Paintball

[cockerpunk]

ok, one at a time here -

Originally Posted by drg

It is possible that that moment is the critical issue. In effect, I'm thinking that if it is ever successful, the spin may be caused by basically the same mechanism as the flatline effect ... the ball rolling along the top of the barrel. HP cited the need for an oversize barrel, so basically the point of contact must be reduced in order to bias the spin. The moment at firing could drive the ball to the top of the tube, though i am not sure what forces would keep the ball there as it travels down the barrel.

I'm thinking the longer the barrel, the more the forces acting on the ball would tend to equalize out, so perhaps a very short barrel would produce the most marked effects?



Wouldn't this only be possible if the guns were constantly in a high-pressure, high-flow state? Wouldn't there be be *some* laminar flow at *some* point? Whether it affects the ball is of course another question.

the moment is the critical issue here.

we used an over bored (by .006 inches) barrel with only a one inch control bore, and the rest of the 12 inch barrel was .709 inches, and we saw no effect. its a short barrel, with a huge back, and a very short control bore. if there was any effect, we would have seen something.

Rn is a function of velocity, viscosity, and area. this describes the amount of energy lost (in the form of pressure) while the fluid is flowing, due to how messed up the streamlines are. what you are thinking of is entrance length, the length of tube that is required to fully develop a flow. now, true, eventually a really choatic flow will slow to the point where the velocity is low enough to yeild a Rn of a smooth flow, but thats becuase the velocity in changing, not becuase length of the flow is in the equation for Rn.

Originally Posted by Frink

First off, good job. Nice to see some effort being put in, but a couple of points that bug me:



You have absolutely no way of predicting the magnitude of the instantaneous acceleration of the paintball in that first instant. I think your theory is fine, but it's just that, a theory. Wouldn't the paintball see the greatest magnitude of acceleration in that initial instant? Might that not have a greater affect than you think?



You can't make claims and expect people not to critic and comment on them. In this test, you essentially made a half-assed backspin bolt and then applied the results to all backspin bolts. It does seem sketchy.

no, your right, we dont have a way to predict the instantaneous moment. if we did, then there would be no need for the test. if we could, i could just write and equation for you to disprove it.

the theory is supported then by our testing, which showed no difference in the range of either of these.

a low blow or backspin bolt is not rocket science, and cooperTs are not magic, they follow the same laws as everything else in this universe. they can lay out there critisims but sometimes it gets frustrating because i acutally know fluid mechanics, i acutally know mechanical system, so sometimes its hard for me to explain where my thoughts are when they are 10 steps beyond someone elses, and they are protesting thought process 3 and im on process 28.

engineers are not good at talking to people! why can't you people understand me!

Originally Posted by HP_Lovecraft

The test was flawed. That is what I was pointing out. I was very specific in pointing out where the error in the logic was. Did you miss the discussion on laminar flow, and turbulence?

The CooperT bolts were well documented to work, producing the same trajectory as the Flatline. It was not some voodoo science.

The complaint for the CooperT bolts was that the barrel had to be overbored. Meaning, if your paint was .688, your barrel would need to be .698.
Those barrels did not exist "off the shelf" so you had to have them overbored for you in order for the effect to work.

I took an alternate approach. I found a good .694 barrel that was overbore, but not enough. Instead of a flat trajectory, it was making corkscrews. Clearly the added friction was effecting the spin axis.
http://montneel.selfip.com/paintball/cvm03.jpg

So I started cutting it shorter, and shorter. At a certain point, the corkscrews stopped, and the trajectory flattened out:
http://montneel.selfip.com/paintball/cvm04.jpg

Like drg pointed out, with the CooperT bolts, barrels have the opposite effect as the Flatline. The Flatline barrel increases the spin, while the barrel used with the CooperT bolt slows it down. Thus, needs to be way overbored, or extremely short. Neither method was popular with people who liked there overpriced barrels.

SO I have no need to "Do my own test", as I've been doing them for the last 20 years. There are also some people who think the Flatline is also fake as well. But, it seems you do acknowledge the Flatline does work?

