Wind Drift

[rsterne]

For several years now I have been telling people that if you drive pellets too fast the Ballistic Coefficient will decrease, and the wind drift will increase.... I recently downloaded the latest version of ChairGun Pro 4, and it has some quite sophisticated models that show how the drag coefficient varies against velocity, from subsonic up to over Mach 3.... These models all have one thing in common, and that is a drastic increase in drag in the transonic region, typically Mach 0.8-1.2.... They have models for airgun pellets, spheres, .22 rimfire, cylinders, flat nosed bullets, and the G1 ballistics model that most BCs are based on.... When you input a BC for a pellet/bullet and select a model, the new ChairGun uses the Drag Coefficient at each velocity as the projectile slows down to calculate the next velocity increment and the drop and wind drift.... If you select a different model, you get a different result.... Likewise, if you select a different velocity, you will be using a different portion of the drag curve, and the results will therefore reflect that....

I set up a file to use four different .22 cal pellets/bullets and after selecting the appropriate model and BC for each, I input various velocities and recorded the wind drift at 100 yards and graphed the results.... I used a 10 mph wind at 90 degress to the line of flight, and here are the results....



This is the base data I used:

18 gr.JSB Heavy, BC= 0.036, GA model (airgun)_
27 gr. .22 Short, BC = 0.084, RA4 model (rimfire)
41 gr. RWS RN (basically a 22LR bullet), BC = 0.12, RA4 model (rimfire)
41 gr. Bob's Boattail, BC = 0.12, Custom model based on the Cd curve from the Kolbe Drag Calculator

The Cd plot from the Kolbe Drag Calculator is in the same format as the models used by ChairGun, so I just copied the data into the Custom model.... If you look up the BCs of .22 rimfire bullets, you will find a horrendous variation, so I took an average, and ended up using a BC which equals the Sectional Density, therefore using a Form Factor of 1.00, and I did the same thing for my Boattail.... Note that the above data is highly theoretical in nature, especially for my bullet, and measurements have not been made to confirm any of it.... I have, however, studied the BC of JSB Exact Pellets, and proven a rapid increase in drag exactly as predicted....

The most interesting thing, IMO, is that all the different pellets/bullets exhibit a minimum wind drift in the high subsonic range.... It occurs just after the drag coeffient starts to increase (the "Drag Divergence Mach Number"), which makes sense because at first the extra bit of velocity makes up for the higher drag, and the bullet soon slows into it's efficient range.... If you look at the velocity where the least wind drift occurs, and even more importantly the range over which it is the lowest, you get the following:

18 gr. JSB: 11.76" @ 900 fps (800-1000)
27 gr. .22 Short: 5.01" @ 1000 fps (900-1070)
41 gr. .22 LR: 3.49" @ 1000 (900-1070)
41 gr. BBT: 1.88" @ 950 (800-1000)

The rapid increase in the drag as the BBT approaches Mach 1 is due to the 60% meplat, a feature not found on the others.... This flat nose drastically increases the release of energy on impact, reducing penetration while creating a bigger diameter wound cavity.... It also lends itself to adding a large HollowPoint cavity without changing the BC other than by the slight loss of weight.... The trade off is that above Mach 1, the flat nose is, literally, a drag.... Fortunately, in airguns, we don't care.... Also note that the wind drift should be less than a .22 rimfire bullet of the same weight right up to Mach 1....

I think this analysis is pretty exciting.... It shows clearly why you should stay below Mach 1 with any airgun pellet/bullet.... and in fact from a wind drift point of view, 900 - 1000 fps is the best place to be, depending on the projectile.... There isn't a lot to be gained by adding 50 fps in terms of flatter trajectory, and every gain there is accompanied by a loss in resistance to crosswinds.... Lighter pellets, or ones with a poorer shape and a lower BC.... will start losing ground at an even lower velocity.... Trajectory we can deal with using holdover and MilDots.... Doping the wind is a lot harder....

Bob

[azoutdoorsman]

Wow, a lot to process.

Your BBT is sure incredible.

If accuracy is good, push the JSB heavy to 1000 fps for best BC and trajectory? Most of my shooting is at 50 yards or less with that pellet.

[rsterne]

I'm surprised nobody twigged to the fact I did all the drifts at 100 yards.... If you did the same thing at 50 yards, you would shift the graphs to a slightly lower velocity.... That is because over 100 yards the pellet is already slowing quite a bit.... If you are only concerned with a 50 yard maximum, 1000 fps is a bit too fast to get the least wind drift for a JSB Heavy.... Of course the wind drift at 50 yards is only 1/4 as great (~3").... The "best" velocity for drift at 50 yards is about 850 fps, and the optimum range more like 750-950.... so I would stay under 950 for an optimized 50 yard gun with JSB Heavies....

