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 Post subject: Velocity and Wind Drift
PostPosted: Fri Oct 14, 2016 8:57 pm 
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I decided to do this thread because of some misconceptions I keep running into regarding wind drift with airguns.... The comments inevitably blame the wind drift that plagues us on the low velocities, and consequent long flight times to the target.... In fact, airguns often operate in a "sweet spot" in terms of low wind drift.... Think I'm crazy?.... Consider this....

The amount of drift is not proportional to the flight time, but rather to the DIFFERENCE in flight time between the real world and what would happen to the same pellet/bullet starting from the same velocity in a vacuum.... The higher the drag, for a given Sectional Density, the quicker the projectile slows down, so the greater the difference between its flight time in air and in a vacuum.... The problem is, that the drag increases many fold as the projectile breaks the "Sound Barrier".... There are several Drag Models, which represent various shapes, and here are a few "drag curves" showing that rapid increase in drag in the Transonic Region (Mach 0.8-1.2)....

Image

If we use a typical drag curve, represented by the G1 Model (the orange line above).... and then use various Ballistics Coefficients, we can plot the wind drift for various muzzle velocities over any range.... I chose 200 yards because that is what is used at the Extreme Benchrest event, just run in Arizona.... Here is what happens for BC's of 0.05, 0.10, 0.20, and 0.40.... which spans pretty much anything we might see in airguns.... These charts are for a 10 mph crosswind, calculated using the JBM Ballistics Calculator.... http://www.jbmballistics.com/cgi-bin/jb ... ft-5.1.cgi

Image

The first thing you will notice is how important the BC is for reducing wind drift.... The biggest influence on BC is the Sectional Density of the bullet, with the shape of secondary importance.... Pellets or roundball might behave roughly like the yellow curve, a chunky slug like a 100 gr. in .357 cal might be like the blue curve, a long thin bullet like a 90 gr. .257 would be something like the orange curve, and a very well designed boattail spitzer might be like the grey line.... The other thing that is apparent is that the drift does NOT get less as the velocity increases above what we usually run with airguns.... In fact, it gets WORSE as we push Supersonic, and you have to reach velocities unheard of with airguns to get back down to the same amount of wind drift we get.... Counterintuitive maybe, but FACT.... Let's concentrate on bullets we might use at the BigBore shoot at the EBR....

Image

The important thing here is to look at the velocity where the wind drift is at a minimum.... For any BC we might use at that event, shot at 200 yards, the least wind drift occurs when using a muzzle velocity of about 900 fps.... True, the curves are pretty flat either side of that (particularly with a high BC).... but you have to push the bullets more than twice that velocity before the drift once again drops to what we achieve with the velocities we already use.... So, when you are cursing the wind when shooting an airgun.... don't blame it on the low velocities we use.... You are looking in the wrong place if you do.... Instead, you need to be looking at a bullet with a better BC.... When shooting around that 900 fps velocity, if you double the BC, you will cut the wind drift roughly in half....

The actual MV you choose will be governed mostly by where your bullet shoots the most accurately.... Anything between 800-1100 fps makes sense, but the closer you stay to 900, the less drift you will have to deal with.... Yes, the trajectory won't be as flat as if you push the bullet at 1050 fps, but gravity is a constant and can be allowed for by zeroing your scope.... The wind is anything BUT constant, so IMO you need all the help you can get....

Bob

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PostPosted: Sat Oct 15, 2016 5:26 am 
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This is as straightforward and understandable an explanation as can be found on the interweb. Thank you for that.


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PostPosted: Sat Oct 15, 2016 10:20 am 
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rsterne: What are the best "windbuckers" to use in .22 cal and .177?.....I shoot at 720 fps with 14 gr. and 950fps with 7.4 gr.I usually shoot at 60 yards and the crosswinds....or should I say swirling winds.....are usually 20 kms or so.My best days are with less than 10k winds but they're far and few between....Confused....should I go heavier in .177 and have a lower fps...although I like the fps to stay in the mid 800s for distance if possible.What would be the best brand and type of pellet exactly.....This constant wind is @%#$ing me off. :(

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PostPosted: Sat Oct 15, 2016 10:36 am 
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Generally a heavier pellet, with its correspondingly higher Sectional Density, will have a better Ballistics Coefficient.... The higher BC will have a greater effect on drift than the slightly lower velocity.... In addition, the above charts were for 200 yards.... At 50 yards, the sweet spot shifts to even lower velocities.... Here is an example at 50 yards....

Image

Note that the optimum velocity has dropped from about 950 fps for the same BC at 200 yards to about 850 fps at 50 yards.... There are two components to the BC, the SD and the Form Factor, which is a measure of the drag of the shape, where lower is better.... They are related by the formula....

