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Aerodynamic phenomenon? Physics of bbs

  1. Aug 21, 2007 #1
    Greetings all,

    I am an avid airsoft enthusiast. For those of you unfamiliar, airsoft is a bit like paintball, but without the gooey mess. In any case, a few friends and I were experimenting with the performance of one of our bolt-action sniper rifles which fires 6mm projectiles weighing .20 grams @ approximately 500fps (give or take a few on the fps, number. Also, the rifle can fire heavier projectiles, but not exceeding .43 of a gram.) During the comparative performance tests, one of the guys noticed that quite a few shots appeared to suddenly veer off target mere inches before striking the bullseye. This occurred in the neighborhood of 150ft, which is the approximate distance just before the bbs begin to drop earthward (total range is probably around 200ft.) We were able to see the effect through the use of binoculars which we were using to help with spotting. I'm wondering if someone knowledgeable in physics/aerodynamic knows of a phenomenon which might explain why the bbs would dramatically veer away meer inches from a flat target (if I had to guess, I would hazard there is some kind of airmass moving ahead of the bb which compresses when interrupted by the target and acts on the bb, but hey, I'm probably way off :smile:)

    I'm also curious if there is a point of diminishing returns when it comes to performance as it relates to such light projectiles? By that I mean, our natural inclination is to upgrade the internals in order to boost the fps at which the bbs are fired so that they will fly more accurately and with increased range. At what point do the realities of aerodynamics dictate the bbs' flight characteristics wont benefit from more speed?

    Thanks in advance for taking the time to consider my questions. Much appreciated!

    TF
     
  2. jcsd
  3. Aug 21, 2007 #2

    Danger

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    Welcome to PF, Triple F.
    My first thought was the one that you mentioned. I would surmise that your projectile encounters a standing air mass directly in front of the target that serves as a 'cushion', sort of like a vertical version of the ground effect that causes an aeroplane to float prior to landing.
    While I'm not sure about any of this, I do think that more information about your projectiles is needed to answer the second part of your post.
     
  4. Aug 21, 2007 #3
    My guess would be that it slows down in and around that 'range' and is effected by its own aerodynamics. If you drop (throw, etc.) a ball into water, the ball travels straight for a while until friction of the water slows it down, and then moves as in a method of 'other' parameters----same as a bullet shot in a tank of water.
     
  5. Aug 21, 2007 #4

    LURCH

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    First and most obvious; fire the same projectiles without a target in front of them. I think you'll find that the projectiles veer off course at roughly the same distance. But you would have to find some way to objectify the experiment. That is, measure the deflection, don't just go with "well, it looked like that one veered... ."
     
  6. Aug 21, 2007 #5
    Thanks guys.

    Does this effect have a name? Is it common to light pojectiles as well?

    Sorry, maybe I didnt explain it clearly. The bbs travel in a straight line toward the target (a sheet of carboard measuring approx. 18in x 12 in, on a pole), but sometimes they are seen to veer away a few inches before impacting the surface of the target. If the bbs are fired away from the target, they simply fly off as expected (straight and in a parabola until they impact the ground.) It is the presence of the target that is somehow influencing the erratic flight path.

    Not practical. Besides, I dont have a good enough video camera capable of high enough film speeds (or an indoor studio for that matter). A guess will have to do.
     
  7. Aug 21, 2007 #6

    pervect

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    I would wonder if the bb's were spinning. Spin can't cause a sudden shift in trajectory, but I recall reading that the path of curve ball (in baseball) will give the appearance to the human eye of a sudden "jump" even though the actual trajectory is curved.

    I think the first thing to try might be to put a whole bunch of thin paper targets in a straight line to attempt to document the actual path of the bb. The problem will be whehter or not hitting the paper significantly changes the trajectory.
     
  8. Aug 21, 2007 #7
    Pervect:

    The bbs veer before they hit the target.

    Let me try to paint a picture. Imagine the bb is a baseball. Imagine the target is a wall. Imagine throwing the baseball at the wall, but just before the baseball hits the wall it suddenly veers in a random direction. Trheowing the baseball the same way down the street would just result in it flying a goodly distance in a parabola and bouncing on the ground.

    To answer your question about spin, yes, there is backspin on the bbs. This is provided by a device called a hop-up (a rubber sleeve in the barrel which protrudes into the bbs path and puts backspin on it.) I dont believe this is a factor in the phenomenon we observed because the phenomenon only occurs in the space directly in front of a flat target (it doesnt happen wen shooting at coke cans, for example or when shooting into empty air.) If the backspin were causing it, it would happen all the time, right?
     
  9. Aug 21, 2007 #8

    Danger

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    I'm not sure if it would be considered a boundary layer situation or not, since the air farther away isn't necessarily moving. It's probably best to let one of the Aero guys like Fred take over from here.
     