Bob

[AmBraCol]

So, are we talking initial (muzzle) velocity or terminal velocity?  Once it starts on its way a pellet's velocity is a steadily diminishing commodity

[rsterne]

The chart is, as labelled, muzzle velocity....

Bob

[Oldblades]

Very nice write up sir. I'm in complete agreement with the optimal speed being around 900-950fps. However theres lots to learn at slower speeds.

I actually got into air guns to learn to read the wind. Also AGs have the most variables to consider. Most centerfires dont need to be concerned with the transonic barrier since they mostly stay above Mach 1 through the duration of the flight.
They do, however, have spin drift to deal with along with a host of other considerations not to be concerned with by the airgunner.

Rimfire target loads deal specifically with speeds just under mach 1. Typically in the 1030-1050 range. That makes it pretty easy having a consistent velocity under mach 1 and generally not having to worry about the transonic crossover. 

What I have learned though is airgunning is tough, no doubt about it. Dealing with constantly changing pellet speeds in a single set, dramatically changes the winds influence. I have an absolute blast setting wind flags out, launching a .177 pill at 750fps and watching through a good 9x scope to watch the winds influence in relation to what the flags 'say' is supposed to happen.

[rsterne]

Unfortunately, even though ChairGun Pro 4 has a new GA ballistics model, I have my doubts that it will be a very good match to, say, a wadcutter.... Although it will produce better results than previous versions of ChairGun, some pellets will not match the model, and the drag divergence mach number (the point where the drag increases rapidly) will be much lower.... This will result in much lower optimum velocities for those pellets....

Bob

[AmBraCol]

The chart is, as labelled, muzzle velocity....

Bob

OK, that was what I was thinking.  However, something's nagging at the back of my mind (back in the dark corner where I've not swept for a while) about WHY you'd want a lower muzzle velocity at a closer range - but it's counter intuitive.  My intuition says that if launching at  950 is good for 100 yards it's at least as good at 50 yards.  Would you kindly remind me what factor I'm overlooking in the equation? Why would one slow it down for a closer shot?  Is it because of the initial slow down of the pellet at faster velocity due to higher resistance at the higher velocity?

[rsterne]

You got it exactly.... Starting at 1000 fps, the "median" velocity over 100 yards is (ie at 50 yards) is 811 fps (and down to 687 at 100).... However, the velocity at 25 yards is 891 fps.... If you dial back the MV to 950 fps for the 50 yard setup, your "median" velocity at 25 yards is 856 fps (783 at 50).... and if you dial it back further to 900 fps, the "median" velocity at 25 yards is 818 (751 at 50 yards)....

The initial large loss of velocity, which is the cause of wind drift, is compensated for over the 100 yard range by the longer time the pellet spends at velocities where the drag is lower.... At shorter distances, you're still losing that first big chunk, but not getting the payback from 50 to 100 yards.... Therefore you are better to back off the MV a bit to get the absolute least wind drift at shorter ranges.... It isn't a huge difference, of course, but measurable....

Wind drift is proportional to the DIFFERENCE in flight time between the real world and in a vacuum.... not just the flight time.... The quicker a pellet loses velocity, the more it will drift....

Bob

[plinker81366]

Bob comparing 800 to 1000 they both have the same wind drift, so as 850 and 950 .. to me it looks like the optimum mv's are at the low to mid 800's

[rsterne]

That would be equally true (800 = 1000), if it were not for the fact that higher velocities have more FPE and a flatter trajectory.... The lowest drift was at 900, but given the choice between 800 and 1000, why not choose 1000 to get more power and a flatter trajectory?.... Surely that would make the higher end a better choice?....

I do, however, have some new information.... I have quite a bit of data on the JSB Exact Series, chrony data at the muzzle and 25 yards at several different velocities from 500 to 1100 fps.... The pellets I used were the 13.4 gr. RS, 14.3 gr. Express, 15.9 gr. Exact, 18.1 gr. Heavy, and the 25.4 gr. King.... which all had remarkably similar drag profiles.... By using that, I have generated a new Custom Drag Profile in ChairGun specifically for that series of pellets.... What I did was keep adjusting the profile until I got the best fit for all my data, so that the BC is as close to a constant for each pellet over that velocity range.... Here is the new drag profile.... It is only valid from Mach 0.4-1.0, ignore the rest....



Using that drag profile, I redid the graph from yesterday for the JSB Exact Series..... Here is what I got....



