Rookie Wind questions about drift velocity and changing wind conditions...

[Nomercy]

Tell me something about wind? Specifically, do you account for "drift velocity" or not? When a bullet is in the wind, it's getting pushed laterally, but when it exits the wind, even tho it's not getting pushed anymore, it still has some lateral "drift velocity" right?

Most of my shooting is done either no wind, or in pretty constant wind areas, but I have a question about wind calls based on "drift velocity"... I have 2 examples from my own hunting experience of shots that I passed on because I couldn't make up my mind about the wind...

For example, a 600yrd shot with a .300WSM 150grn on a whitetail doe. 400yrds of open flight with a full value wind left to right 15mps, but then enters a valley with zero wind (slight standing swirl at the mouth, but gentle). I figured the hold over at 16" for 400yrds. (400yrds/100*15mph)/15= 4MOA=16" at 400. My question is, where does the bullet go next? I'm estimated at 400yrds, the bullet had about 8" per 100yrds lateral drift velocity... So over the next 200yrds, I estimated another 16" of drift, total of 32"... But some of my buddies say I shouldn't account for "drift velocity", so the adjustment should have been just 16" instead of 32".... Not wanting to potentially miss by 16", I passed on the shot...

My next example, I spotted a coyote across the "tip" of a horseshoe valley from one side to the other with a "valley wind" flowing like a river through it 10mph gusting to 15-20mph. 350yrds across one leg, then 70yrds across the "peninsula",and another 325yrds across the other leg. Total shot was 745yrds. 0-350 under full value 10mph right to left, 70yrds no value (in my face), then 325yrds of full value 10mph left to right. Accounting for the winds individually, I'd expect 8" drift to the left from the first wind, then 8" back to the right in the 2nd wind. Ignoring drift velocity, since the bullet velocity is slightly lower in the 2nd section, I would push slightly to the left of zero. However, if I account for drift velocity, I would expect an additional 1.5" of left drift in the 70yrd "no value" section. So I would expect 9.5" of left (0-350 + 350-420yrds), and 8.25" of right drift (420-745), so I would adjust 1.25" right. The coyote was moving through pretty quickly, so by the time I decided it was only 1.25" diff between both methods, he was gone.

So what's the verdict? Do you account for the lateral velocity across NON-wind areas or not? (essentially do you assume the bullet lines back out once it exits the wind, or do you expect it to keep drifting laterally?)

 

[whiskey08]

for a rookie wind question, that is a graduate level equation. With spin drift, coriolis effect, now drift velocity. My head is gonna pop.

there are some ballistic caculators that allow multiple wind entries (i think). I would check one of those. The other would be to just keep shooting it and learn from experience.

I live out west and shoot lots of long range stuff in winds that usually start at 10mph, I use an old army equation to determine drift and it has always worked. granted that i never considered velocity drift. I just do the math and shoot and it works. (most of the time)

 

[Nomercy]

Whiskey... I use the same "army" formula for calculating drift off-the-cuff, and it's how I figure the drift velocity (figure out the drift at target range, then plus 50yrds and minus 50yrds, shows the rate of change of the drift, aka lateral acceleration, so I can pin down an approximate lateral velocity).

I guess my point is, in my opinion, even after the bullet exits the wind region, it will still have, and keep, a lateral velocity, which I believe has to be accounted for, but I haven't had enough opportunities to shoot across SIGNIFICANTLY changing wind zones. I've done a lot of shooting where it's ALL one direction, but stronger or weaker in certain areas, but those two instances above threw me for a loop.

I guess another example would be sniper's shooting down a city street with wind perpendicular to the road. The bullet will take wind as it passes each cross street, but not between buildings. In my opinion, once the wind "bumps" the bullet at the cross street, it'll still keep drifting sideways even while it's flying between buildings, protected from the wind. The drift velocity would remain constant, instead of accelerating, but it'll still drift...

Or am I way off base? Does the bullet stop drifting sideways from line of sight when it exits the wind? Or does it continue to drift due to it's lateral drift velocity?

 

[BoydAllen]

In short range benchrest (1-200 yd.), matches feature columns of wind flags, one in front of each firing position. Often, flags along any given line, from bench to target, are in complete disagreement, and switch positions from moment to moment. Until you have spent some time observing one of these spectacles, you really have no idea how complicated the wind really is, and how oversimplified imagined models are.

Within the lore of this particular sport, it is generally thought that what the flags nearest the shooter are showing should be given greater weight in estimation of the bullet's deviation from the point of aim. I think that this is consistent with the thought that horizontal momentum caused by winds that are not parallel to the line of sight does not magically evaporate when the force that produced it stops.

 

[Nomercy]

Boyd, so if I'm reading your 2nd paragraph correctly, or rather your last sentence in fact, that you're agreeing with me that the "horizontal momentum", what I'm calling "drift velocity", continues even after the bullet exits the wind condition, correct?

