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Thread: Paintball Spin Physics - Getting to the final Answer

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  1. #1
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    Ok this is what you do with the data.

    1. look at the marks on the ball and determine how far it rotates between each flash exposure. Use that,the distance between shots and the known FPS to determine what RPM's the ball is rotating at. When your done with this you should be able to answer the following questions:

    1. What is the balls RPM?
    2. Is there only one spin axis or does it corkscrew on two spin axies?
    3. Does the spin maintain, speed up or slow, down range?
    4. When the ball spins it has higher surface speed on one side and lower on the other. Using the RPM tell me the difference in surface speed, this figures into the Magnus effect.

    Once you do this homework then we will go on to correlating the spin to the X,Y ball position in flight.

    Don't post until you have this info, the data is in front of you not on PBN.

    AGD

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    Paintball Spin Physics - Getting to the final Answer

    Gentelmen,

    I have posted a framework of info in the data thread. This is from an extensive study we did in the early 90's. The data is representative of our findings.

    Currently on the table:

    Spin is the only major factor accounting for paintball inaccuracy. Promoted by Pbjosh

    Closed bolt operation has an effect on overall accuracy. Promoted by Glen Palmer.

    The paintball flight is subject to "knuckleball effect".

    Spin may or may not be possible because of the liquid in the paintball.

    Barrels have something to do with accuracy.

    Seams have something to do with accuracy.

    Balls distort with the impact of the air blast.
    Balls distort when leaving the barrel.

    Ok lets have a pointed discussion on the subject. For those of you just reading this, this thread is a continuation of the "closed bolt" thread found here.

    original thread

    AGD
    Last edited by AGD; 11-18-2002 at 03:34 AM.

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    BJ,

    You can eliminate the possibility of the ball rotating more than 180 deg in my pictures. We spaced the strobes at varying widths to determine the general range of spin RPM's on most balls. From that info we spaced them as you see them in the pics so we could measure the spin without missing a turn. Bravo BJ! You get my respect for actually looking at the images and trying to figure something out.

    Josh,

    So you are saying that the pics are not good enough data but we should continue to debate what we have experienced? You really think high speed video is required? How about we just go through the motions and see what we get?

    AGD

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    Redkey,

    There is a LOT more data, so much that I am not about to put it up. We did this extensive study in the early 90's. The data presented is representative of our findings and if properly analized should point to some pretty strong conclustions.

    In fact I already have all the answers to all these questions. I am going through the motions with all of you to see if the general public can, or is willing to, understand the reality of paintball accuracy.

    Flanders,

    Where are you from?


    AGD

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    Pstan,

    This is about getting the facts and involves more than just an open or closed bolt issue. In paintball this is as close as we get to publishing.

    BJ,

    Nice job, that's the kind of effort I was looking for. The result of your investigation points out that the magnus effect is SMALL. So if the expected deviation from the Magnus effect is 1% and in tracking the paintball we see a deviation of 10% then we can safely say that another force besides Magnus is acting on the ball. We don't have to actually see it to make sense of it.

    Do not let the string fool you, it was streached tight between two posts and only looks curved because the mirror was not perfect.

    Everyone concentrate on test number 114 because we have the additional data for the flight path on that one.

    AGD

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    BJ,

    Tippman as far as I know just did empirical testing and never measured anything.

    The ball is moving slower down range, I would have to try and find the exact FPS. You can get close by shooting over a crono at 40 ft with a ball that leaves the barrel at 280.

    Now that I think about it we most likely spaced the strobes wider apart so we could measure the spin in the same place.

    Glenn (sorry about the n),

    Intersting that you scraped off most of the powder half way down the barrel. Did you dry fire the excess powder out first? We have never seen that happen in our tests, I might have to repeat them. We have seen the ball on initial launch bang sideways into the bore but most of the time we just got two streaks.

    Yes we are departing from the closed bolt issue temporarily. We are trying to come to terms with the influences on the ball so we can sort them out and rank them.

    AGD

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    Glenn,

    There is a good reason to rank them. If you find out that a force acts on the ball ONLY after it leaves the barrel and that the barrel has no influence on this force, this is important.

