Paintball Spin Physics - Getting to the final Answer

[nino_fs]

Hello all I have just spent a good part of two days reading this thread and find it very interesting. The one thing that I always found unusual about Tom's testing is that they did not find any improvement in accuracy by spinning the paintball. Now from what I understand spinning the ball should cause a tornado like vortex to form behind the ball rather than the random vortex shedding which is occuring behind a non spinning paintball. I was very confused by the fact that imparting a spin with it's axis along the line of flight did not improve accuracy. Then I got to thinking and I believe I may have come up with something. Now this is only mind experiments and I have no testing to back anything up so I will state this as a hypothesis.

Tom said they spun the balls to 10 000 rpm and that is the same speed a flatline reaches according to Tippman. Now from my basic in my head calculations I found the circuference of the paintball to be about 2" remember this number I will becoming back to it.

Now the air is flowing over the paintball at 300 fps or 3600 inches per second. Now to try and overcome this flow pattern we would most likely need the horizontal motion of the air(The air moving over the paintball along the direction of the spin) to match this velocity or exceed it. So 2" circumference means the air travels 2" per rotation. So that means we need 1800 rotations per second or 108000 rpm which way exceeds that which tom imparted to the ball.

Now I accept that this is a rediculous speed but may it be possible that at a certain speed the rifling(which is the driving force for the tornado like trailing vortex) will overpower the random vortex shedding and we will have a much more accurate shot.

Basically why I am putting up for debate is the reason rifling the ball did not work for Tom was the ball just was not spinning fast enough?

[WorrNemesis]

Wow, nice job guys! I know don't fully understand everything, espcially those formulas, but I believe that I have comprehended enough to possibly contribute (plese tell me if I'm wrong). When I finally reached the end (3 days worth of reading) and got to nino_fs's post, I think you were on to something. I may be talking way over my head, but anyway, I remember reading that the vortex sheds every 4", which could be totally off since I coudn't find this when I went back to double check. With this, I figure that in order to improve accuracy, you'd need to rotate the paintball once per shed, and that would be 54,000rpm. Once a test has been done with a paintball rotating at that speed, I'll be satisfied that spinning will have no effect at all, unless I have no idea what I'm talking about.

[pbjosh]

[Lurker27]

With respect to the air footprint...Could a backspin effect actually have the additional benefit of a longer, less randomly shedding profile?

[WorrNemesis]

Yes, it was a very fun argument. I was hoping that there way a possible way to increase accuracy in some way. I think I understand a bit better, although it's kind hard since I haven't taken physics yet in school. Thanks!

[bjjb99]

Um... Josh? How does a paintball travel only 15 feet in one second if it's moving at 300 fps?

At 3000 rpm, you have one rotation every 0.02 seconds. At 300fps, that equates to one rotation every six feet, for a twist of about 1:72".

I believe you're right on the mark with respect to high spin rates and the fragility of paintballs. At some point the shell will separate and spray fill all over the place. Anyone out there have access to a force tester that can measure tension? Care to destructively separate two halves of a paintball for us?

BJJB

[nino_fs]

wow,

I thought this post was dead but I guess I should check back more often. Now I do agree and have always thought that the speeds I am suggesting to rifle a paintball at are completely impossible simply because the paintball would shatter. I was just throwing out an idea as to why it seemed that rifling did nothing when it should have some effect of the flight of the projectile. Also I had never thought of how rifling may actually create a large air shell around the ball which might be cancelling out any benefit from the rifling.

I have no idea if my idea has any merrit as I am only half way through my fluid dynamics course and only starting on flow. I also doubt we will get into the topics of vortex sheading in this course anyway. I don't really know if the vortex shedding can be controlled but I would hazard a guess that it is not completely random and uncontrollable. What I would say is that it is so complex that science and engineering currently have no explanation for it other than it seems to be random. It would be interesting to try and produce the high reynolds number flows in a much slower flow of a more viscous fluid. And then by introducing dye and trying different things one might be able to see how they can attempt to control the vortex shedding.

Even if it was possible to force more shedding in one direction by a backspin that could be extremely useful since it might then be possible to reduce the drift of the ball.

But this is all speculation and has no scientific backing. I may get back into this topic in the following weeks trying to find something in the University Library. So any ideas please post as I will see what I can find out.

Cheers to everyone who posted on this thread.

[pbjosh]

Thought I had replied to that-

Yes, duh, 15 ft apart is from 20bps- two subjects and totally different thought (or lack of) process.