BC = SD / FF

So you can see that the SD (which in a given caliber means the weight) is a major component in the BC.... For pellets, they can be grouped into a few main types by nose shape; flat nose (wadcutters) with a poor FF, round nose (hemispherical) with a good FF, pointed (not a lot better than flat), and other (hollowpoint, specialty, domed, etc.) which can be anything between bad and fair.... Generally, for bucking the wind, you want a good round nose pellet like the JSB Exact series.... and the heavier the better, within reason....

When comparing different calibers, the SD is most helpful.... You can calculate it from....

SD = Wt. / 7000 / cal^2 .... with the pellet weight in grains and the caliber in inches (squared).... For good round nose pellets, the FF is typically about 1.5, so you can estimate that the BC will be about 2/3rds of the SD.... Here is an example....

14 gr. in .22 cal.... SD = 14 / 7000 / 0.22 / 0.22 = 0.041.... and the BC should be about 0.027
7.4 gr. in .22 cal.... SD = 7.4 / 7000 / .177 / .177 = 0.034 .... and the BC should be about 0.023

So for your choices, the 14 gr. in .22 cal at 720 fps should have a slight edge over the 7.4 gr. in .177 cal at 950 fps.... but not a lot of difference.... I think a heavier pellet in .177 would be the best choice.... For example, if you go with a 10.3 gr. the SD would increase to 0.047, and the BC to about 0.031.... You should be able to drive that at just over 800 fps, putting you closer to the optimum velocity, and with a better BC.... I would try the JSB Exact Heavy and see how they work in your gun.... You can also try the 8.4 gr. Exact, it has a reputation for excellent performance in the wind, and accuracy in many guns....

Bob

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Airsonal;
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Proud Member of the 2000+fps Club!


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PostPosted: Sat Oct 15, 2016 11:12 am 
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My 2 cents.I am also from the frozen north and can attest to these seemingly endless winds. Sometimes the grass part way downrange blows to the left and near the target farther down the grass is blowing right..How do you set your scope for that? Lol.Just want to add that I have tried many pellets in windy conditions and the rounds are the best.I stocked up on the very pellets you mentioned.JSB exacts.


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PostPosted: Sat Oct 15, 2016 11:19 am 
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rsterne wrote:
Generally a heavier pellet, with its correspondingly higher Sectional Density, will have a better Ballistics Coefficient.... The higher BC will have a greater effect on drift than the slightly lower velocity.... In addition, the above charts were for 200 yards.... At 50 yards, the sweet spot shifts to even lower velocities.... Here is an example at 50 yards....

Image

Note that the optimum velocity has dropped from about 950 fps for the same BC at 200 yards to about 850 fps at 50 yards.... There are two components to the BC, the SD and the Form Factor, which is a measure of the drag of the shape, where lower is better.... They are related by the formula....

BC = SD / FF

So you can see that the SD (which in a given caliber means the weight) is a major component in the BC.... For pellets, they can be grouped into a few main types by nose shape; flat nose (wadcutters) with a poor FF, round nose (hemispherical) with a good FF, pointed (not a lot better than flat), and other (hollowpoint, specialty, domed, etc.) which can be anything between bad and fair.... Generally, for bucking the wind, you want a good round nose pellet like the JSB Exact series.... and the heavier the better, within reason....

When comparing different calibers, the SD is most helpful.... You can calculate it from....

SD = Wt. / 7000 / cal^2 .... with the pellet weight in grains and the caliber in inches (squared).... For good round nose pellets, the FF is typically about 1.5, so you can estimate that the BC will be about 2/3rds of the SD.... Here is an example....

14 gr. in .22 cal.... SD = 14 / 7000 / 0.22 / 0.22 = 0.041.... and the BC should be about 0.027
7.4 gr. in .22 cal.... SD = 7.4 / 7000 / .177 / .177 = 0.034 .... and the BC should be about 0.023

So for your choices, the 14 gr. in .22 cal at 720 fps should have a slight edge over the 7.4 gr. in .177 cal at 950 fps.... but not a lot of difference.... I think a heavier pellet in .177 would be the best choice.... For example, if you go with a 10.3 gr. the SD would increase to 0.047, and the BC to about 0.031.... You should be able to drive that at just over 800 fps, putting you closer to the optimum velocity, and with a better BC.... I would try the JSB Exact Heavy and see how they work in your gun.... You can also try the 8.4 gr. Exact, it has a reputation for excellent performance in the wind, and accuracy in many guns....

Bob




Thanks....Jsb's have just been ordered..…Im hoping for even BETTER accuracy in these constant 20 km winds.

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PostPosted: Sat Oct 15, 2016 11:47 am 
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I must be missing something. Normal BC is presumably for the direction a pellet is travelling in, right? But the wind is pushing on the side of the pellet, which has a totally different shape. So wouldn't we need to account for that?

