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PACa Mk.II, French airgun

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Lockednloaded » Mon Sep 20, 2010 7:17 pm

Wow! very impressive to say the least. I love everything about it, the pump, the piston, and the high pressure.

Your Check Valve seems very compact and well made, any details on its construction?

I think the on thing that would really put this gun over the top is some proper sights. A red-dot or some steel sites would be practical, but a proper scope would look amazing!
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Unread postAuthor: maverik94 » Mon Sep 20, 2010 9:27 pm

Hmmmm. I'm liking the spool valve and especially that onboard pump...you could have the QDV activated pneumatically with a schrader valve, blowgun, or even a pushbutton or hammer valve...Anyway. Nice build!
Also, I like how you're wearing gogles and gloves when you're shooting!
Just a thought. You could put a medium-strength spring behind that trigger just so you don't have to push forward the trigger after every shot to reset the valve. Very impressive I must say!
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Unread postAuthor: G-man » Tue Sep 21, 2010 1:49 pm

@killerbanjo:

I don't thing that I got enough vocabulary to explain you the function of a QDV, so maybe someone of this forum could do it ;)
That said, I think that I'm gonna work on a tutorial for my forum this weekend, and there will be animated schemes, I'll show you.

The pump is quite simple.
-the anti-reversal is made with a schradder valve, on wich I take out the spring. So it opens easily when you pump.
-the piston has been machined from a piece of brass, and the joint has two position: while pushing, it moves back and the piston is airtight.
While pulling, the joint moves up and let the air flow inside the little holes you can see on the picture, so there's no vacuum effect.

This was not the original system, that's why the piston looks strange.



@Lockednloaded:

What do you call "chek-valve" exactly ? I don't understand what it is. The anti-reversal ?
Also, I tried to attach a scope last day, but it's quite difficult...
There's not many possibilities for attachin a rail on this rifle.

@maverik94:

I don't think that using a pneumatic triggering system is a good idea, it would add a lot of pieces and complexity, for an infime gain.
For me, the main interest of this firing system is it's simplicity ! ;)

In fact, there IS a return spring !
But he's not strong enough to push the piston back...
That's not a big problem since it takes about 1min to reload the gun ;)
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Unread postAuthor: JDP12 » Tue Sep 21, 2010 2:23 pm

Very cool g-man. It has a very appealing look to it as well.

and a check valve is essentially an anti-reverse valve.
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Unread postAuthor: maverik94 » Tue Sep 21, 2010 4:25 pm

I don't think that using a pneumatic triggering system is a good idea, it would add a lot of pieces and complexity, for an infime gain.
For me, the main interest of this firing system is it's simplicity !

It's cool. It was just an idea. After all, it would add efficiency, and eliminate the need for a return spring (as the air would act as one), but if you're happy with it, it's perfectly fine! After all, it's your own gun right? :) Anyway, that is a very cool gun you've got there man. i hope to see more from you real soon![/quote]
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Unread postAuthor: wyz2285 » Tue Sep 21, 2010 4:26 pm

nice way to quick open a valve :) have think about this to but i thout it hard to build.and it's very cool to have that hp pumb :D
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Unread postAuthor: maverik94 » Tue Sep 21, 2010 4:32 pm

Please explain the pump too

Im guessing its a sturrum (Spelling) pump that uses a bought check valve?

I'm guessing you mean a "stirrup pump"?
Here is a how-to page on how they work and how you can make your own. hope that helps :wink:
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Unread postAuthor: G-man » Tue Sep 21, 2010 4:52 pm

@Maverik94: Uh, I didn't want to seem to reject your proposal without consideration, I'm not saying that you're wrong or whatever else !
I'm just saying that the actual system is way powerful enough for me, I'm not trying to reach the max efficiency by adding pneumatic mods ;)
I have a preference for mecanichs systems too, I don't like the feeling of a pneumatic trigger.
Sorry if I offended you in any way, my level in english writing doesn't allow me a lot of nuance in my words...

The pump in your link makes me think of my first HP pump !
http://i87.servimg.com/u/f87/11/27/23/97/116.jpg


Here's a quick schematic of my pump:
Image

The little yellow thing in the middle is, as I said, a schradder valve, on wich I take off the spring so make it much easier to pump.
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Unread postAuthor: Gun Freak » Tue Sep 21, 2010 5:48 pm

maverik94 wrote:Hmmmm. I'm liking the spool valve and especially that onboard pump...you could have the QDV activated pneumatically with a schrader valve, blowgun, or even a pushbutton or hammer valve...Anyway. Nice build!

