Paintball dynamics

Discussion in 'Physics & Math' started by Frud11, Feb 14, 2008.

  1. Frud11 Banned Banned

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    I played paintball with a bunch of other dudes out on some former farmland with all these prefab shacks and a few trees around. I noticed that firing at the lower edge of a window (just a cutout in a wall) would mean the paintball deflected downward, it was repeatable, and it meant I could hit someone hiding inside, or ducking below the window.

    What's going on here? is it because the paintball is spinning? I could repeat this "scattering" off the window edge despite the rotation of the pistol.
    The "bullets" travel slowly enough that any wind deflects them, but despite this, the "spin" or whatever, seems to be unaffected, and every shot I got close to the lower edge of the window deflected downward. I thought it was something to do with the way paintballs are made, as hemispheres that get joined, so there's a thickness or a ridge, which must affect the dynamics when it gets propelled out a barrel...?
     
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  3. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    I think Bernoulli's effect---same thing that makes a baseball break, or a soccer ball bend. The spin creates a slightly negative pressure on one side of the ball. The way the ball spins depends on the characteristics of the barrell.

    http://www.youtube.com/watch?v=V26Fbsrpcus

    Just a guess, though.
     
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  5. nietzschefan Thread Killer Valued Senior Member

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    This is correct.

    I've had paintguns curve left and right. Often(esp dirty) barrels have broken paint shells and actual paint in the barrel - perhapson one side and yes often settling at the bottom of the tube, Thus causing downspin.

    Get the "ref" to clear your barrel good. Or do it yourself.

    When this happens to me - I go rambo mode(which can be really effective with paint guns lol). Paint balls don't hurt THAT much even from 10 feet, and if people don't accept a mercy kill from that range - I don't care if it's grandma - fuck them.
     
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  7. Frud11 Banned Banned

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    If the paintball's spin is caused by a dirty barrel, why doesn't rotation of the barrel affect the perceived deflection?

    Is it because the central "ridge", or "equatorial mass" (an artefact of paintball manufacture), always orients itself the same way? I was seeing a gyroscopic effect, maybe. Haven't exactly tested this exhaustively, except it didn't seem to depend on which window, just as long as it was the lower edge; maybe the width of the "slit", or the ledge (think camo-painted plywood sheets with square holes cut out), would alter the dynamics, too.
     
  8. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    You mean you physically turn the gun upside down and fire, and get the same result? How does this work---I thought the things were gravity loaded?
     
  9. Pete It's not rocket surgery Registered Senior Member

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    The paintball actually touches the lower edge of the hole in the wall on the way through?
     
  10. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    Hey pete....long time no see!

    Perhaps this is it.

    The paintball doesn't fit exactly in the barrel, i.e. it is a little smaller than the diameter of the barrel. So the paintball rests on the bottom of the barrel, and any spin that the ball gets is forward spin. This explains why the paintball breaks down.

    In order to get less down-spin you could lubricate the inside of the barrel. Of course, this may mean that the ball has less spin, and thus a less predictable path---like a knuckleball.\\http://www.youtube.com/watch?v=X3tWTKL6xNM
     
  11. Roman Banned Banned

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    Way back in the 19th century, this guy wanted to show that musket balls did, indeed, curve out of the barrel when they were shot, due to the spinning of the ball.

    So he stood around and fired muskets through lined up sheets of paper all day.
     
  12. Vkothii Banned Banned

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    No, it has to be within a couple inches (maybe ~10cm); if it hit the edge, it wouldn't hit the dude crouching below the window. The guns were all gas-charged, and hired from the place - they sell CO2 cartridges as well as magazines of paintballs. One of these things "decorates" someone pretty painfully if you get close--we all got a few "medals" for bravery under fire...takes about a week or so to fade.

    I only rotated the pistol sideways, because firing with any accuracy is pretty difficult when it's held with the stock pointing upwards (maybe I could try hanging upside down?).
    I figured that, since the effect appeared to be "rotation-independent", then gravity has to be involved. The spin imparted by the pressure difference--(if there's a gap above the paintball before it's fired) seems the most likely explanation, or it's to do with the gyroscopic effect, or the fact that it's filled with liquid, or some combination--is gravity-dependent...?
     