The test was well done, but based on a false premise. If your happy with that, then who cares? But just don't called the "Backspin Bolt" a myth, or busted in any form until you do a proper test.

no, i did not miss your laminar flow and turbulence. i however (unlike you) know fluid mechanics. Rn has nothing to do with spin. only forces and the instantaneous moment are the big factors here.

please, send us one of your backspin guns. if you dont trust my ablity to re-create a backspin bolt, lets see yours. i have a feeling even when i go out and use a setup identical to yours you will still protest, so until i use your exact setup, there will be complaints. as i said before most of the barrel we used was .709, even more overbored than your setup. it was short, and it yeilded no corkscrews, no ever range, nothing.

our test is simply based on a magnitude here. even if you backspin setup was no ideal, it still should have produced some backspin (esp with the overbored setup) and yet we saw no difference. none. zero. there was no change in the flight, no corkscrews, no nothing.

[punkrex]

Originally Posted by Schmitti

Back to science... you are using Reynolds to it's fullest extent.... which makes it that much harder. In my work I deal with stationary solids and the moving fluid.... you are dealing with a moving solid and a moving fluid... too far beyond my skills.

Just show me where you want the house, utilities, rail road, etc and I'll figure out the rest.... advanced physics... Forget it!

E

me and my buddies always joke around and say that civils are dynamically challanged.


Sorry to jump in so late but I can shed some light on why golf balls have dimples. The dimples cause the ball to fly further because they are two types of drags, pressure drag and frictional drag. Yes adding more surface area increases fritional drag but this is negligable. When a golf ball is hit it is at such a speed that it enters turbulent flow (higher re#), but by inducing a little surface friction, the ball travels a little slower (re is a function of velocity) and enters a laminer flow region which actually allows the ball to fly farther because it is no longer in a turbulent region (higher pressure drag).

sorry for the hack explanation, I don't have my old fluid texts with me right now but you get the gist.

[brycelarson]

I'll phrase it a little more politely than CP.

We would love to see anyone out there perform a test on their optimized setup OR send stuff to us to test. If you've got a couple of bolts and a chrono - it's pretty easy to set up.

Based on the pic posted by Tom222 - that Pro teams bolt looks to have more flow in the center than the bolt we used. Additionally, the venturi we used had more flow closer to the bottom edge. If the spin is created by the flow hitting the bottom of a ball cupped by the bolt - I think ours did it better than that one.

CP and I had several discussions about thresholds. It's possible that there was an improvement in range - but at 100' if it isn't significant enough to measure, then that additional range isn't going to have any benefit in play.

Said another way - if at 100' the additional distance wasn't noticeable, then it's not useful. I know that the flatline works - I've shot them and been shot by them. The effect is instantly obvious - there was nothing on that magnitude happening with the back spin bolt.

And, as CP said, using the CCM carbon fiber barrel should have been a great test bed for the spin - over-bored control back - 1" long and an extremely over-bored barrel.

[cockerpunk]

Originally Posted by punkrex

me and my buddies always joke around and say that civils are dynamically challanged.


Sorry to jump in so late but I can shed some light on why golf balls have dimples. The dimples cause the ball to fly further because they are two types of drags, pressure drag and frictional drag. Yes adding more surface area increases fritional drag but this is negligable. When a golf ball is hit it is at such a speed that it enters turbulent flow (higher re#), but by inducing a little surface friction, the ball travels a little slower (re is a function of velocity) and enters a laminer flow region which actually allows the ball to fly farther because it is no longer in a turbulent region (higher pressure drag).

sorry for the hack explanation, I don't have my old fluid texts with me right now but you get the gist.

yup, it changes the boundary layer of air surrounding the ball. the boundry layer is fluid that "sticks" to the ball as it flies, changing the manner in which the streamlines go around the object.

dimples work by changing the profile of the boundary layer, and thus the way the fluid flows around the ball.

bryce, i have offered to split shipping with HP if he sends us the backspin gun. i honestly hope he sends it to me.

