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Author Topic: Velocity and Wind Drift  (Read 1480 times)

rsterne

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Velocity and Wind Drift
« on: October 14, 2016, 06:57:05 PM »
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)....



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/jbmtraj_drift-5.1.cgi



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....



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
« Last Edit: October 14, 2016, 08:56:38 PM by rsterne »


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rsterne

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Re: Velocity and Wind Drift
« Reply #1 on: October 15, 2016, 10:21:49 AM »
Here is a similar chart for a typical pellet at 50 yards....



Note the optimum MV is about 100 fps lower than the same BC at 200 yards, because the velocity won't decay as far with the shorter distance....

Bob


PS, I will unlock this topic so that you can ask questions and I will try and answer them to the best of my ability.... PLEASE STAY ON TOPIC or I will lock it to prevent clutter....
« Last Edit: November 16, 2016, 08:45:08 AM by rsterne »
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olddog

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Re: Velocity and Wind Drift
« Reply #2 on: November 30, 2016, 06:39:00 PM »
Bob;

Was that chart made with the GS drag model?  I'm having issues duplicating the numbers unless I use the GS drag model.

Thanks;
Mike
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rsterne

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Re: Velocity and Wind Drift
« Reply #3 on: November 30, 2016, 08:32:11 PM »
Which chart?.... The first one was made with G1, the second post, for pellets, I thought I had done using the GA profile.... but you're right, the drift is too much.... I'll redo it.... Here it is for JSB 18.1 gr. pellets at 50 yards, using a BC(GA) of 0.036, in a 10 mph wind....



It still goes through a minimum wind drift at 800-850 fps....

Bob
« Last Edit: November 30, 2016, 08:46:25 PM by rsterne »
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olddog

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Re: Velocity and Wind Drift
« Reply #4 on: December 01, 2016, 03:38:53 PM »
That minima you found at 850 is what I found interesting as well.  That's what had me making drift charts.

Thanks for the great information as always.  You have given me another point to ponder when I tweak the regulation of this Condor.
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rsterne

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Re: Velocity and Wind Drift
« Reply #5 on: December 01, 2016, 06:56:24 PM »
As you extend the range, the optimum muzzle velocity increases.... as you want the pellet to spend as much time in "low drag" mode as possible.... However, at 200 yards with a pellet the optimum is still only about 950 fps (see the BC = 0.05 on the first chart).... It depends on the pellet, of course, but the minimum drift at 100 yards would probably occur with a good pellet, like a JSB Exact, at around 900 fps.... One thing to remember is that the curve is pretty flat across the bottom, so going 50 fps higher than the absolute bottom won't change the drift much but will give you a flatter trajectory....

One thing that isn't taken into account in the GA drag model is that the drag increases faster than it predicts in the Transonic region (Mach 0.8-1.2).... Some measurements I have made indicate that at Mach 1 the GA model may underestimate the drag by 30-50%.... While this won't affect the middle of the curve, it will raise the right hand end.... ie the drift at 1100 fps will be significantly more than the graph shows.... All the more reason not to push over 1000 fps with pellets....  :o

Bob
« Last Edit: December 01, 2016, 06:58:15 PM by rsterne »
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