  10. Aug 21, 2007 #9
    I'm having some trouble with the idea that you can see this 'veering' so well, just a few feet before the target at 500 fps----and at different ranges?


    can you tell me about how you do this?
     
  11. Aug 21, 2007 #10

    pervect

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    I'd expect backspin to cause the bb to go up or down, not left or right. Fuzzy tennis balls, for instance, "float" when given backspin. I'm not sure if a smooth bb would "float" or "sink" when given backspin, but I'd expect the motion (due to the magnus effect) to be in the vertical plane. The magnus effect can change sign depending on the surface characteristics.

    If your backspin isn't perfectly vertical, but has some horizontal component, this would cause some component of the resulting motion to be like that of a curveball.

    In the case of a baseball, the "break" in the baseball is in large part an optical illusion. The baseball actually takes a slightly curved path, but the eye and brain falsely interprets the gradual curve of the path as a sudden break.

    Thus, I would think that the first step would be to better instrument the actually path of the bb to see exactly how it is travelling, rather than relying on your eye. The simplest thing to do would be to use a piece of paper (i.e. a paper target) as a "position detector" - the hole in the paper show where the bb was. Unfortunately, this has the potential to interfere with the trajectory of the bb, so your first experiment would have to demonstrate, using two widely separated sheets of paper, how much impact putting in or removing the first sheet of paper affected the measured position of the bb when it went through the second sheet.


    Anyway, I suspect part of what you describe may be similar to the known illusion of the curveball suddenly "breaking", but at this point it's just a suspicion. To really nail down whether or not an actual break is occuring, you need some better data, hence the suggestion of using paper targets to track the bb's actual course.
     
  12. Aug 21, 2007 #11
    If they're BB guns, there's usually no rifling in the barrel, the BB's are smaller than the inside diameter of the barrel, and the BB can roll along one side (inside) of the barrel, giving the BB 'spin', sometimes quite a bit. Switch to pellets (they tumble)--that may help the accuracy.
     
  13. Aug 22, 2007 #12
    My bet is that the distance is about where you hop up induced back spin is defeated by air friction and without the gyro stabilization, combined with the light weight of the plastic bbs any puff of air will deflect it noticably. Depending on your bb maker a little bit of mold sprue will do wierd things too.I have a couple of airsofts and everyone of them has a definite "maximum accurate range" for the diff weight pellets. A heavier premium pellet will fly straighter further.
     
  14. Aug 22, 2007 #13
    The "bb's" aren't travelling at 500 fps by the time they reach the target. They decelerate all the way to the target due to friction. And I must have missed the part about different ranges.

    If they can see it, they can see it. If you shoot the gun and see it happen it's not an "idea", it's an observation. If you doubt the accuracy of the observation that started the discussion then please explain why you think the observation doesn't seem possible. I've fired a few weapons in my time and when you're looking down the trajectory "tunnel" from the sight of a gun that fires slow projectiles it's common to view the drops and turns at or near maximum range because the projectile has slowed down so much by then that you can see it and you're also watching it from behind rather than from a side view so your eyes aren't really forced to follow at the speed of the projectile. Try it sometime and you'll believe.

    As to the sudden turning before the target I'd blame the wind bouncing off the target. If you're shooting at or around max range it doesn't take much to throw the bb off course and the target could be deflecting wind just right to accelerate or contribute to the distortion of the bb's path that normally starts at that range anyway.
     
  15. Aug 22, 2007 #14
  16. Aug 22, 2007 #15
    Thank you, everyone.

    You're right, Idjot. The phenomenon occurs near the end of the bbs flight path (just before they start falling earthward). There is no way they are moving at 500fps by this time. Also, our tests were conducted at a constant range of approx. 150 ft for those who thought I indicated otherwise.

    Thank you. As I wrote above, we were using binoculars to help us spot hits on the the target. This made seeing the phenomenon easy. We didnt notice it with the naked eye, but I attribute this to the fact that the bbs are small and white, the target was mostly white as well, and it was bright day light outside.

    Pervect:

    The trouble I have with your theory that the bbs are breaking like a curve ball is that the phenomenon we witnessed doesnt happen in the absence of the target. If your theory held, wouldnt the effect also present tself when bbs are fired at an open range without obstruction? The bbs only veer as they come within a few inches of a flat target, so this cant be caused by "breaking" alone.

    Idjot wrote:

    Sounds simple and I cant believe I overlooked it, but you might be right there. The flat surface of the target could be deflecting wind back at the bb or across the bbs flight path. At the ranges we were shooting, there isnt much energy left for the bb to power through a good blast of wind. I'll have to find a suitably large indoor space to put the theory to the test.
     
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