[Frink]

Originally Posted by cockerpunk

the theory is supported then by our testing, which showed no difference in the range of either of these.

a low blow or backspin bolt is not rocket science, and cooperTs are not magic, they follow the same laws as everything else in this universe. they can lay out there critisims but sometimes it gets frustrating because i acutally know fluid mechanics, i acutally know mechanical system, so sometimes its hard for me to explain where my thoughts are when they are 10 steps beyond someone elses, and they are protesting thought process 3 and im on process 28.

engineers are not good at talking to people! why can't you people understand me!

If your test is flawed, it supporting your theory means nothing.

FYI, I'm a Mechanical Engineer. The thing I hate about Engineering students and brand new engineers is the cockiness that inevitably comes with them. You might be on step 28, but you damn well better realize that it's entirely possible you took at wrong step at turn 3.

I don't see anything wrong with your experiment, other than the test is completely flawed. To say that plugging the bottom holes on a venturi bolt is the same as a backspin specific bolt is not proper scientific method.

On a side note, haveing taken Fluid Dynamics and Computational Fluid Dynamics, the fact that you say you "Known Fluid Mechanics" makes me My Professors didn't "Know Fluid Mechanics" you take your best guess, then your surprised as hell when the CFD model shows you something completely different.

Just remember, being an Engineer does not always make you right.

[cockerpunk]

Originally Posted by Frink

If your test is flawed, it supporting your theory means nothing.

FYI, I'm a Mechanical Engineer. The thing I hate about Engineering students and brand new engineers is the cockiness that inevitably comes with them. You might be on step 28, but you damn well better realize that it's entirely possible you took at wrong step at turn 3.

I don't see anything wrong with your experiment, other than the test is completely flawed. To say that plugging the bottom holes on a venturi bolt is the same as a backspin specific bolt is not proper scientific method.

On a side note, haveing taken Fluid Dynamics and Computational Fluid Dynamics, the fact that you say you "Known Fluid Mechanics" makes me My Professors didn't "Know Fluid Mechanics" you take your best guess, then your surprised as hell when the CFD model shows you something completely different.

Just remember, being an Engineer does not always make you right.

no, it doesn't. i think we all wish it did though ...

our quest here is for the truth, not to convince HP his gun does not work.

CFD does get crazy ... my problem is always with the programing, not the fluids aspect. i seems to write better and faster in MatLab, but its still a PITA.

i honestly do not see any difference in a "true" backspin bolt, vs plugging a bunch of ports, becuase after all, a "true" backspin bolt is simply a bolt with a big plug in it. if it shoots at speed, and has only openings below the centerline of the ball, then is should create backspin, of some type. more correctly, it should produce something if the hypothesis is wrong. and if we just can't find any difference, any measurable, noticeable difference, then my theory seems like to correct one. hell, we should be able to stick the bolt in upside down and watch them go straight into the ground too. around corners ...

if HP would send us his gun, we could get the final word.

again, our quest here is the truth, not to put down our version of the facts and press them on everyone. we are all students of paintball physics, regardless of education.

[Railgun]

A truly nice discussion. And I have to agree with Frink and HP that trying to guess what's going on in the sort of conditions found in the barrel is not going to be easy. But there are a few things that sort of make sense given all the testing and HP's last post with that short barrel.

I should also mention that I've designed and built model aircraft for a LOT of years and as part of that I'm a pretty keen student of aerodynamics. Although I'm comfy with test results and graphs I stop at doing the highly complex math required to actually predict airflow.

First is that the Magnus effect sure does work. I've seen balls from flatline barrels actually curve upwards before starting to arc over to a more normal line.

Second is that CP is right. I think we can forget about laminar flow inside the gun. There's just too many transitions and turns the air takes so it's really down to rapidly changing pressure gradients and localized transitory flow patterns.