On short range matches, yeah, I'm plenty familiar with disagreement and quick changes between range flags, but typically for those matches and those short ranges, estimating the appropriate direction and magnitude of the wind isn't too difficult (ok, let the flaming begin!). But those changes in direction are usually relatively short, say instead of being 2 or 3 conflicting range flags over 200yrds, I'm curious about 200-300yrds worth of range flags conflicting with 200-300 yrds worth of OTHER range flags over a 600-1600yrd shot. Messing up the correction at 200 is usually a few tenths of an inch error for me, but having a 16" error (see example 1 in my initial post) is a whole different ball game...

 

[Erik Cortina]

When wind acts on a bullet, it doesn't simply make it drift, it changes the bullet path. So, after the wind quits affecting it, the bullet will continue to travel in that path.

The closest flags "dictate" the direction of where the bullet is going to go and how much, the farthest flags "modify" the path, but don't change it, unless the wind down range is much stronger or has a much greater value than the close up wind.

Hope that makes sense.

 

[BoydAllen]

Yes, it is my belief that the direction of the bullet is determined by the sum of the forces acting upon it, at any given moment, some of which are momentum, whether it be from the wind, or the bullet's initial velocity and direction as it leaves the muzzle. Of course, at extreme ranges it becomes impractical to set out flags every 25 yards, which can lead to errors in our perception of the wind down range. Of course, this is why it is common for sighter shots to be allowed in various disciplines. If shooters could simply note the available information and make the necessary calculations, there would be no need. As it is, the problem is sufficiently difficult to make matches "interesting".

 

[brxbrad]

Good Question but.....Why do people pay to go up tall buildings and then put money in binoculars to look at things on the ground?

 

[whiskey08]

I think everyone makes perfect sense.

If your counting on a first round hit at extended ranges, then all crosswinds and changes should be factored in.

Nomercy

with the new formula compared to the older one, the "constant" changes for different ranges. It is in fact taking into account that a bullet doesnt drift a set number of inches per hundred yards. The drift accelerates the further you get for a few reasons. "Velocity drift"??

using the formula, the difference between 800yds and adding an additional 50 yds for velocity drift (850yds) is three inches.

If I use the new constant of 13 at 800 yds the drift comes out the same as if I used the old constant of 15 at 850yds .

Im sure Im not making any sense but to make it short, if your changing the "constant " with the appropiate range then I think your drift is mostly accounted for.

 

[Nomercy]

Thanks guys for bearing with me. My big confusion is coming from shooting through wind into relatively long "dead zones". I really don't like to be TOO reliant on "sighters", since I like to be able to to make cold bore shots while hunting just as well as I make "verified" shots at a match. So I tend to be a little over-analytical for my estimations...

Ok, so I'm an engineer, so I'll admit that I'm guilty of carrying over my professional experience into my "hobbies", so I can't help but analyze a bullet's flight the same way I would a physical issue at work, so here's where I'm coming from... (Warning, it's gonna get nerdy!)

Consider the classic "falling ball" example. A ball is dropped from a rest (0fps) and gravity accelerates it to fall. Gravitational acceleration is 32fpsps, so after 1 sec, the ball will have a velocity of 32fps. However, the ball will have only fell 16ft. If gravity were magically shut off, the ball would continue at 32fps, so after 2sec, it would have fallen 48ft (16ft in sec1, and 32ft in sec2). Similarly, if a bullet is 2" off at 100yrds (2MOA), it is likely actually travelling at 4" per 100yrds, so when it enters a dead zone, it would then travel ANOTHER 4" off of zero by 200yrds, for a total of 6" (3MOA).

So, if I apply that line of thinking to my 600yrd shot example, 400yrds of 10mph wind, then 200yrds of NO wind, I would estimate at 400yrds 2.66MOA of drift, or a hold over of 10.66" [(400yrds/100*10mph)/15 = 2.66MOA, @400yrds = 10.66"]. However, if I look at 350yrds, I estimate 8" of drift (2.33MOA), and at 450 yrds, I estimate 13.5" (3MOA) So between 350-400, the bullet drifts 2.66" in 50yrds. Between 400 and 450, it drifts 2.84" in 50yrds. So taking the average, I estimate that the actual lateral speed (drift velocity) is about 2.7" per 50yrds, or 5.2" per 100yrds. Basically, the POSITION of the drift, is still behind the SPEED of the drift. When I plot it out graphically, my way comes up correct, but what happens on paper isn't always what happens in real life, and I haven't had enough opportunities to shoot through significantly long "dead zones" to have "felt it out" myself.