  13. BenTheMan Dr. of Physics, Prof. of Love Valued Senior Member

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    I don't think it's gravity. It's probably travelling too fast to have a large effect.
     
  14. Vkothii Banned Banned

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    Just found a site that looks like it does a fairly complete aerodynamical analysis of paintball flight - haven't found anything yet about the downwards deflection when crossing the horizontal edge of a vertical barrier, but the spin is (apparently) caused by pressure difference (not any gap, just the difference behind and in front of the projectile when it's still moving down the barrel).
    Bernoulli's principle and the Munro effect are involved.

    http://home.comcast.net/~dyrgcmn/pball/pballIntro.html
     
    Last edited: Feb 17, 2008
  15. chris4355 Registered Senior Member

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    I play paintball at least once a week, in fact im going to go play in 10 hours = D....

    there are many factors that can cause a paintball to spin, from old paintballs, dented paintballs to cheap paintballs. A dirty barrel, wind or a velocity thats too high or too low can also cause the paintball to behave oddly. Also, another thing you may want to take into account is that the paintball diameter needs to be the same as the barrels, and depending on the barrel size you should get different sized paintballs, a paintball is not supposed to roll out of a barrel from one end of it to the other, its actually supposed to barely get stuck inside.

    as for why it suddenly spins downward the second it flies over a window is beyond me. I would guess you might have seen it wrong. Did it spin downwards when you just shot in plain sight?
     
  16. Challenger78 Valued Senior Member

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    Wow. Bernoullis can do that? , here I was thinking that it was only good for earaches.
     
  17. Vkothii Banned Banned

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    No, there was a bit of wind. But I managed to hit several "enemies" with the deflection shot through a "window", reliably and repeatedly - it wouldn't deflect off the vertical edges, only the lower edge.

    Note: this wasn't exactly an experiment, but it was something I could verify was occuring, ( >20 times, through various windows). It may have had something to do with the particular pistol, I didn't get to swap with anyone - they were all too busy running around yelling and so on. I didn't empty the gun trying to explore paintball flight - I was trying to shoot people.
     
  18. Pete It's not rocket surgery Registered Senior Member

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    How much deflection was involved?

    How far were you from the window?
    How far above/below the window's lower edge was the gun?
    How far behind the window was the target?
    How far below the window's lower edge was the impact?
     
  19. Vkothii Banned Banned

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    There was considerable deflection, I could hit someone crouching below the aperture, or barrier.

    I was about 30-40 feet (10-12 meters) away.
    The lower edge of any window was roughly in line (or a couple feet lower, maybe), with my line of sight.
    Not sure how far behind the window anyone was, but when I fired 2-3 test shots into an empty "building" (to test that it was happening), a shot aimed at the lower edge should have hit the rear wall, but would angle into the floor. These shacks were no bigger than 2 x 2 meters, I'd say. There was a bit of horizontal deflection from the wind, too.

    If the ball had a reverse spin, would that explain the interaction with the barrier?
     
    Last edited: Feb 18, 2008
  20. Pete It's not rocket surgery Registered Senior Member

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    10,167
    I don't know much about fluid dynamics, but I'm really surprised that it would make that much difference. Need more input!
    About how high above the floor is the lower edge of the window?
    About how far from the back wall did the test shots land?

    Any idea of the muzzle velocity of your gun?
     
  21. Vkothii Banned Banned

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    The windows were all barely high enough to just conceal an average-sized person. Maybe 1.2 meters, but it varied and the dirt floors weren't flat either. Couldn't see where any test shots or misses landed, but I know I hit more than one human target, because of the "ow" factor. A gas gun can only reach a max 300 fps (90-something m/s). I estimate the gun I had was maybe 60-70 m/s.
     
  22. Pete It's not rocket surgery Registered Senior Member

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    How do you know these test shots hit the floor?
     
  23. D H Some other guy Valued Senior Member

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    Hmm. Other threads made me suspect that Frud11 and Vkothii are one and the same person. This thread makes that suspicion much more plausible.
     

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