Paintball Spin Physics - Getting to the final Answer

[SlartyBartFast]

My next plan was to make some with hollow fronts I could fill, and with hollow rear ends to move the weight around.

Just drill out the back and push a ball bearing into the front.

[manike]

Just drill out the back and push a ball bearing into the front.

Good idea!

I'll look for the largest bearings I can push through from the back whilst still keeping a decent wall thicknes.

[SlartyBartFast]

I'll look for the largest bearings I can push through from the back whilst still keeping a decent wall thicknes.

I only meant to increase the weight to the same as paintball with the weight further foward.

That size sounds like you can remove the moniker "less-than-lethal".

[manike]

I only meant to increase the weight to the same as paintball with the weight further foward.

That size sounds like you can remove the moniker "less-than-lethal".

I'll check but the weight is currently quite comparable to a real paintball.

I also want to make some rounds that are as accurate as possible. I won't be firing at anything other than a target though.

It isn't a "less-than-lethal" gun. It's a "less lethal" gun. Big difference.

[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]

Well, Yes and No guys, but your thinking:

Vortex shedding is the same thing your hand does when it moves through water, the whirlpools behind your hand are the vortices produced. This is the air flow and action that would happen around just about any sphere that is moving through air, or even water.

While spinning the paintball in a direction parallel (like a bullet does) to the path of the paintballs travel might even out the vortices produced, the rate tested is far above feasible considering the fragileness of a paintball.

Looking at 3000rpm’s, you have a rate that is 50 times a second. In the 15ft the ball travels in one second (at 300fps), you have three twists in every foot, or one every 4 inches. 1:4” ratio. That is far above what is produced in a rifle, with rates like 1:48”. Since exaggerated (by 12 times) rotation is not producing the wanted (or even expected) affect other items are in play. These items seem to trump basic ballistic bullet physics.

How they are (if directly correlated like WorrNemesis suggested) affected by the parallel motion of a paintball is another matter. Since spinning a bullet stabilizes it, that alone could simply cut down on parasitic drag, from no spin or slowly rotating bullet not being stable enough to cut a clean path through the air. Since the sphere shape of a ball is not the best choice for a projectile, trying to adapt the physics of a non sphere designed projectile to a paintball is most likely a lost cause, even if it has been a fun argument.

One thing possible missed is the affect of a rotating sphere to produce an exaggerated boundary layer, in affect acting like a larger object as it moves through the air. This could actually produce more inaccuracy, as you would have a light object with a now even larger air profile footprint.

Food for thought.

Josh