GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: JungleShooter on July 25, 2021, 12:46:37 PM
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Suited-for-the-barrel Pellets Perform Badly in the Wind? WHY? HOW? ❓
I figure this might be the sub-forum that a reliable answer can come from.... 👍🏼
I should explain the issue:
(1)
So, we know that each barrel "likes" certain pellets.
(2)
So we test a bunch of pellets to find the "magic pellet" for our barrel — the suited-for-the-barrel pellet.
We do the testing in near perfect conditions, because if we did it with wind gusting our precision results would get messed up by the wind.
(3)
Then, we shoot those suited-for-the-barrel pellets in normal conditions — with WIND.
➔ Now, people have reported occurences where those barrel-suited pellets actually do not shoot good in the wind.
➔ WHAT?!? 😱
("do not shoot good" refers to group size — not the amount of wind drift — let's assume that the comparison is between pellets of similar BC — since comparing a wadcutter with a dome at 50y in the wind would be pointless.)
❓ I'm puzzled! WHY is this? HOW can this be? 🤔
Matthias
Here is just one report of such an occurrence, and I have read a fair share over the years:
https://www.airgunnation.com/topic/yrrah-pellet-rolling-does-it-work-a-study/#post-1051349 (https://www.airgunnation.com/topic/yrrah-pellet-rolling-does-it-work-a-study/#post-1051349)
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Unless going with a heavier weight, faster fps or moving to a slug,.... it is what it is. The pros learn to read the wind,... period. It makes no sense to move away from your guns favorite pellet expecting something magic to happen.
Your link refers to a wind tunnel,.... which is abut ideal for pellet selection. Most of us are not that lucky.
Good luck.
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Case in point of doing some testing of late in the Taipan Vet .177
Using this rifle to try it out as a Field Target rig and found "Accuracy Wise" the JSB Monster 13.4 weight pellets shot the best at > 20 fpe power limits.
The Original Straight waist AkA: trash cans accurate as they are MOVED like crazy in the wind being shot at @ 815 fps .. Bummer !!
The Newer "Redesigned" monsters which appear to shoot with the same BC value DON'T MOVE around nearly as much in the wind at the same speed … Go figure ?
SIDE PROFILE and AREA presented to the wind is very likely the reason IMO.
Scott S
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A different "moment" about the CG for the aerodynamic CP will cause a different amount of yaw as the pellet exits the muzzle in a crosswind.... This can cause not only "aerodynamic jump" (vertical dispersion due to a crosswind), but also "lateral dispersion" from the yaw itself.... The result is a larger group in a crosswind, and not just horizontally as you would expect....
Bob
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Just because pellets have the same weight and BC does not mean they have the same wind drift if tested against each other. There are a number of reasons why not, to do with the limits of the BC system, the inconsistencies of the wind making testing very difficult and the stability of the pellet and its ability to damp out yaw.
The methods generally used for BC calculation based on measured velocity drops or POI drop at different ranges cannot take account of the differences in the drag curves for the pellets. In calculating a BC all the pellets are assumed to have one basic drag curve shape which is not true. Combine the subtle changes in drag curve shape between the pellets and the variation in the wind speed and direction along the trajectory, and the pellets can easily have different impact points. This is where you need purpose drag curves for the different pellet designs, rather than a BC.
Wind, no matter how gentle it is blowing, is never constant in both speed and direction. I managed to get wind speeds and direction measured every second for a couple of hours on one of my firing trials. The results showed that variations from one second to the next could be 2-3mph wind speed with a change in direction. It was not a particularly windy day. On top of the variability from second to second, the average wind changed during the hour as well. When testing firearms, in order to try to minimize wind effects when comparing projectiles we would fire them alternately i.e. one of projectile A followed by one of projectile B then one of A then B etc. in what were called interleaved serials. To get a true result, you need to repeat the test on different days a minimum of three times.
Finally, there is pellet stability. When a pellet leaves the barrel it has to turn into the relative wind as explained here. https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.msg155928900#msg155928900 (https://www.gatewaytoairguns.org/GTA/index.php?topic=169459.msg155928900#msg155928900) When the pellet turns to face the combined airflow, it does not just turn exactly the right amount and stop, it will continue to yaw about the correct angle until the yaw is damped out. The differences in the damping of different pellet designs will give differences in total drag, the force producing down wind drift, which combined with the wind variability, will change group size. There is also the pellet cross wind vertical deflection, which is a function of stability. This will be different for different pellet designs giving different group sizes with wind variability.
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FWIW: I live in Canada and am used to shoot low power airguns (under 500 fps). In my humble experience the aerodynamics plays a big role in stability in windy conditions. I found domed pellets to be better overall performer compared to wadcutters and pointed pellets in up to 15 mph winds. Maybe ''playing'' with pellet weight/lead hardness/size can help. HTH.
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The depth of the rifling grooves can have a big effect in the wind, deep grooves spin up a ring of air around the pellet giving it a bigger profile.
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That's an interesting idea, James, although it should not apply right at the muzzle, because the pellet is "screwing" it's way into the air, as the twist rate imparted by the grooves in the barrel matches the forward velocity.... However, a large distance downrange, when the forward velocity has decayed at a greater rate than the RPM, I suppose it might have some validity.... Miles, do you have a comment about this?....
Bob
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The only difference I can see it making is to very slightly increase the spin damping coefficient. The spin velocity at the pellet surface is very small compared to the forward air velocity, which will probably not notice any slight changes induced in the boundary layer air.
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I’m no way near qualified to even engage in this conversation, but, there’s one thing that I haven’t seen any of y’all mention. And I think it has to come into play here. And that is barrel harmonics. Every barrel, even one’s from the same make and model gun, may vibrate at slightly different frequencies, thus impacting the experiment with other variables.
Richard
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True, and very important to accuracy.... but that shouldn't be different when the wind blows....
Bob
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True, and very important to accuracy.... but that shouldn't be different when the wind blows....
But for the “suited for barrel pellets “ it seems highly relevant.
Bob
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Do you sometimes get anwers to questions, and you end up not liking the answers...? 🤣
Yeah, the truth is sometimes a bit inconvenient....
But personally I rather believe an inconvenient truth — than to believe a convenient lie.
So, thank you all for clarifying this issue for me! 👍🏼
I'm very thankful for our GTA forum and the collective wisdom that it offers to the WORLD of airgunning.
And it is truly special that PEOPLE take time out to reflect on questions and issues, and make the effort to answer them and offer suggestions.
Thank you so much!! It's great to have people around like you guys! 😊
Matthias
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Pity we don't have access to video equipment like this; that enables muzzle to target video capture: https://youtu.be/xpJ8EoGmLuE?t=489
Now imagine a clear "wind tunnel" that enables cross winds to be varied in speed and direction, and applied anywhere or everywhere down range...
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There is a system called Flight Follower which was used to track 5.56mm bullets. Tracking and seeing small projectiles is a lot more difficult than it is for the big stuff, but it can be done. It is hugely expensive to use.