One other shooter I spoke with about this says I should assume that the 2.66MOA will continue carrying on beyond 400yrds, but will not increase, so at 600yrds, the POI would be 16" off of a zeroed shot. However, that's counting on 2.66" per 100yrds, NOT the 5.2" per 100 that I calculated above. In my case, I would estimate the strike to be 21" (10.66 + 2*5.2). 16" vs 21" is a pretty big difference.

So what would you estimate? Am I just way off base by using the lateral speed of the bullet?

 

[BoydAllen]

Scroll down:

http://www.stevespages.com/page8e.htm

 

[necchi]

I'm tryin to wrap my head around this too, to sum it up,
I don't think so.

When the wind stops, the bullet will continue on the altered path without further drift.
If it does continue it's drift it would be for just a short distance from the momentum, but cease, and drift would not be part of a linear continuance.
Look at things that are blown by the wind. A leaf doesn't continue drifting away from a tree when the wind gust stops, it just falls. Or even lighter things like Poplar or Thistle down.

 

[Forum Boss]

One thing a beginner should understand when learning about wind drift is the "Rule of the Square". What this tells you is that, given a constant, full-value (90 degree) wind along the bullet's trajectory, the effect of wind equates (roughly) to the square of the increase in distance. This explains why it is WAY more difficult to shoot a 1/2 MOA group at 500 yards compared to 100 yards. It we take those distances (100yd and 500yd), the increase in distance is 5X so the increase in drift is 25X according to "Rule of the Square". That may seem crazy, but it is very close to the solutions provided by the best Ballistics calculators. For example, here is what JBM predicts for a 6mm, 105gr VLD Berger, launched at 2900 fps, using G7 model, 10 mph wind speed, full value (90 degrees), 70deg temp, 500' altitude:

Drift at 100 yards: 0.6 inches
Drift at 500 yards: 15.6 inches

Rule of Square predicts 25x drift increase
JBM calculates 26.0x drift increase (15.6 divided by 0.6)

Pretty darn close eh?

For more about the Rule of the Square, read this:

http://www.6mmbr.com/winddrift.html

 

[6BRinNZ]

Maybe purchase Brian Litzs book, this explains a lot of the cause and effects, what the bullet is actually doing in flight be it subject to wind or not.

Brian has also posted a question on this forum reguarding what information is needed for his next iteration. It could be worth while to post on that as well.

Certainly you will getting a ballisticians answer to your questions.

 

[BoydAllen]

Necchi,

The two examples that you gave are greatly different than a bullet, which has compact shape and is dense. Perhaps a billlard ball that has accidentally brushed against another ball would be a better illustration. Once a bullet's initial line of flight has been modified, it has no memory of its previous direction. Discussions of the effects of discontinuous crosswinds are largely guesses, because real knowledge of wind conditions at various distances is thin to nonexistent, but my guess is that if you look at a bullets flight as a string of segments, its position as it leaves a particular segment is a combination of where it was headed as it entered it and the additional forces that acted upon it while traversing it.

 

[Nomercy]

Look at things that are blown by the wind. A leaf doesn't continue drifting away from a tree when the wind gust stops, it just falls. Or even lighter things like Poplar or Thistle down.

A leaf doesn't continue to drift in the wind because it has a a substantial surface area (high form drag) and a low momentum (low mass), AND because gravity is working against the drift (leaves typically loft due to low pressure in the wind, and higher pressure in the stable air against the ground).

My question isn't about increasing drift over long wind effects (but thanks for all of the links to articles), it's about how to account for drift across "dead zones" after a wind zone nudged the bullet. With shorter range wind changes over a long shot, I'd figure an average wind speed and appropriate value over the range, but for a long dead zone, I expect the bullet will keep some sideways momentum and continue to drift. Over 20ft, this probably wouldn't add up to much, but over 200yrds, I expect it to be pretty substantial.

Another anology is a deflected bullet. If your bullet strikes something in it's path, the object exerts a deflecting force. Even though the force only acts for a VERY short time, the angle of the bullet's flight is forever different. If the deflection is 1MOA, then it will continue to carry on at that "angle" forever more, not line back out. The physical deflection might only be 1/10th of an inch, the the angle of deflection is what really screws the pooch.

I guess my point is, what angle do you use? In my 600yrd example, the bullet is off by 2.66MOA at 400, and is actually travelling at an angle of 5.2MOA, so I'd expect it to stay 5.2MOA off from that point on. (5.2MOA from the end of the wind, not as viewed from the shooters position).