Also, why does the speed of the pellet in the forward direction affect the drift rate from a crosswind?

Lastly, you said:
"The amount of drift is not proportional to the flight time, but rather to the DIFFERENCE in flight time between the real world and what would happen to the same pellet/bullet starting from the same velocity in a vacuum"

That makes no sense to me...the wind is acting on the projectile from the instant it leaves the barrel, so why isn't the drift a function of flight time?

There is an interesting look at factors influencing wind drift at http://www.backcountrychronicles.com/ballistic-factors-bullet-wind-drift/. In his example the biggest factor is distance, then velocity, then BC. But that was for a rifle, it could work out differently for an airgun.


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PostPosted: Sat Oct 15, 2016 2:00 pm 
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The concept that it is the DIFFERENCE in flight time that relates to drift.... ie how much the bullet slows down in the air.... is not something I dreamed up.... Do some studying on the subject, and you will find out that is what occurs.... I wrote this thread precisely because most people don't understand the basics of wind drift....

Something for you to consider.... If time of flight was the governing factor, then the following bullets would all have the same drift....

BC 0.40 @ 900 fps.... Range 200 yds.... Time of Flight 0.693 sec.... Drift 6.2"
BC 0.40 @ 1800 fps.... Range 347 yds.... ToF 0.693 sec.... Drift 22.0"
BC 0.20 @ 900 fps.... Range 193 yds.... ToF 0.692 sec.... Drift 10.1"
BC 0.10 @ 900 fps.... Range 182 yds.... ToF 0.693 sec.... Drift 16.8"
BC 0.05 @ 900 fps.... Range 165 yds.... ToF 0.695 sec.... Drift 27.3"

The link you gave was for Supersonic flight, and if you look at the drag curves, in the area of his data (about Mach 2.5) the drag coefficient is decreasing with velocity, so you would expect drift to decrease as velocity increases.... I agree that distance is the primary factor in wind drift (drift increases by the square of the distance), but for us, BC is the next most important, and additional velocity can be counterproductive....

Bob

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Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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PostPosted: Sun Oct 16, 2016 7:46 am 
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cbf123 wrote:
I must be missing something. Normal BC is presumably for the direction a pellet is travelling in, right? But the wind is pushing on the side of the pellet, which has a totally different shape. So wouldn't we need to account for that?

Also, why does the speed of the pellet in the forward direction affect the drift rate from a crosswind?

Lastly, you said:
"The amount of drift is not proportional to the flight time, but rather to the DIFFERENCE in flight time between the real world and what would happen to the same pellet/bullet starting from the same velocity in a vacuum"

That makes no sense to me...the wind is acting on the projectile from the instant it leaves the barrel, so why isn't the drift a function of flight time?

There is an interesting look at factors influencing wind drift at http://www.backcountrychronicles.com/ballistic-factors-bullet-wind-drift/. In his example the biggest factor is distance, then velocity, then BC. But that was for a rifle, it could work out differently for an airgun.


Those are logical questions that most people would have until you look deeper into what's happening to the pellet. At 10mph/14.7fps the air striking the pellet from the side is relatively inconsequential to the force of the air the pellet is encountering at the front 648mph/950fps.
If you imagine the air going around the front of a pellet at those high speeds the difference in air density on the windy side compared to the lower density on the other side is greatly magnified and will effect it's direction.
It's a little like a curve in a baseball. In that case it's the spin on the ball that's creating a slightly higher pressure, due to friction, on the side spinning forward and less pressure where it's spinning rearward. Even at those relatively slow speeds the small pressure difference at the front of the ball can have a large effect on it's direction.
It could also explain why a better Form Factor, which would have less friction moving through the air, would be less effected by the wind. A higher SD should help any pellet shape.

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PostPosted: Mon Oct 17, 2016 1:40 am 
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Thanks guys that's been a fascinating couple hours of reading.

I found this to be particularly useful as an explanation of what is actually happening to a projectile in a crosswind: http://www.exteriorballistics.com/ebexplained/articles/article2.pdf

This article from the same site covers the specific formulas involving last time and crosswind drift:
http://www.exteriorballistics.com/ebexplained/4th/532.cfm

Incidentally it looks like the.22LR folks are in the same boat as us airgunners, where increased muzzle speed gives worse wind drift.


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PostPosted: Mon Oct 17, 2016 9:06 am 
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100% correct, target velocity (subsonic) .22LR ammo drifts less at 100 yards than does high velocity (supersonic) ammo.... The HV falls subsonic at about 75 yards....

Bob

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Airsonal;
Too many! Springers, Pumpers, CO2, but I love my PCPs and developing them!
Proud Member of the 2000+fps Club!


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