Multiply by 20 the spin and it seems that the difference between the rifle twist and Tom's paintball test is alot closer.

One thought about vortex shedding and randomness:

Due to the "Chaos Effect" the shedding will be in a random order due to affects (air condition before ball passes, any shape irregularities in the ball, manipulation of the Matrix) too small to measure. These almost average out, and end up producing the accuracy/inaccuracy we see.

To distruct the Paintball in a manner that would demonstrate the affects of spin we just need to get a varible speed dremel or other tool, and glue a paintball to the end!

Go ahead, do it! (wear your goggles!)

Josh

[hitech]

To distruct the Paintball in a manner that would demonstrate the affects of spin we just need to get a varible speed dremel or other tool, and glue a paintball to the end!

Go ahead, do it! (wear your goggles!)

Now where did I put that dremel [looks around...]

[nino_fs]

I was wondering if anyone knew the names of some books that discuss this random vortex shedding phenomenon. I ask because I tried to find information at the library but was unable to find anything as the library is huge and I don't really know what to look under.

[Lurker27]

I don't believe its random. It's a function of reynolds number, and the path will change based on this... at low Re, no vortex shedding, then as it steps up, you get a zigzag pattern, and then, at high Re numbers that are useful to us, a helical pattern.

Also, I've been thinking about rifling again. It seems to me that it would have some serious advantages with regards to accuracy if done correctly.

Prerequisites would be a marker with a spiky breech pressure to try to deform the balls, and a fairly tight rifled barrel.

You'd get the uniform aerodynamic profile, due to the averaging out of seam position.

You get paint void, which would cause off-line weight distribution, spun out due to centrifugal force, effectively normalizing weight distribution of liquid fill about the barrel axis

I'm not suggesting that the vortex shedding effect wouldn't occur, but I think that you COULD eliminate some variables. I hear very good things about the hammerhead. I think its irresponsible to wave one's hand in the air and dismiss spheres as poor aerodynamically.

They're what we're stuck with. Let's deal with it.

[SPECTER24]

that was a lot of reading! I was waondering if any body has tryed a different shaped paintball like the ones that Nelson technologies, Inc. has on ther web site? http://www.nelsontechinc.com/encapsulate.html

[mysteryfish]

Most amazing [paintball] thread I've ever read. Inspiring.

Can someone confirm that I'm understanding correctly:

- Because of (random) vortex shedding, we see a "random walk" effect on the paintball over the course of it's trajectory, which is (in summary of this thread) the "mystery force" that accounts (in significant part) for the spread of the impact point over the shot distances typically found in paintball.

- The spherical shape of a paintball doesn't lend itself particularly well to what I'll call "bullet physics", because at the rotation speeds necessary to counter-act the "random walk" a paintball would disintegrate

Thanks to anybody who's still sleeping on this thread with one eye open!

I recently dove head first into a mechanical engineering program, driven in part by a desire to understand and eventually be able to create things like paintball guns, and recently discovered how deep the "rabbit hole" goes. This has been a reassuring read, but a bit overwhelming, considering the unbelievable depth of knowledge some of you have collected. I sometimes wonder if I have that kind of room in my head...

(By the way - I fully recognize that this thread has seen no activity in 3 months... so, feel free to mention it anyways, I don't know the policies about ressurecting dead threads here, but if anyone feels like typing a bit more, I'd appreciate it )

[nerobro]

On the bright side, knowing why isn't necessarily any better than knowing it happens. Many sucessfull paintball guns have been built on love and pasion rather than design and technology. (Stroker, cocker, sterling)

And if you wanna know how bad "engineering" can be. Look at the PVI shocker. And the seven years of slow guns that followed it. ;-)

Just be prepared to do a lot of testing. And make sure the testing is repeatable. YOu also have the whole of AO behind you. Decades of shared experience is available to you.

[hitech]

Can someone confirm that I'm understanding correctly:

- Because of (random) vortex shedding, we see a "random walk" effect on the paintball over the course of it's trajectory, which is (in summary of this thread) the "mystery force" that accounts (in significant part) for the spread of the impact point over the shot distances typically found in paintball.

That pretty much covers it.

- The spherical shape of a paintball doesn't lend itself particularly well to what I'll call "bullet physics", because at the rotation speeds necessary to counter-act the "random walk" a paintball would disintegrate

No. The paintball can stand the rotation. It just doesn't do any good.