Is Wind drift linear?

B.L.E., it is the time of flight between two points in the bullets trajectory that determines how far from the line of sight the bullet drifts sideways.

If you run a decent ballistics software program for a given bullet at a given muzzle velocity with its range increments set to 5 yards and maximum range at 1000 yards, it's easy to find out why. Find several range bands with 1/10th second time of flights. Those closer to the muzzle will be longer than those nearer the target. See how much drift the bullet has in each range band. That'll show you how much the bullet drifts at right angles to the line of sight for each 1/10th second of time of flight.

This is what I think's reality. Gonna run Berger's software on a 30 caliber 150-gr. bullet with a .450 BC leaving at 2700 fps then check its drift for each 1/10th second of flight to 1000 yards. I'll post the results but it's gonna be a while before they're up on this thread. Gonna be gone for a couple of weeks and internetting ain't all that great via satellite feeds.
 
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B.L.E. said:
A bullet traveling through the air never feels a side wind hitting it. If you are traveling at 1000 mph, what you experience is 1000 mph wind hitting you from the front. The natural wind of the air you are moving through adds, subtracts, or shifts the angle that this wind hits you by a fraction of a degree.
This "apparent wind" slows you down in the direction that the apparent wind hits you so if its direction shifts by a fraction of a degree, you slow down in that direction instead of the direction of your ground path.

Think about it this way and it becomes obvious why it's how much you slow down and not the time of flight that determines your course deviation.

As a bullet slows down, that wind angle also gets bigger, going from a small fraction of a degree to maybe a degree or so. The fact that slow bullets experience a larger headwind angle shift than fast bullets do explains why slow bullets drift more when the decelerations are equal.
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Well, sort of.

The bullet is not being "slowed down" by the wind. There are two vectors, friction and wind. Neither cares that the other exists, though the effect is as you describe.

It's actually not any different than considering the forces separately though. The bullet doesn't care that its going forward at 3,000 fps or hanging stationary in gravity free air. If the wind blew it would have the same effect. (Ignoring other forces for simplicity)
 
Exactly...that's what I'm saying.

It's simply the fact that a bullet's velocity slows- exponentially- as it goes downrange.

The higher the ballistic coefficient, the more velocity it retains, and the less it is affected by wind drift, compared to bullets with lesser BC's...and the "straighter" (meaning less curve) the line of flight.

Fairly simple concept...
 
B.L.E., it is the time of flight between two points in the bullets trajectory that determines how far from the line of sight the bullet drifts sideways.
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If that was strictly true, then the .300 Whisper (essentially a .223 necked up to .30 caliber) shooting that 220 grain Sierra hollow point boat tail match bullet with a BC of .608 with a muzzle velocity of 1040 fps should have drifted way more than the .17HMR shooting that 17 grain bullet with a BC of .125 at 2550 fps.

Time of flights at 200 yards
.300 Whisper .598 seconds
.17 HMR .324 seconds

Wind drift in 10 mph crosswind
.300 Whisper 3.8 inches
.17 HMR 15.7 inches


Lag time, the amount of extra time added to time of flight due to slowing down increases exponentially even if the velocity decay is linear.
To understand this, visualize driving 60 mph and someone going 80 mph passes you. Even though you don't decelerate, he keeps getting farther and farther ahead of you and the lag time between the two cars increases.

The only way for a bullet to stop the increase in lag time after being slowed down would be to somehow re-accelerate back to the original muzzle velocity and stay at that velocity, then the lag time would be constant from that point onward.
 
It's the time of flight for a given bullet, not between two bullets.

A given bullet will drift twice as far in 0.2 seconds as it will in 0.1 seconds.
 
We could probably eliminate all this conflict and confusion if we were to take our rifle and a given bullet to the range, shoot it in different conditions and at different ranges and SEE WHAT HAPPENS.

Silly concept I know.:D
 
A headwind will slow a bullet down, a tailwind will speed the bullet up a bit.
When a bullet transitions from sonic to subsonic, is a time when a wind can have a significant effect on bullet deflection.
 
A headwind will slow a bullet down, a tailwind will speed the bullet up a bit.
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A tail-wind doesn't speed up a bullet. The bullet is slowing from the millisecond that it is no longer under the influence of pressured gases in the barrel.

The effect of a tailwind or headwind is exactly the same as if the bullet had the same speed difference in stationary air.

In other words, a 3,000fps bullet with a 20fps headwind will slow at exactly the same rate as a 3,020fps bullet in stationary air.

The same bullet with a 20fps tailwind will slow at the same rate as a 2,980fps bullet in stationary air.

The bullet has no idea if it is moving or the air is moving around it.

Of course, the actual effect at any given instant is a bit complex, requiring calculus... as Δt approaches 0 and whatnot...
 
A tail-wind doesn't speed up a bullet. The bullet is slowing from the millisecond that it is no longer under the influence of pressured gases in the barrel.

The effect of a tailwind or headwind is exactly the same as if the bullet had the same speed difference in stationary air.

In other words, a 3,000fps bullet with a 20fps headwind will slow at exactly the same rate as a 3,020fps bullet in stationary air.

The same bullet with a 20fps tailwind will slow at the same rate as a 2,980fps bullet in stationary air.

