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Pneumatic QEVs on eBay.

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: POLAND_SPUD » Tue Oct 19, 2010 5:18 am

dwell time - that tells you for how long the valve stays open. this is used for hammer valves not piston valve for obvious reasons. Change it to, let say, 100ms the valve stays in the open position until you fill the gun again
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Unread postAuthor: r00kie » Tue Oct 19, 2010 5:20 am

Ah, of course, I didn't even give that value a second thought. It's up to 734ft/s now, thanks POLAND_SPUD. :D
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Unread postAuthor: Crna Legija » Tue Oct 19, 2010 5:20 am

the valve should be set as as Barrel seal if your modeling a QEV. generic is a sprinkler valve i think
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Unread postAuthor: r00kie » Tue Oct 19, 2010 5:26 am

Ah, I see. I was quite confused by those options, I went for generic as I heard it was for a modified sprinkler valve, and the QEV I ordered works in the same way.
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Unread postAuthor: POLAND_SPUD » Tue Oct 19, 2010 5:26 am

Generic let's you enter any data you want. If you switch it to barrel seal then you have to enter the mass of the piston and pilot volume etc. and the programme calculates the opening time for you

Generally speaking, 'generic' is more convenient to use
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Unread postAuthor: r00kie » Tue Oct 19, 2010 5:29 am

I'd rather have accurate data than convenience, but seeing as I haven't got the QEV yet, I can't crack it open to work out those values, so perhaps I'll leave it as 'generic' until I do. :)
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Unread postAuthor: POLAND_SPUD » Tue Oct 19, 2010 5:37 am

I guess I shouldn't put it as 'any data you want' :-D

If you use barrel seal method then you've got to enter all that data and it will give you something close 2.5ms anyway. So if you're modelling QEV's then it is more convienient to use generic and just enter 2.5ms than to measure everything
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 19, 2010 5:45 am

Much better :) I usually set the dwell time to 999. Also, 2.5ms opening time is a little bit optimistic, 15-30ms will give you a more realstic prediction
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Unread postAuthor: r00kie » Tue Oct 19, 2010 5:46 am

Bugger. I guess I'm not leaving it at 2.5ms then. 15ms dropped the performance quite a bit, down to 546ft/s.
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Unread postAuthor: POLAND_SPUD » Tue Oct 19, 2010 5:49 am

Personally I usually enter 5 or 7ms. The drop in performance is minimal, so yeah 2.5ms is a bit optimistic but it's not that it changes the results that much
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 19, 2010 5:53 am

Better that then putting your hopes up and being disappointed with your launcher.

Don't look at the actual velocity data though. Play with chamber size, barrel length, operating pressure etc, and see how it affects performance (eg double barrel length gives you 25% more muzzle energy) and work accordingly.

Also, it's worth looking at the energy, not the velocity. Remember that kinetic energy will be 4 times greater if you double velocity.
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Unread postAuthor: POLAND_SPUD » Tue Oct 19, 2010 6:03 am

oh and don't forget that the speed of sound is a limiting factor. As the velocity increases the efficiency drops. So you can get a lot more ME by switching to heavier ammo (and generally speaking heavier ammo flies further as it looses it's speed at a slower rate than a comparable lighter ammo)
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Unread postAuthor: r00kie » Tue Oct 19, 2010 6:08 am

I have played with chamber and operating pressures, I'd like to keep the barrel at about it's current length.

While on the subject of pressures, I know you've done a lot of work with epoxy JSR, do you think it's worth using that as a lazy alternative to brazing copper? What kind of pressure would I be able to safely kick it up to with epoxy and brazing?

200 psi was just a value I threw in because it's the maximum pressure I can reach with my current tools, but I can always go out and get a fridge compressor if the gun will take higher pressures.

POLAND_SPUD, I'm planning on using some steel ball bearings, but as I don't have them yet, I can't weigh them. I'm planning on ordering them from here, they're a bit pricey, but have a good range.
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Unread postAuthor: POLAND_SPUD » Tue Oct 19, 2010 6:11 am

Then use this table. It's in polish but you should be able to work it out
(diam. in mms, diam. in imperial units and then the mass in grams)

http://www.gewa.com.pl/page.php?d=kulki1wymiary
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Unread postAuthor: r00kie » Tue Oct 19, 2010 6:14 am

POLAND_SPUD wrote:Then use this table. It's in polish but you should be able to work it out
(diam. in mms, diam. in imperial units and then the mass in grams)

http://www.gewa.com.pl/page.php?d=kulki1wymiary


Very helpful, thanks mate. Judging by that, I can assume that my projectile will weigh 7 to 8 grams. :)
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