be linear? First off wind, as much as we would like it to be is not a linear force even though we squeeze it into a linear equation best we can, second time of flight would be a factor of bullet drift off of line of flight at any given range? I am with Lowlight, human factors trump this...<O</O

 

[damoncali]

It's linear because it's not due to wind in the sense that you're thinking. It's an angular "jump" caused by a wind disturbance. It's a similar effect to having an unbalanced bullet. The net result is a constant angular deflection. With the unbalanced bullet, you have no idea which direction it's going, so you get a circular group. With aerodynamic jump, a side wind always pushes the bullet up or down (depending on which way the wind is blowing).

Whether or not human factors trump the effect depends entirely on how small the target your'e shooting at is, and whether or not a half MOA or so matters.

Edit: By "linear" I mean "close enough to linear". None of this stuff is truly linear.

 

[jim_k]

Let us also imagine that when you sighted your rifle in at the 200 YD range there was a 5MPH wind from the right.. You already went to the dog town with your elevation zero off 3/8" high to start with.
Little stuff does add up..

DTubb

I think you mean "a 5 MPH wind from the left". BTW, John Henderson from Frog School shooting team says "Hi." Something about a blind date.? I've worked with John for the past 20-some years. Jim

 

[DTubb]

If you ever have the opportunity to shoot a crosswind jump dedicated range (500 yd) you will forever be changed in your outlook of the wind as a vertical component.
Human error included.
Dtubb

 

[marduk185]

my coriolis compensator device mounted directly to me countersniper already accounts for this issue all day long if i do my part. otherwise, righty tighty lefty loosey criss cross applesauce. O.A.F!