If it were pneumatically modified then it would no longer be a QDV, it would be a normal piston valve, and I believe you would lose power. I think Technician1002 would agree.
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Unread postAuthor: maverik94 » Tue Sep 21, 2010 7:40 pm

Uh, I didn't want to seem to reject your proposal without consideration, I'm not saying that you're wrong or whatever else !
I'm just saying that the actual system is way powerful enough for me, I'm not trying to reach the max efficiency by adding pneumatic mods Wink
I have a preference for mecanichs systems too, I don't like the feeling of a pneumatic trigger.
Sorry if I offended you in any way, my level in english writing doesn't allow me a lot of nuance in my words...

Nonono. you didn't offend me in the slightest! :) As I said, it's your gun, and if it's doing what you want it to, then there's absolutely no reason to change/modify it.
If it were pneumatically modified then it would no longer be a QDV, it would be a normal piston valve, and I believe you would lose power. I think Technician1002 would agree.

Idk...I think as long as there was minimal dead volume and an efficient pilot valve, you could get a considerably faster opening time...though I can't prove it, that would be my guess though. Does anyone know for sure?
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Unread postAuthor: killerbanjo » Tue Sep 21, 2010 7:51 pm

G-man wrote:The pump is quite simple.
-the anti-reversal is made with a schradder valve, on wich I take out the spring. So it opens easily when you pump.


-the piston has been machined from a piece of brass, and the joint has two position: while pushing, it moves back and the piston is airtight.
While pulling, the joint moves up and let the air flow inside the little holes you can see on the picture, so there's no vacuum effect.

This was not the original system, that's why the piston looks strange.




Thanks for that, the tutorial sound great, be sure to post a tutorial when your done.

Just want to say again how much i love your gun :P
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Unread postAuthor: Ragnarok » Tue Sep 21, 2010 8:01 pm

Gun Freak wrote:If it were pneumatically modified then it would no longer be a QDV, it would be a normal piston valve, and I believe you would lose power.

I'm sorry, but no. All credit to Tech - the QDV does meet the criteria of having a faster opening time than a conventional piston valve, but the effect that has on final muzzle energy will be imperceptible.

As I have said many times, improving opening time has diminishing returns, and well made piston valves are almost invariably past that point.
A QDV has advantages (and some disadvantages to go with it - there are no free lunches), but these aren't really in the realm of performance, regardless of what popular opinion might think.

TL;DR* - Piston valves and QDVs are almost identical as far as power. Don't use it as a reason to choose between them.
*If so, smack yourself in the head for being a bloody lazy git. That was only a short post.
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Unread postAuthor: maverik94 » Tue Sep 21, 2010 8:06 pm

How would a QDV have a faster opening time? As I see it, when you're pulling open the QDV with your finger as in this gun, you are opening it with, say, a max of 30 lbs of force. And if you had a pneumatically actuated valve with 400 lbs of pressure it would have ~ 400 lbs of force pushing it open (depending on the seat diameter, etc). Wouldn't that give you a much faster opening time? Or is my logic totally messed up and am i making a fool of myself lol :D
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Give me a lever long enough, and a fulcrum on which to place it and I shall move the world.
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Unread postAuthor: saefroch » Tue Sep 21, 2010 8:20 pm

A QDV has no pilot valve. You're assuming instant opening time on the pilot valve with your estimation. A normal piston valve also has some resistance to the piston moving back produced by the not-yet-vented pilot volume, a QDV does not. GGDT actually models all these effects, if you punch in a piston valve at like 1,000 psi and look at the graph of valve opening speed, you can see the slope of the line change. You're not making a fool of yourself, as Rag said, there's almost no difference in the final product.
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Unread postAuthor: maverik94 » Tue Sep 21, 2010 8:23 pm

A QDV has no pilot valve. You're assuming instant opening time on the pilot valve with your estimation. A normal piston valve also has some resistance to the piston moving back produced by the not-yet-vented pilot volume, a QDV does not. GGDT actually models all these effects, if you punch in a piston valve at like 1,000 psi and look at the graph of valve opening speed, you can see the slope of the line change. You're not making a fool of yourself, as Rag said, there's almost no difference in the final product.

Ok, I gotcha, so they're pretty much the same as far as performance goes. I gotcha.
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Give me a lever long enough, and a fulcrum on which to place it and I shall move the world.
–Archimedes
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–Carl Denham

Current Project: None, I'm in Spudremission.
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