The bullet has no idea if it is moving or the air is moving around it.
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It's also the same if you are in calm air and shooting from a vehicle going 20 fps. If you shoot forward, the vehicle's speed is added to the bullet and if you shoot rearward, the vehicle's speed is subtracted from the bullet. If you are firing at a target that you are chasing and the target is going as fast as your vehicle, it's exactly like shooting into a 20 fps headwind, same drop, same target impact speed.

Come to think of it, when I shoot at a range, I don't even have to think about the fact that the air I'm shooting through is actually traveling about 860 mph. It just seems calm to me because I and the ground under me and the targets I shoot at are also going about 860 mph from the west to east as the earth turns.
All wind is relative. The bullet can't tell the difference between wind caused by its motion and wind caused by the weather.
 
Vertical Deflection --- Head winds tend to slow the bullet because of added air resistance and the resultant drag. This is why the point of impact is lower. Tail winds cause just the opposite, thus making point of impact higher. This is called vertical deflection and is not as pronounced as horizontal wind deflection. At 1,000 yards a .30-caliber match bullet will have about a 1/2 minute vertical deflection up with a 10-mph tail wind.

With a 10-mph head wind, the deflection will be about 1/2 minute down. A half-value head or tail wind will be about 1/4 minute deflection. There isn't any set formula or system to help figure vertical deflection, and most shooters don't even consider it. Sierra Bullets' ballistic computer program gives all the vertical deflections, even with the oblique wind angles. If you want exact figures for all ranges and wind angles, this program is the only way to go.
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Quotes from the book: Dead On, Authors -- Tony M. Noblitt and Warren Gabrilska, Paladin Press
 
Yes, but a tailwind doesn't "accelerate" (in the common meaning of the word, technically any change in speed is an "acceleration") the bullet. It causes it to lose speed more slowly.

For instance, a bullet fired with a 20mph tailwind will decelerate as if it's muzzle velocity were 20mph slower than it really is.

A bullet with a 20mph headwind will decelerate as if it's velocity were 20mph faster than it really is.
 
As promised....with Berger's software; interesting stuff indeed. It's all based on a single wind speed at all trajectory heights above a horizontal line of sight.

8310186563_890750a0d7.jpg


Concusions: In a crosswind, bullets move:

* sideways faster as range increases.

* downrange less for each 1/10th second flight time.
 
That's a lot of drift between ~900-945 yards, 9" in 50 yards..
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Even if the wind had been dead calm from 900 yards out to the target, it would still have drifted nearly 9 inches between 900 and 945 yards. The bullet travels a curved path in the wind, once it leaves the wind, it travels a straight path, but that straight path still diverges from the original path of the bullet.

It's like a bullet that got deflected one moa by hitting a twig right in front of the muzzle. The deflected bullet flies in a straight line but it misses the bullseye more and more the farther away the target is. One inch at 100 yards, two inches at 200 yard, 3 inches at 300 yards, etc, etc.
 
1stmar, that bullet at around 900 yards is moving sideways about 13 times faster over not quite half the distance tha it does for the first 100 yards.

I gained a better insight to wind drift after I made this spreadsheet.

One other interesting thing is if the wind stops blowing on the bullet when it's at the 316 yard range. It'll still keep moving 1.81 mph sideways all the way to the 1007 yard target for 1.3 seconds. In that 1.3 second time, it'll drift about 35 inches from the line of sight.

If the wind didn't start blowing until 682 yards where it's velocity is 1588 fps, its drift sideways for the last 325 yards to the 1007 yard target will only be 15.30 inches according to Berger's software. Which kind of shows that wind during the first third or thereabouts causes more drift way down range than wind in the last third of target range.
 
Very interesting chart, that bullet is moving off target by~ 2" in every 10yards at that distance. Incredible...really highlights the skill required to shoot long distance and judge wind correctly. Bart do you have any similar charts of different calibers for comparison! Say something heavier, 50bmg and something lighter, 223 or 22-250?
 
1stmar, no, that one's the first one I've made. Took about an hour or two to get the formula's figured out for each column, run Berger's software then type its values in for the data columns so the segment colums would have good data.

Now that it's done, making another one for a different bullet's BC and velocities would take less time. Here's one for a .22 bullet at 3400 fps.

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Thanks Bart, so what techniques do you use when the wind is not blowing constantly. Do you make the dope change to compensate for the wind and the fire as many shots during a breeze as possible or wait for wind to die down, minimize the dope change and only fire during lulls?
 
I think the best thing to do if you're in competition with shoulder fired rifles and the wind's at higher speeds is to keep your shots hitting about 1/2 MOA upwind of center. Higher speed winds tend to gust more quickly than they fall off so those gusts will blow your shot to the other side of the 10-ring. When the wind speed falls of, it typically does more slowly so one can notice it and make a correction on the sights or hold off more readily.

On the other hand, long range benchrest matches shooters oft times put their 5 or 10 record shots downrange during a lull in the wind. Firing one shot every 10 seconds or so during a period of near constand wind speeds can do wonders. Such is life when the rifle's fired in free recoil and doesn't have to be reshouldered after each shot.

A lot depends on the range characteristics. Some are notorious for surprize gusts and others are more gentle. In hunting situations, just do your best and take your best shot.
 
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