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07-28-2008, 09:54 AM #1
Pre-Charged Pneumatic Air Rifle valve+physics questions
Is anyone here familiar with PCP air rifles? Especially the more powerful ones (such as the Career Dragon or Sam Yang rifles).
I'm designing my own one, but since I haven't had a chance to disassemble a PCP rifle, I have no general idea about the dimensions of the valve or the transfer port, nor the weight of the hammer, so that the air flow would be optimal. I guess the physics behind this is not very simple, requiring knowledge of fluid dynamics, but for me the most important thing is that it would work - preferably so that it'd be useful for shooting
Let's consider the following setup: A regulated reservoir of 800 PSI, a simple poppet valve, a hammer plus a spring, and .45 caliber barrel of 28 inches length.
Now, the transfer port should be as short and as large a diameter as possible. In my design the diameter of the port is 4 inches - this shouldn't be a problem.
However, I can't decide what diameter the valve stem head should be so that it would pop open upon impact of the hammer AND allow adequate amount of air to escape through the transfer port to proper the bullet to around 700-800 fps. This of course depends on the weight of the hammer and the spring strength. However, there must be some kind of recommendations?
Also, since there is no downstream pressure at all, the venturi effect comes into play at the point where the air escapes through the valve, being the narrowest part of the air passage. How narrow should the passage be at minimum?
If anyone here has done this kind of stuff and explain how you've done it in your project, or disassembled a big bore PCP rifle, I'd most appreciate for info. Of course I could go do all this through pure experimentation (which is what I'm eventually going to do), but I'd like my design to make even somewhat sense before I start machining the parts
The attachment shows the very basic principle of the valve design
07-29-2008, 02:00 AM #2
07-29-2008, 03:32 PM #3
You need this book.......
"The Modern Pneumatic Airgun" by H. M. Buckley. It is a complete manual on designing and building a PCP airgun. Buckley is British, and he self-published the book. I think it will answer all your questions and/or will help you find your own answers, safely. His design is rated for about 3000 PSI. The book can be purchased directly from the author, but has been intermittedly available. It has also been occasionally in stock at several UK book shops. The only contact info for Buckley I can find is: [email protected]. It is worth the trouble to get it. I got my copy at www.tidelinebooks.co.uk last year, but they don't show it in the current web page.
07-29-2008, 04:38 PM #4
Thanks both of you I registered to the airgun forum and am still waiting for being accepted to it.
This book would likely be THE bible I need, so I guess it's time to try to lay my hands on it (will be difficult). Is it a copyright violation to answer if I ask how the author designed the PCP valve in his book? All I need is a few dimensions, to satisfy my curiosity...
08-01-2008, 08:25 PM #5
That wouldn't be copyright violation since the responder isn't copying the book, just revealing what is in it. Search this forum for other info on the author, he was recently reported as being available to sell the book, as far as I remember.
08-07-2008, 08:40 PM #6
Probably the BEST online resource for discussing anything about making your own PCP airguns is LD's forum over on Delphi. Larry is a pretty helpful guy if you do your homework and search the archives first.
08-08-2008, 11:38 PM #7
The forum on Delphi as mentioned above, is a REALLY god place to look, but really heed the advice. Read as much as you can, and do your own homework and be prepared to be called upon to show your work, just as in school.
Guys that show up there and ask such questions as you have here, should be prepared to become the centerpiece of an internet scorchfest.
Really. Homework. It's dead easy to find a LOT of information and dimensions, just by pounding Home Made Airgun into Google.
There are a lot of talented guys out there, that have made a lot of really great pieces of work. Most of them have done a lot of development work, and few have bothered to get it all on the record for the likes of us to see. Some have though.
Take a look at airgunartisans.com IIRC, it's a site with a lot of different makers stuff on it. Some info can be gleaned, some you have to buy or build and try yourself.
04-13-2009, 05:38 PM #8
here is a website where the Buckley design has been made and looks like it works good.