    If that force makes up a large percentage of the inaccuracy of a marker, then whatever you do in the gun could not make a significant improvement.

    AGD

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    Glenn,

    If a barrel did it's job perfectly every time and 100% of the spread was due to external forces then it would be a waste of time to try and improve it.

    The problem, as I see it, is that people spend 300 dollars for a barrel not knowing if it will make a 1% or 50% difference.

    In general I have to ask you, how much of an increase in accuracy have we really seen in 15 years? Given the fact the barrel prices have increased by 10x and are now honed and sized to perfection, what are we getting for the money?

    While you may be willing to spend big dollars on a 1% improvement most will not or at least would like to know what they are getting.

    AGD

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    All,

    Now I just want to point out to everyone that Glenn and I are having a very pointed discussion about the subject at hand. We are both presenting facts, opinions and points of view.

    It is refreshing to have such a discussion without it breaking down into name calling and disregarding the others opinions. This is why I love Glenn, he is a no BS guy. Doesn't take it and as importantly doesn't give it.

    This type of discussion represents what Deep Blue is suposed to be all about.

    Thanks Glenn.

    Tom

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    Ok back to the battle,

    Glenn, I am purposely leaving the valving air blast thing out of this becasue it's one of the things on the table to be examined at the end.

    You do make a point that todays groups may in fact be closer due to the better propellent, aka compressed air, than a decade ago. I also thought about why you may be getting a full powder scrape off the inside of the barrel. It could be because todays balls are much rounder than 10 years ago. I know they could be off by 20 thou in 92.

    I will absoulutely give you the valve tuning issue if it concerns tighter velocity spread out the barrel. If it affects down range flight you would have to explain that to me.

    AGD

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    Pstan,

    Good comeback! Yes BC can affect things in flight. Problem is that all spheres have the same drag coefficient with is something like .7 or .8. The nylon balls being perfectly round to better than 1000th of and inch and missing the seam should fly straight and true but they don't. In fact the shot group for nylon balls is hard to differentiate from regular paint.

    This would seem to sugest that the seams, drag coefficient differences and small size variations are not a substantial portion of our X force.

    AGD

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    BJ,

    NICE JOB!! Good analysis. Ok so NOW we are seeing real data that points in a direction, it just seems that the direction is not what everyone expected. So is the way of science.

    So please lets assume for a second that this 114 data generaly represents what's going on with paintballs. (I happen to know it does)

    The next claim is that maybe the ball is kicked sideways right out of the barrel. If this were true you would expect to see it deviate in one direction and that direction would be significantly increased in the second camera trap. If you look at the picture called "3D interpretation" you will see the balls position plotted as relative movement from one pic to another. If you look at the first position in the second trap (flash #6) you will see that the ball trended LEFT from the #5 flash postion but it hits WAY RIGHT at the final target backdrop.

    So if we interpret the data correctly the ball "S" curved in flight. This is not consistent with a ball knocked off course at launch.

    We are getting better lets stick to it.

    AGD

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    BJ,

    YES!! You correctly state that spin is a continuum. At the level we see in 114 it has no substantial effect but with the Flatline or wet barrel the magnus effect overcomes whatever the mysterious force "X" is.

    So lets talk about spin for a minute. Spin is a FORCE or magnus effect or whatever you want to call it. This force has the characteristic that at high spin rates (flatline) it always pushes the ball off the centerline of the barrel. So I am going to make the broad statement here and say that spin has a force vector randomly perpendicular to the line of flight. I think this is reasonable but I am open to debate.

    If your ball spun every shot (again flatline rpm) but the barrel did not induce the spin in one direction like the flatline just in random orientation, you would end up with a target grouping that looks like a doughnut. This is because the spin wants to pull it off the centerline. Ok hold this thought.

    We now know some things about the mysterious force "X". We know it has a magnitude because it can overcome the force of low rpm spin. We also know its magnitude is lower than flatline rpm spin. Now remember the doughnut pattern from spin? We get that because the spin force is not completely random, itís always pointing away from the centerline. When we look at standard shot groups only affected by ďXĒ (clean barrel, low rpm spin) they are randomly distributed, this leads me to interpret this force X as random, meaning it might not affect the ball at all on one shot but throw it off the next. This I think we agree is completely consistent with what the paintball world sees. So if you agree with what I say here, we are looking for force X to have a magnatude that varies randomly from zero to something less than flatline rpm on every shot.