 

[Nomercy]

One thing a beginner should understand when learning about wind drift is the "Rule of the Square". What this tells you is that, given a constant, full-value (90 degree) wind along the bullet's trajectory, the effect of wind equates (roughly) to the square of the increase in distance. This explains why it is WAY more difficult to shoot a 1/2 MOA group at 500 yards compared to 100 yards. It we take those distances (100yd and 500yd), the increase in distance is 5X so the increase in drift is 25X according to "Rule of the Square". That may seem crazy, but it is very close to the solutions provided by the best Ballistics calculators. For example, here is what JBM predicts for a 6mm, 105gr VLD Berger, launched at 2900 fps, using G7 model, 10 mph wind speed, full value (90 degrees), 70deg temp, 500' altitude:

Drift at 100 yards: 0.6 inches
Drift at 500 yards: 15.6 inches

Rule of Square predicts 25x drift increase
JBM calculates 26.0x drift increase (15.6 divided by 0.6)

Pretty darn close eh?

For more about the Rule of the Square, read this:

http://www.6mmbr.com/winddrift.html

Just some "fun info" behind the rule of the square. Again, it's gonna get a little nerdy...

Any constant acceleration can be modeled by a "square" principle, whether it's bullet drift or bullet drop due to gravity. In a contant wind, the "Form Drag", aka wind force, exerted on the bullet will be constant. Fd = 1/2*p*A*Bc*v^2, where Fd is the drag force, p = air density, A = area of the bullet (presented to the wind), Bc = bal coef, and v = wind speed. So if air density, bullet size and shape (A and Bc), and wind speed doesn't change, then the force acting on a bullet doesn't change. Since the Force is constant, and the bullet weight is constant, Newton's 2nd law (F = m*a) show the acceleration of the bullet by the wind is constant.

So then it becomes a simple high school physics problem... If acceleration is constant, then displacement can be calculated by:

X = 1/2*a*t^2

where X is the displacement, a is the acceleration, and t is the flight time

So if we assume our bullet doesn't slow down, then the time it takes to fly 1000yrds will be 10 times longer than it takes to fly 100yrds.

For simplicity, if a = 20 feet per second per second, and we say a bullet has a MV of 3000fps, then it takes a bullet 0.1s to fly 100yrds, and 1s to fly 1000yrds.

The displacement at 100yrds is 1/2*20*(0.1)^2 = 0.1ft = 1.2" at 100yrds.
The displacement at 1000yrds is 1/2*20*(1)^2 = 10ft = 120" at 1000yrds.

120" = 100 x 1.2" = 10^2 x 1.2".

The major reason for the descrepancy in professional calculators and the rule of squares is that in the real world, our bullets slow down. A bullet DOESN'T actually travel 10x as far in 10x the time. If we calculated the ranges based on flight time instead of using 100yrd increments, the drift at those ranges would more closely follow the rule of squares.

Additionally, it takes some amount of time for the wind force to achieve it's maximum force on the bullet (known as boundary layer developement). As the wind first acts on the bullet, it does not act with its full force because the flow profile (air over the body) has not yet regularly formed. So at short ranges, the actual drift on the bullet will be slightly LESS than what would be mathematically expected based on the steady-state force of the wind.

So accounting for the velocity reduction of our bullet and the transient "wind slip" at SHORT ranges, it is more likely that the rule of squares will be most accurate for estimating drift at ranges where the velocity doesn't change much, and the "reference drift" i.e. shooting at 100yrds to determine drift for 1000yrds, should be significantly down range to have established steady-state drift force.

Ok, enough nerd-talk... I'm off for the range...

 

[BoydAllen]

1. Do you have any sort of wind flags?
2.Lacking accurate information about what the wind is doing any place except perhaps where you are standing, how do you expect to calculate something useful?
3. Theory and calculation cannot make up for lack of data.

 

[Nomercy]

1. Do you have any sort of wind flags?
2.Lacking accurate information about what the wind is doing any place except perhaps where you are standing, how do you expect to calculate something useful?
3. Theory and calculation cannot make up for lack of data.

I use range flags when I'm on the bench, and I typically post range flags at known positions on my hunting areas, or at least tie flags to natural items, like trees, fences, fence posts, etc. I've been shooting competitively off and on (3 position, buffalo bore long range, silhouette, and NRA smallbore) for 15yrs, so I still use every opportunity I can to practice my fieldcraft (relating what the Kestrel says compared to mirage, range flags, and natural indicators like grass, trees, etc).

My luck is that I live, shoot, and hunt in pretty flat land, so having significant switch winds is a relatively new experience for me. Out of all of my years hunting, I can only recall these two instances where I was so unsure about the wind correction that I passed on the shot. I honestly assumed it would be a pretty cut and dried question that some of the "hill shooters" could answer off the cuff.

I realize that wind shooting is as much of an instinct as it is a science, but I figured that while these long section switch wind shots are pretty rare around here, there has to be areas in the country where they're a day to day occurance?

 

[BoydAllen]

It would seem to me that you have two alternatives...use your first shot as a sighter (if you can see where it hit) or get closer. Good hunting