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Do pellets tumble ?
- Thread starter Twiceshy
- Start date
I can not say i have seen pellets tumble, but i have seen them spiral wildly ( JSB Beasts out past 70 M )
With the sun behind you it look as if the pellet swell up to a huge size as its rear end is just whipped around ( i assume )
I have also noticed some times that a pellet will have a frisbee like flight path, so in the scope it look as if the pellet hook into the target, sort of like a baseball curb ball.
If it was strait up / down i would say that the flight path, but i have seen it in the horizontal plane, or close to that.
This do mean accuracy or at least repeatability is out of the window as holes will be appearing very erratic.
I have also tried 24 grain .177 slugs, and having them hit pretty darn close to where i was aiming, but instead of making round 4.5 mm holes in the paper they made 4.5 X 10 mm holes, which i believe is what people call key holing.
As the accuracy was sort of OK at 130 yards, i dont think the slug been tumbling all the way there, but clearly at arrival it was 90 deg of ideal trajectory.
With the sun behind you it look as if the pellet swell up to a huge size as its rear end is just whipped around ( i assume )
I have also noticed some times that a pellet will have a frisbee like flight path, so in the scope it look as if the pellet hook into the target, sort of like a baseball curb ball.
If it was strait up / down i would say that the flight path, but i have seen it in the horizontal plane, or close to that.
This do mean accuracy or at least repeatability is out of the window as holes will be appearing very erratic.
I have also tried 24 grain .177 slugs, and having them hit pretty darn close to where i was aiming, but instead of making round 4.5 mm holes in the paper they made 4.5 X 10 mm holes, which i believe is what people call key holing.
As the accuracy was sort of OK at 130 yards, i dont think the slug been tumbling all the way there, but clearly at arrival it was 90 deg of ideal trajectory.
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@dgeesaman tomorrow I should be free. Between the Aerodynamics of Ping-Pong balls, shuttle-cocks, dimpled spheres Freezbies, Canterbury Tails, AGN, and Emily Dickinson, it has been a 6 week LSD trip.
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On the first of these references, it is unfortunate that the tests were all carried out on relatively high nose drag pellet designs, all with recognized poor BC values. This led them to make the wrong conclusion on where the majority of the drag is generated. The wind tunnel tests on pellet shapes I initiated when working also gave some spurious results from pressure measurements (though they were flare stabilized bomblets, not pellets, honest guv)
As for using CFD for pellets, if you were to make a list of projectile characteristics which cause CFD accuracy problems, pellets have them all. It does produce pretty pictures, which look impressive, though.
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I am eagerly awaiting your peer review paper proving the perceived errors in the papers I referenced are actually errors as well as one additional paper ~ Nicos Ladommatos - Influence of air pellet geometry on aerodynamic drag .On the first of these references, it is unfortunate that the tests were all carried out on relatively high nose drag pellet designs, all with recognized poor BC values. This led them to make the wrong conclusion on where the majority of the drag is generated. The wind tunnel tests on pellet shapes I initiated when working also gave some spurious results from pressure measurements (though they were flare stabilized bomblets, not pellets, honest guv)
As for using CFD for pellets, if you were to make a list of projectile characteristics which cause CFD accuracy problems, pellets have them all. It does produce pretty pictures, which look impressive, though.
Thanks!
Troy
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So you two are both basically telling us pellets suck. And we need to be shooting slugs? Lol
Sorry. My attempt to inject some humor
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Thank you all for your replies and thoughts. I read them and considered all the possibilities offered. Peashooter described the nonrounded holes as keyholing, which is a great way to put it. Beerthief said to use thicker paper for the target, excellent advise. I make targets from posterboard/foam presentation board from dollar tree. Mounted on a DIY pellet trap made from a storage bin. I shoot from the same spot, 25 yards to the target. Spiraling has been suggested, along with deformed/defective pellets. Which makes sense, seeing as all pellets came from the same tin. Washed, lubed, and weighed, and shot at the same session. With the pellets that hit in the rings are nice round holes, as the ones off the rings are keyholes. Perhaps considered flyers. Again, Thanks to all, it was great to see so much response to my post. I am still learning, Thank you!
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When pushing my .177 10.3 grain long waisted pellets too fast I found that after they passed the apex of their trajectory they would drop into the target nose high and "keyhole", this was at 60 yards. At about 70 yards they would spiral.
So I slowed things down, went to a standard shaped 9.57 grain diabolo and tuned for N50 competition.
My original intention was PA Cup 100 yards but it was obvious the pellet-chamber-bore-twist rate-crown-speed combination was not going to let that happen. When I slowed the pellet down the keyholing and spiral went away but the flight time was too long with too much interference from the wind for me to be accurate enough to be competitive.
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Well having now read aerodynamic research papers on: boat tailed bullets used for long range competition, diabolo pellets, baseballs, and Ping-Pong balls...... l would say comparing a diabolo to a slug is similar to comparing a Ping-Pong ball to a baseball.So you two are both basically telling us pellets suck. And we need to be shooting slugs? Lol
Sorry. My attempt to inject some humor
The only similarity is they all are propelled into air.