I have to take some exception to some of the terms being tossed around though. There's a lot of mention of the Reynolds number being a key factor almost as though it's some sort of force that acts on the ball. But it's not. Instead it is merely a measure of how the ball sees the air around it. It's also just one factor of how the air flows within the passages inside the valve, bolt, around the paintball and in the barrel. It's a factor that when considered with other factors we can use to help understand the flow of the air within the gun and around the ball. Also once the ball leaves the gun it can be used again with the other factors to help predict the strength of the magnus effect. But on it's own it doesn't really mean much other than as part of other considerations. HP, I don't know where you got that graph in that one post but it would require a lot more explanation of what and how and where it applies to be of any use. For example, in my model work we use the Re of the wing to work with some other equations to predict airflow patterns at various speeds and sizes of the wings. But on it's own it is only a general and vague comment on how "sticky" the air seems to the wing flowing through it.

HP's work on that barrel sure helps to hit home. It would seem that much like the angle of a golf club hitting the ball under the Center of Mass can generate a spin the initial air blast hitting the ball down low can lift it and set it to spinning. I suspect that equally effective is the lifting of the ball into contact with the upper portion of the bore so that simple friction aids the spin. However as we all seem to agree the flow get's pretty chaotic as the ball moves a short way down the barrel. At that point the pressure behind the ball is likely flowing past it thanks to the overbore size and creating a thin aircushion to reduce friction. However from that corkscrew result it would seem that there are still some collisions or near collisions with the wall that generate some additional unwanted spin. It's interesting that it seems to work better if you can set the spin and then chuck the ball out of the barrel as soon as possible so it can't be affected by the later part of the barrel.

Golf ball dimples generate some turbulence. One of the observations in aerodynamics is that a thin turbulent layer of flow over a surface acts like a "glue" to the more laminar air flowing just a short distance away from it. A smooth ball produces a large turbulent wake behind it when there's no turbulent boundry layer. Adding the dimples to encourage the air to form this turbulent boundry layer makes the air stick to the round surface to a much greater degree so the wake is only roughly 1/2 the diameter of the ball rather than around 0.8 the diameter. This results in a lot less drag. But because you "coated" the ball with this "sticky" boundry layer it aids the Magnus effect and any amount of spin will have more effect on a ball with dimples than one without.

Now here's where the Re on the free flying balls comes in. Re of an object in free air is a function of the size of the object and the speed. I'm not sure just how fast a golf ball travels but it seems to be near to par (sorry ) with our paintballs. But the golf ball is roughly 3.5 times larger in diameter. That means the golf ball will have a much higher Re than our paintballs. Again, on it's own the Re is just a descriptive number that indicates how the air will react to the surfaces of the ball. But the number can be used in further equations to get a better idea of how the air is flowing around the two balls.

Now as part of this scale effect and how the air flows around the balls we can look at what is needed to generate that golf ball like turbulent layer. But because airflow doesn't "scale" in a linear way we can't just scale down the dimples from the golf ball and use them on our paintballs. It just doesn't work that way. Similarly the one seam on our paintballs may or may not provide enough of a trip to generate a proper turbulent boundry layer. But even if it did there's only one seam so as the ball tumbles in its flight the generation of the turbulent flow "glue" while be sporadic and cyclic in it's creation and the ball will be pushed around sideways in flight and not follow a true path. A description that seems to apply to far too many brands of paint.

Which all comes down to the idea that the low blow bolts do seem to work when coupled with the proper factors around them but you need to look at the whole paintball gun setup. As HP found using a big overbored barrel seems to help a lot.

[cockerpunk]

we used a massively overbored barrel a control bore (one inch long) .006 or .007 over and then the front (11 inches long) of .021 inches overbored.

no corkscrewing, no range increase, nothing.

[c6quad]

it was a nice experiment but i don't think it proved that backspin bolts don't work. it proved that your attempt to make a backspin bolt didn't work but that's about it. the myth is far from being busted.

here's a pic of a Cooper-T backspin bolt for a KP.

[Clearush]

so the air burst is 90% directed towards a very bottom edge with a small center port.

The location of the venturi holes being higher up on the face of the bolt could make a significant difference on initial forces on the paint.