    Importantly we see it overcoming the slower spin WHILE THE BALL IS IN FLIGHT. This is the next point of debate so I will start. If the X force happened in the barrel or say a foot from it, then the spin no matter how small should affect the flight path in a direction consistent with the axis of rotation as it flies down range. To state it another way, if the ball in flight was not being affected by X then spin should be the major factor causing deviation. From another point of view, the force X has to be happening while the ball is in flight because in 114 it pushes the ball in two different directions while its going down range. So in order to argue against this you have to explain how something in the gun or barrel can affect the ball down range as we see in 114. The one thing the barrel can do is impart spin to the ball and that affects it down range but since we have that under control you have to come up with something else. Fire away.

    Redkey,
    Yes we are basing this on unseen data, if when we get to the end, someone wants to see more I will gladly post it. I am quite confident that the brains on this forum, once they comprehend all the factors, will be satisfied with the answers. Like you said they had this same problem with the musket balls its not new stuff

    Yes we did test nylon balls, the shot group is #5 in the 8 box shot group pic. The scale of the in flight positions is proportional to the diameter of the ball. So if the diameter of the ball is .680. It deviates to the left less than one ball width in the 114 flight path.

    I see that I forgot to post the tiff files for you to download hi res versions. I will try and do that asap.

    Glen,

    You have to explain the muzzle blast effect in context of the smoke pictures. It is not apparent to me that there is a pressure jet exiting the barrel right when the ball leaves.

    Damn this is good stuff....


    AGD
    Last edited by AGD; 12-04-2002 at 04:30 AM.

  14. #14
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    I appologize, I thought I had posted the tifs when I first put up the pics but I must have forgotten. Please download the tif and look at the pics, they are much clearer than the lousy jpg's in the thread. Sorry about that, you cant get good data from lousy pics. Here is the link and its in the data thread.

    AGD


    spin data

  15. #15
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    Black,

    Yes of course you are right about the DC being different at different R. You all should look at the site Black pointed out. Very informative especially figure #3....


    AGD

    The trip wire thing is cool too, I will have to look at that more.

  16. #16
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    Originally posted by bryan
    I dont know much about the magnus effect, but i do know that a knuckleball goes crazy in flight.
    Bryan,

    Ah, I see you have brought up the point about a force we have not discussed in detail yet!! Bravo!! Let's see, mysterious force "X" affects the paintball randomy in flight, knuckleball effect happens randomly in flight....

    Could there be ANY connection? I'm just not sure....I have to wait and see how the forum examines this.

    AGD

  17. #17
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    Originally posted by Paladin
    There is nothing at all mysterious about the forces that act on a paintball in flight. (even though I don't know how to explain it in scientific terms)
    Glen,

    The discussion so far has lead to the knowledge that spin does not correlate to where the ball hits down range. The fact that the ball changes direction in mid flight indicates that something, "X", is affecting the ball in flight. If you want to say that the barrel is causing this you have to make a reasonable argument on what it is. We all know the barrel can induce spin but this has been discounted in this case study.

    Your claim of seam problems is discounted because shooting round nylon balls of the same weight shows no improvement in accuracy.

    We were proceeding down a path that was getting somewhere but we seemed to have stalled as we got closer to the truth.

    AGD

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    Hitech....those pictures (falling to knees).....ARE BEAUTIFULL!.....(tears come to eyes)..we have arrived at the altar...

    AGD
    Last edited by AGD; 01-03-2003 at 11:07 PM.

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    YES!!!!! YESSSS!!!! I LOVE YOU GUYS!!!!

    We have arrived!! I didn't think it would ever happen that players would take the time to investigate the science behind paintball flight to its logical conclusion but the magic of AO comes through.

    YES vortex shedding happens in the R numbers we are talking about.

    YES we can verify this because paintballs make a buzzing sound when they go by.

    YES this happens with perfectly round nylon balls too.

