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Achieving consistent muzzle velocities

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: ramses » Wed Jul 15, 2009 3:23 pm

@btrettel: if you do write that ballistic simulator, please share it with this community!

The darts could be traveling nose-high, but only if the center of gravity was behind the physical center, while still being in front of the center of drag.
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Unread postAuthor: btrettel » Wed Jul 15, 2009 7:25 pm

The empirical data I've seen suggests the Cd (of a blunt cylinder) will be essentially constant up until about Mach 0.5. As I don't intend to shoot a Nerf dart any anyone at that velocity, constant Cds are a very reasonable assumption.

Yes, the angle up would be potentially a source of lift. I tried to keep my barrel level, primarily because most Nerf guns are shot straight on, so this should have minimized the effect, though, I'm thinking it might have contributed to the variance as a slight deviation could still generate significant aerodynamic force.

Recording both range and hang time should provide enlightening results.

As for my simulation, sure, I'll release it if it's finished. I like sharing. Hell, I'm writing a short paper deriving the formulas used in my pneumatic gun simulation and describing its operation in detail right now. I'll do that for the external ballistic simulation too if its released.

For kicks I uploaded my old simple external ballistic simulation here: http://trettel.org/bags/code/proj_0.py

As I wrote it back in December 2008 and didn't comment the code much I don't know everything about the code, but someone might find it useful.
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Unread postAuthor: jimmy101 » Wed Jul 15, 2009 11:17 pm

btrettel wrote:The empirical data I've seen suggests the Cd (of a blunt cylinder) will be essentially constant up until about Mach 0.5. As I don't intend to shoot a Nerf dart any anyone at that velocity, constant Cds are a very reasonable assumption.

Yes, that Cd is OK as long as the cylinder is like a Nerf dart. If the cylinder is solid then the Cd is way off. A solid cylinder would be unstable and when it starts to tumble the Cd will go way up.

Fin and COM/COD stabilized rounds tend to have a more constant Cd then do spin stabilized rounds, especially at long ranges.
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Unread postAuthor: btrettel » Thu Jul 16, 2009 1:52 pm

I feel really stupid now. I did my confidence interval wrong; the actual CI is 0.28 to 0.40. This is more reasonable.

The discussion definitely wasn't worthless though. :wink:
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Unread postAuthor: PVC Arsenal 17 » Fri Jul 17, 2009 2:35 pm

btrettel wrote:The original test setup used a 3/8" Deltrol QEV with a Clippard MJVO-3 valve as the pilot valve. Clippard's catalog says the valve has a flow rate of 14.0 scfm at 50 psig and 25 scfm at 100 psig. This is good for Nerf but is inadequate for this testing.


I'm not 100% sure, but I think that flow rate you read refers to the flow through the inport to the outport, not from the outport to the atmosphere via exhaust ports. What matters in your tests is the rate at which the air behind the QEV can be dumped, not the rate at which your chamber can be filled.

I could be wrong, however. It occured to me that that might be the case when I was comparing the MJVO-3 to a similar valve made by Pneumadyne.

If I am right, though, you may need to call Clippard for that information or conduct a test of your own.
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Unread postAuthor: btrettel » Tue Jul 21, 2009 2:17 pm

Could be true, but either way, the valve seems to do its job fairly well so I don't intend to replace it. I actually didn't care too much about getting a specific flow rate as long as the valve was better than the crappy one I used earlier.

At the moment I'm focusing on finishing the gun part of this Nerf project so I can focus on something else. The tests and optimization are pointless until I have a working product.

PVC Arsenal, I read that you intend to use the tiny QEVs Clippard sells. How well did the tiny one you got from Pneumadyne work? That one would have a Cv of about 0.3 and the Clippard ones are about 1.2. Is this adequate flow?
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Unread postAuthor: PVC Arsenal 17 » Wed Jul 22, 2009 2:10 am

The Pneumadyne valve has noticably poor flow. It should work for what I want it to do, but I ordered a Clippard JEV-F2F2 just to compare performance. That will be here tomorrow, so I'll post my findings then.
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Unread postAuthor: PVC Arsenal 17 » Wed Jul 22, 2009 2:37 pm

Image

Top row: Clippard JEV-F2F2, Clippard MJVO-3
Bottom row: Pnuemadyne QE11-M-44, Pneumadyne A11-31-44


Both Clippard valves are smaller and more compact than the Pneumadyne valves. I don't notice any difference in performance between the two push button valves, but I would guess that the Clippard exhausts slghtly faster because it has two large holes in its body whereas the Pneumadyne exhausts around the button. Both valves have the same threads around the button, which I find convenient because I can use the Pneumadyne Thumb Operator as a trigger on both.

The QEV's are a different story. Clippard's QEV beats Pneumadyne's hands down. I can hear and even feel a difference in kick between the two valves. The Clippard definitely opens faster and dumps more air.
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Unread postAuthor: Ragnarok » Wed Jul 22, 2009 2:43 pm

PVC Arsenal 17 wrote:The Clippard definitely opens faster and dumps more air.

Not to be pedantic (well, actually, I am being pedantic), but I'd be a little surprised if you can notice the difference in the opening times without sophisticated measuring equipment.

Doubtless you can tell that the flow is different, but these valves will be opening in handfuls of milliseconds, far too fast for a human to identify.
Reaction time to piloting might be a different matter, but actual opening time...
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Unread postAuthor: POLAND_SPUD » Wed Jul 22, 2009 2:47 pm

clippard is all brass isn't it ?

I wouldn't feel comfortable while using fittings or valves made from plastic...
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Unread postAuthor: jeepkahn » Wed Jul 22, 2009 2:57 pm

POLAND_SPUD wrote:clippard is all brass isn't it ?

I wouldn't feel comfortable while using fittings or valves made from plastic...


i BELIEVE THE PNEUMADYNE'S ARE ANODIZED ALUMINUM...ooPS...Toolazy to retype.. Plenty strong enough IMHO...
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Unread postAuthor: PVC Arsenal 17 » Wed Jul 22, 2009 7:01 pm

Ragnarok wrote:
PVC Arsenal 17 wrote:The Clippard definitely opens faster and dumps more air.

Not to be pedantic (well, actually, I am being pedantic), but I'd be a little surprised if you can notice the difference in the opening times without sophisticated measuring equipment.

Doubtless you can tell that the flow is different, but these valves will be opening in handfuls of milliseconds, far too fast for a human to identify.
Reaction time to piloting might be a different matter, but actual opening time...


Bah. Clippard QEV > Pneumadyne QEV. That's the bottom line.

Clippard setup actuating a small cylinder

Pneumadyne setup actuating a small cylinder

Both valves have a tendency to only discharge a portion of the chamber volume, causing the cylinder to remain in its compressed position, when the button is not pressed down completely. It reminds me of shortstroking and AutoCocker. The Clippard is more prone to this because it has a longer button stroke. Both are still capable of cycling the cylinder at impressive rates. If I can get someone to hold the camera for me, I can demonstrate.


Btrettel: You got your MJVO-3 off eBay, correct? I got my first one off eBay too, but it's leaking. The smaller o-ring needs to be replaced. I bought a brand new one from Clippard when I was ordering my QEV just for the heck of it. There seems to have been some slight changes in the design from whenever my first one was made, but the dimensions are still the same.
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