Diabolos and slugs are shot, Ping-Pong balls and baseballs are smacked. But once they are on their way each does its own thing its own way.
Lesson learned......use what works best for the game your playing.
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I am afraid you will have to wait a very long time then. I did not suggest that the reports were wrong, just that the pellet shapes chosen led to a biased conclusion. This can be easily seen and checked in the calculated drag coefficients, which are higher than those derived from free flight measurements. The tunnel drag coefficients appear to be around 20% higher than measured free flight values.
Trying to use a wind tunnel to measure data from something as small as a 4.5mm pellet is extremely difficult, this is why most wind tunnel models are large scale for small projectiles. The result of using the small actual projectiles is that the Reynolds number is incorrect as can be seen in the tests where the test Reynolds number is quoted as 50,000 as compared to around 85,000 to 90,000 for free flight. This is a significant difference, especially around the values used, where things change rapidly. The flow from external blower tunnels is also often not very uniform. We would occasionally use a large blower tunnel for certain equipment operating tests such as fin deployments etc, but we would not use it to measure pressures or forces. Considering the equipment used, the results are good.
We used wind tunnel tests extensively on projectiles for experimental purposes, though the wind tunnels were much larger and more capable than the university tunnels used for pellet tests. The models used were also much larger than pellets or the actual projectiles to try to match Reynolds numbers, and thus the results were closer to free flight. However, the final data was always produced from free flight measurements, as it was well known long before I was working, that the tunnel data on projectiles was not sufficiently accurate for fire control systems or range tables.
That is interesting as it matches the results of modelling spiralling on heavy pellets, and appears to be due to the gyroscopic stability factor being too high, making the pellet sluggish in reacting to the changing airflow direction as the trajectory curves down.
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This is a great observation as to when it is happening and what it is doing to effect the impact on target. Pellets can and do get "blown around" by an exssive amount of air leaving the bore directly after the pellet leaves. Commonly referred to as waisting air because the extra air isn't being used to propel the pellet at that point. As an experiment, try turning down rhe power just a little to see if the gun is a bit quieter and no more tumbling flier's or using heavier lead projectile's if you can't adjust power.
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Thank you for your advice. I have a break barrel .177 so the power is not adjustable. I contacted the manufacture for the recommended pellet weight, which they said 7.9 gr. However, I got a tin of Crossman domed 10.3 gr. Which when weighed out they are all over the place. The 10.6gr seem to shoot the best, from this batch. I am going to try a different brand but not sure what weight to try.
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Hard to say which, but bigger or tighter in the bore seem to be working better. Maybe try some medium/heavy weight slugs for accuracy too.
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I would like to try slugs, seem like the longer slug would run truer. Like a boat tail in a powder burner.
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Keyholing, can we agree that a keyhole doesn't indicate that the projectile was tumbling, only that it didn't hit perpendicular to the target paper? I'm thinking the angled impact can rotate the projectile enough to tear a keyhole in the paper. I see similar shapes when I shoot a close paper target off to the side, say from a 45 degree angle, and then the keyholes all face the same direction.
Great discussion and information, thanks!
Great discussion and information, thanks!
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Well, I do not fully agree with you when you say pellets are "not drag stabilized". Yes, drag is not the only form of stabilization on a pellets but a large percentage of the stability, and the ability to "self correct//stabilize", on a pellet comes from the fact that the center of gravity (GoC) of a pellet in flight is in front of the center of drag (CoD), change that by giving a pellet less drag at the back by giving it more of a boat tail and you will see the stability disappearing VERY quickly.
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All types of stability, including aerodynamic and gyroscopic stability, are dependent on moments, not forces. The drag force has practically zero moment about the CG, whereas the normal forces produced by the flare when at an angle of yaw produce correcting moments about the CG. Drag has little or no contribution to the centre of pressure position, which is dominated by normal forces produced by the pellet shape. For your information, the flat based slug also has much higher drag at the base than it does at the front, yet it is not aerodynamically stable.
The true aerodynamic stability of pellets was explained in this thread. https://www.airgunnation.com/threads/aerodynamic-stability-of-pellets.1276895/
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You will have to watch your barrel twist rate with slugs. The twist rate will dictate a maximum length for the slugs which can be stabilized. Boat tails make slugs less stable and require a faster twist rate to be stable than the equivalent cylindrical based slug of the same length.
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The drag produced by the flare is there all the time, but it becomes more pronounced, and is directional when the pellet is disturbed (in yaw) and that is what causes the pellet to try and auto correct. That is the beauty of a traditional pellet and why it will even work to some extend from a smooth-bore barrel. The same "drag cone" principal was used to stabilize and auto correct other larger projectiles fired from barrels and dropped from aircraft though these days other "active" correction methods are preferred. It is a field I was involved in on large calibers (mostly 150mm and air to ground weapons) in the mid 1980's.
The base of a slug is VERY different from that of a traditional pellet so not sure why you bring that into the discussion. The main differences between slugs and pellets are the relative positions of the CoG, CoP and CoD thus a slug needs to be spin stabilized, a different discussion altogether.
Edit: I want to add, using something you wrote as a reference to prove your point is a principal really frowned upon in academic circles so sorry your link carries no weight.....
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