    Now for the last step. BJ mentioned that well maybe its just oscillating so it really doesnt have an effect on accuracy. Here is the last thing to look up. Its called "The drunken mans walk". Its a statistical model that fits what we are talking about here to a tee. Go get it!!

    A very excited,

    AGD

  20. #20
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    Ok look at these applets. They demonstrate where the ball will end up (usually off center) at the end of the random walk.

    http://math.furman.edu/~dcs/java/rw.html

    Lets imagine that the applet above allows you to "fire" a ball down range. If we are shooting a ball 80 feet, and we assume that the vortex gets shed and takes a "Step" every 4 inches (as proposed above) then we would get about 250 steps. This is too few to see anything in the applet but lets input 500 steps and then run and clear the program a bunch of times to see the effect.

    Now notice that if you didnít know any better (and couldn't see the small movements) the balls would look like they were CURVING away from center. Also notice that some go right down the middle. Every once and a while you get a real "flier" that curves way off. Does this look familiar??

    Here is another applet that records were each ball "hits" at the end of the walk. Every hit gets recorded as a red bar that builds vertically. The red bars are distributed to the left and right of center. This is only one dimensional but you should be able to get the idea. Hit start and let it run for a while.

    http://stp.clarku.edu/simulations/on...onal-walk.html

    Once you get enough "hits" your graph will look like a bell curve. This means that MOST of your shots will be pretty close to center and you will get fewer and fewer hits the farther you get from center.

    Stack up enough hits and use the gaussian button on the right to see how well your shot pattern fits the bell curve. Fits pretty good after a bunch of trials right?

    Now here is the leap. The fact that it fits a bell curve means that the results are RANDOM!! The gaussian fit is a test for RANDOMNESS. Why is this so important?? After all its what you were expecting right? Itís important because if itís random, then NOTHING IS AFFECTING IT!!! If it were the bolt, or the barrel or anything else that could improve accuracy it would not make a bell curve. Well actually the turbulence is affecting it but in a completely random way.

    So lets think about it, the ball gets pushed around by the bolt, the air blast, the barrel and whatever else is in the way until it leaves the barrel. Now its alone by itself in the free air and instead of clear sailing to the target it gets viciously attacked by the ďXĒ factor Shredding Vortices. Hundreds of times it a couple of seconds it gets pummeled mercilessly back and forth through the air. After a thorough beating it ends up on target randomly distant from the center.

    Now I ask you, after this thrashing, this pinball machine of air flow, this twisting vortex tail of tormented accuracy, how does the barrel, bolt or air blast make your paintball more accurate??

    AGD

  21. #21
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    There is some historical, anecdotal evidence for fast firing rates to produce tighter groups. In the early 90's Paintball Consumer Reports International clamped a minimag in a vise with an auto response trigger and fired full speed at a target.

    The results stated that the configuration was the most accurate they had ever tested. This is just a commentary without any verification but I thought it was interesting.

    I personally think there might be a chance that a drafting paintball might stay in line better. (thats why I always shoot fast )

    Lots of good stuff here guys, this is really facinating!!

    AGD

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    If you can't find a good factor to calc the lateral force due to asymetric flow then it might be possible to ball park it. If you take a snap shot of the shedding vortex as it's pealing away you will see that it has somewhat of a wing shape.

    There should be laminar flow around the outside of this turbulent area causing the area encompased by the ball and vortex to flow air like a wing. In other words, the air flowing around one side of the ball travels farther than air flowing around the other side. This is how a wing creates lift.

    If you can find a wing lift program that we can plug the shape into then it should give us some idea of the lifting force. If you can find a real calculation then forget this.

    AGD

  23. #23
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    I could sit here and rattle off all of my Fluid Mechanics knowledge, but this explains things pretty well.

    http://www.princeton.edu/~asmits/Bicycle_web/blunt.html

    Pstan- The difference between the bullet and the paintball is that one is a streamlined body and the other is considered a bluff body, respectively.

    Tom- Not for all spheres. Take a look at the graph. It's a function of the Nr (Reynolds Number), but for all paintballs shooting through air you can consider it the same.

    I haven't been reading this thread, but when I find the time I'll look it over and see if I can add some input. It seems like an entertaining conversation thus far.

