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Old 06-27-2008, 11:47 AM   #11 (permalink)
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Originally Posted by Schmitti View Post
Oh man... you had to go and pull out the graph on me... you bastard you!
But I did admit you were right... At work, everything complex is abstracted, by policy. It truth, I know nothing about fluid dynamics.

I talked to Chauncey about the subject in the early 90s. He told me it came about when working on improved PGP bolts. He would occationally get extremely erratic results. After much tinkering, it was determined that the cause was the "rimfire" bolt of the PGP.

If tweaked, it could be directed to the base of the paintball to produce a backspin, like golfballs and baseballs.

But originally results were still inconsistent. That was improved by using an extremely curved bolt-face. Almost 1/2 the paintball fits into the face of the bolt. This was critical because as soon as the air leaves the bolt, it IMMEDIANTLY becomes a turbulent mess and spreads around the paintball.

using a curved base meant that the backspin port at the base of the bolt would directly contact the base of the paintball. This was really the only way to get the threshold spin amount needed to generate magnus force.

The side effect was the barrel needed to be overbored, which was tough in the early 90s, and required a custom barrel. Thats not a problem today, but the CooperT bolts were hard on paint then, when shells were thicker. That would be a big problem today
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Old 06-27-2008, 12:24 PM   #12 (permalink)
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Couple of thoughts:

Would a textured/dimpled ball be more receptive to being spun by air?

Is there a way to possibly decrease the tendency, as you say, for air exiting the bolt face to become turbulent almost immediately? Perhaps lengthening the channel, angling it, changing the pressure curve?

EDIT: Seems like there might be some method to the cupped face of the CooperT bolt then?

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Old 06-27-2008, 12:32 PM   #13 (permalink)
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Is there a way to possibly decrease the tendency, as you say, for air exiting the bolt face to become turbulent almost immediately? Perhaps lengthening the channel, angling it, changing the pressure curve?
Schmitti would be the expert.... ahem...

But I wonder if we could make a honey-comb channel, with microfine tubes that ran inside the bolt.

Imagine, a hole in the base of the bolt-face that was about, say 1/4".
Inside that hole was many 1/32" tubes. This would get ride of the turbulence, and give a sharp directional blast.

This would probobly be easy to make with a ultra-tiny drill bit, and a drill press.
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Old 06-27-2008, 12:34 PM   #14 (permalink)
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There is a correlation between dimpled balls and backspin.... I don't have it handy but a quick google should produce what has already been studied.

In paintball guns it will be very hard, if not damn near impossible to get laminar flow. A straight flow path will help though (nelson... where as a sheridan/sniper/cocker has a flow path with multiple bends). Any time you have a change in direction you will induce turbulance. Also a quick change in diameter will induce turbulance. Both of these situations are prevelant in paintball guns... in the passages between valve and barrel.. and in the change at the bolt (flow passage) to the barrel.


EDIT: I read your post too fast... or you changed it... either way... you want turbulant to hopefully induce spin. As in above it's already turbulant flow... there isn't probably too much more you could do with out actually restricting flow (and thus dropping pressure) that could make it more turbulant.

E
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Old 06-27-2008, 12:40 PM   #15 (permalink)
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Quote:
Originally Posted by Schmitti View Post
In paintball guns it will be very hard, if not damn near impossible to get laminar flow. A straight flow path will help though (nelson... where as a sheridan/sniper/cocker has a flow path with multiple bends). Any time you have a change in direction you will induce turbulance. Also a quick change in diameter will induce turbulance. Both of these situations are prevelant in paintball guns... in the passages between valve and barrel.. and in the change at the bolt (flow passage) to the barrel.
Is it really the flow path that causes turbulence or the pressure/volume/velocity? I mean no one would accuse a combusion engine's inlet or exhaust tracks of being terribly smooth paths, but apparently you can get significant laminar flow through those ... no?
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Old 06-27-2008, 12:52 PM   #16 (permalink)
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Velocity and the pressure are not independant of flow path. Flow path will induce changes in velocity and in pressure. The changes in velocity and pressure can produce turbulance.

Also... which has more relavance to the flow in a paintball gun... pipe flow or flow through a linear fan turbine engine system?

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Old 06-27-2008, 03:26 PM   #17 (permalink)
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This is great - exactly what paintball is missing - scientific discussion instead of hype - I love it.

As for what kind of flow is behind the paintball - isn't the bigger issue the fact that once the ball has moved away from the bolt whatever the path of the air - the ball is simply moved down the barrel by the pressure difference behind and in front of it?

The bolts we used had significant curvature - so the ball would have been snugged up to the bolt - at least on the one where the barrel matched the paint.

If you have to drastically overbore - AND you have to hold the ball tightly against the bolt to acheive the backspin - is that even feasible?
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Old 06-27-2008, 03:48 PM   #18 (permalink)
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Stepping back to look at an earlier statement of mine to answer and earlier question...

With a golf ball.... I was thinking about this a minute ago. You actually want to hit low on the ball... th elower you hit on the ball compared to direction of travel, the more spin you impart from the hit. The dimples help impart lift... not backspin. The backspin is already imparted on the ball from the club hitting the ball.

So an evenly dimpled paintball similar to a golf ball would just have lift imparted on it. If you got it spinning backwards then the dimples would theoretically help in flight by providing a little bit of lift.

With your question of over bore and ball against the face....

Start plugging hules in the face of a venturi barrel starting at the top.

Backspin is really achieved from uneven flow over the ball... such as with an undershot bolt like the Cooper-T bolts. They had no hole bored up the centerline, it was actually a channel milled into the bottom of the bolt that allowed the air to move between the bolt and the barrel so that the air would hit the bottom of the ball causing it to start to spin backwards at the same time it was pushed out of the barrel.

I think it's more important to get air moving linearly along the barrel striking the ball low so that you cause the back spin and get the ball moving forward. I would think the goofy wired barrel might actually work well in this application.

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Old 06-27-2008, 05:01 PM   #19 (permalink)
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Quote:
Originally Posted by Schmitti View Post
With a golf ball.... I was thinking about this a minute ago. You actually want to hit low on the ball... th elower you hit on the ball compared to direction of travel, the more spin you impart from the hit. The dimples help impart lift... not backspin. The backspin is already imparted on the ball from the club hitting the ball.
My understanding is that the dimples do not create the lift in themselves, but purposely induce turbulence. That turbulence is required to generate magnus force. This is why baseballs have seems, golfballs have dimples, etc.

Even for the hopup kits on airsoft, metal and plastic BBs have similar seams to paintballs, though much smaller.

So-
No spin + no dimples: No lift
Spin + no dimples: NO lift
No Spin + dimples: No lift
Spin + Dimples: lift (via magnus, not bournulli)

This, of course, is well out of my area though.
But Chauncey said pretty much the same thing as you. The hole had to be small, on the curved bolt face, at the base base, aimed between the ball and bottom of barrel.

He have to be careful, since we are talking about 2 seperate areas:
What happens in the gun. (ie via CooperT, GalataticZ, Flatline/Apex,Undertow, Alien, etc)
What happens in the air. (ie paintball seam, fps, trajectory, etc)
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Old 06-27-2008, 09:30 PM   #20 (permalink)
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My understanding is that the dimples do not create the lift in themselves, but purposely induce turbulence. That turbulence is required to generate magnus force. This is why baseballs have seems, golfballs have dimples, etc.
also my understanding - the dimples create a buffer of air (turbulence) around the ball - and since air has less friction on air than on the ball - the sleeve of air around the ball lowers drag overall.

does anyone have a picture of the face of a commercial back spin bolt - or one that you consider successful?
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