You will notice that the drag increases faster over Mach 0.8 than using the model built into ChairGun Pro 4.... This shifts the curve to the left about 50 fps, putting the lowest wind drift at 850 fps, and the rapid rise starting at about 950 fps.... Assuming the considerable data I have accumulated is more accurate than the generic data in ChairGun Pro 4, then this suggests that for the least possible wind drift the best velocity range is between 800-900 fps, although I will continue to push towards 950 with my own guns just to get that little bit more FPE and flatter trajectory.... However, it does fit better with my experience than the previous graph which showed that velocities up to 1000 fps were not too bad for drift....

Other pellet shapes will produce different curves, but I think this is the best data I can provide at the present time for the JSB Exact Series....

Bob

[JeepEvil]

Excellent information as always Bob, so if I am getting the information correctly the wind drift is less at higher velocities however you lose velocity much more rapidly (depending on the BC) at the higher velocities which makes the pellet more susceptible to wind drift at the lower velocities... so idealy you want to push the pellet as fast as possible while maintaining as high a ballistic coefficient as possible to maintain the velocity of the pellet over the longest distance. which in your oppinion is the FT standard below the transonic range of roughly 880-900 fps (depending on pellet and rifle)?
JeepEvil

[Oldblades]

The easiest way that I associate the wind effect is to think of a spiralling football being thrown crosswind. Thinking of shooting a garden hose also works.
The turbulence in the transonic crossover is a whole nuther animal which you want to steer clear of, being going up through it or dropping into it. 

[rsterne]

JeepEvil.... Not quite.... If the drag coefficient was a constant, then pushing the pellet faster would results in less wind drift.... However, it is not a constant, particularly once you approach Mach 1, the drag at least triples.... This creates an optimum velocity at which the pellet will have the least wind drift.... Since over a long distance (I chose 100 yards) the pellet is constantly slowing down, then you have to start faster than the "optimum" velocity in order to be closest to the optimum for the longest time, and hence end up with the least overall wind drift....

You are correct that the FT guys have figured this out, by using just under 900 fps, at their 55 yard maximum range, they are operating not only in the sweet spot for the BC and minimum wind drift.... they are also pushing the pellet about as fast as they can (to get the flattest trajectory) and staying out of the transonic range above Mach 0.8 (900 fps) where localized shockwave formation can cause buffeting and loss of accuracy....

Bob

[JeepEvil]

Thank you Bob so as you stated in your above post you will likely continue using 950fps is this an ideal speed for longer ranges? is the increased early velocity drop in relation to the wind drift at this speed worth the trade off? I do not mean to pester just enjoy having you do all the technical work and reaping the benefits lol.
That being said I am pushing jsb 8.4's at 950-60 currently as my gun was tuned for the crosman 10.5's I just received my jsb 10.3's and H&N barracudas 10.65's Im sure I will do some tinkering with my velocity and calculating my BC but if 950 is a good starting point that is where I will start, than slowly drop it to see what changes happen at different ranges to my BC. unless ofcourse you have done this and found that there is no significant difference from 950 down to 880.
Thanks
JeepEvil

[rsterne]

The BC is better at under 900 than over.... but I think the small increase in wind drift if worth it to get the extra reach for my long range guns which are set up for varmints out to 100 yards.... I only use 700 on my Grouse Gun and 850 for an under 50 yard gun.... I also use the JSB Heavies in any gun that will shoot them well to decrease the wind drift through their superior BC....

It comes down to a compromise.... Some guns lose accuracy over 850-900 fps, so it makes no sense to push them faster.... I have a few guns that seem to hold accuracy to 1000 fps, so I tune to 960 or so....

Bob

[JeepEvil]

Perfect I know at this tune my gun is stacking the 8.4's at 40 yards at 950-60fps I typically do not use it over 50 yards but would like to be able to reach out so I will see what it likes to do with the JSB heavies and barracudas will shoot it at the current tune (around 870-880fps with the heavies) and check the bc at various ranges than adjust up and compare the differences as opposed to starting high and bringing it down.
I see in your numbers that the velocity drop from 950 and 900 at 25yrds is only a difference of 12 fps and at 50 is 18fps while staying approximately a 30+fps higher velocity, and the difference in velocity between the two at 25 to 50yrds is only 6fps so the tradeoff definitely seems worth it -18fps short range for +30fps at longer ranges of course than efficiency comes into play I guess I will be shooting a few more pellets than I was hoping hahah.

[Oldblades]

Just came across this page on wind drift and practical application. Pretty good and reasonably short read.

http://www.shebbearshooters.co.uk/files/influence_of_wind.pdf