  24. #24
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    How's about an animation of this effect



    Apparently...

    This animation shows the shedding vortical structures at Re = 300 computed using Jeong and Hussain's lambda-2 method. Along with the hairpin vortices revealed by the numerical streaklines, oppositely-oriented induced hairpins can be observed shedding from the sphere.
    You can imagine how the effect of this would cause the ball to follow that 's' shaped path.

    manike

  25. #25
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    Last time I checked (years and years ago now...) paintballs averaged between 50 and 54 grains. I tend to use 52 grains as my average when doing calculations. They may weigh differently now?...

    manike

  26. #26
    Somewhere along the line (in this thread) I thought I saw it mentioned that just about every ball fired through a paintgun experiences the magnus effect to some varying degree. I think it was further stated that the problem is that there isn't anyway to control which direction the effect causes the ball to travel and is essentially random. A description stated that due to this occurrence that the shot pattern of paintguns would look like a donut - or, if a straight line was run through the exact center of the bore to the impact point, the Magnus effect would cause the ball impacts to hit at varying distances from the desired point in a complete ring around that actual point of aim.

    So if this is the effect of uncontrolled Magnus, why not try to control it? The flatline does (in an unintended fashion - they aren't trying to "control" anything - just to induce the effect to utilize the result) but is not "adjustable" and is severe. The Z-Body for the 'mag was designed to produce a similar effect as the flatline - but differs in how it accomplishes this. The Zbody uses a "friction plate" that is adjustable from zero to maximum (I guess where it would crush a ball) and I think most people turn it up to a high degree attempting to reproduce the Tippmann result.

    What if a similar system was used to put just the slightest amount on magnus on every ball fired from a gun? Instead of going to far and putting a "major" amount on - why not just apply enough to be certain that effect was always consistent? That should theoretically reduce the group size which is another way of saying "increase accuracy."

    This might not be the holy grail - but I'm wondering if the line of thought is reasonable. I guess my thought is, "if paintball guns are inherently inaccurate due to variances (inconsistencies) in the shape/size/form of the projectile, and those variances in the projectile are unchangeable and must be accepted - why not try to control how the projectiles behave by forcing their behavior to be more consistent?" This is based on the huge assumption that the varying magnus I described above is caused by the imperfections in the balls themselves.

    -Calvin
    From a poster at PB Nation:

    ""Jim, back to your cave. Bob Long is on the batphone..."

    MY FEEDBACK

  27. #27
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    Wow, I just read this thread, from page 3 (yeah, I skimmed a bit) and this is just incredible. Only on AO could you see this kind of thing. Absolutly amazing.

    It's awesome that we've found out (apparantly TK's always known) about this "reynolds number" but more importantly (to me anyway ) how do we combat this? or Can we?

    Is the future of paintball going to see differantly shaped paintballs?

    My Trading Feedback

    "The only thing necessary for the triumph of evil is for good men to do nothing."
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  28. #28
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    I don't think it has anything to do with the fill of the paintball. It's simply a naturally occuring phenomenon that happens when you've got a non-spinning object that's traveling though the air at high speeds/reynolds numbers.

  29. #29
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    Right, keeping velocities consisitant is the only thing that we paintballers can do in order to make our guns more accurate.

    the graph that's on applet one shows where the ball would land, assuming that velocity is consistant, and that nothing else is effecting the ball's flight.

    now having inconsisitant velocities is alltogether pointing the ball in a differant direction, wheter it be higher (higher velocity) or lower (lower velocities). It's like moving the graph itself around, and then re-drawing your random points.

  30. #30
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    Ah, I see. a denser fill/heavier ball would be less effected by the vortex shredding since it carries more momentum than a lighter ball.

    I believe that there are regulations concerning the weight of a paintball. A heavier ball packs a bigger punch at 300 fps than a *EDIT*lighter*EDIT* ball, so safety concerns are there.

    Unless you lower the speed at which you fire the ball. You might actually not lose any range since the heavier ball loses velocity slower than a lighter ball.

    Hmm... How about a weighted 10 gram nylon ball? That could test our theory... I would be more than willing to bet that it would be more accurate.

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