GTA
All Springer/NP/PCP Air Gun Discussion General => "Bob and Lloyds Workshop" => Topic started by: Kmanca1 on January 24, 2022, 12:05:32 PM
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In my several years of lurking, I've never seen this question referenced. And, I have no idea where the proper place to ask is....so I'm asking here.
Way back when, THEY - that would be the nebulous 'they' - used to say that you couldn't really overload a blackpowder gun. Because, the powder was slow enough burning that, upon firing, the majority of the 'overload' would be pushed out the barrel BEFORE it had time to burn/explode.
NOTE: I'm not suggesting anyone try it to confirm it! ;)
So....similar-but-different. Is there a point, in our tinkering, where the air pushing a projectile reaches its' maximum effectivness?
I mean, if you have a 40cc plenum, ALL of that air will be expelled behind the pellet/slug and aide in its flight.
If you have one of the 'new' FX 80cc plenums, the same happens.
I was just reading RALLYSHARK's post, creating a 129cc plenum for his FX mutant, and I wondered....
When or where is the maximum effectiveness reached?
Will the entire 129cc be expelled-----and push the pellet with the ENTIRE 129cc?
If you were able to create a 500cc plenum, would the ENTIRE 500cc push the pellet?
Or....is there a point, like the blackpowder scenario, where it doesn't make any more difference. Say, beyond '130cc', any remaining air in the plenum is wasted. It doesn't 'PUSH', it's just along for the ride?
I'm sure there's a formula for that. Some trig-a-bra-ic type of equation to figure it out, but I'm no mathematician.
I just have a weird mind that wanders and wonders.
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I'm sure there is but I have no idea what the formula is. I would think pressure, barrel length, barrel ID, and possibly velocity would be the main factors.
Following
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p2 = (v1 * p1) / v2
where p = pressure and v = volume
That is the formula that basically will explain why plenum volume makes a difference. I won't explain much but the less the pressure difference the less the loss
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That precludes that PCP's operate as DUMP guns ... which they do not !
Dump being such as a POWDER CHARGE and all of its created energy / pressure expelling during said shot cycle.
The reason behind LARGE PLENUMS in a PCP is based on the Energy / Pressure used is just a Portion or Sip of whats available. With that sip you lose not only volume but pressure being the regulator simply Can Not supply air volume or pressure to replace whats used quickly enough. So .... MORE PLENUM volume has the "Average pressure maintained" during shot remaining higher due to the volume offered by the larger plenum :o
Simply put ...
Scott S
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That's an interesting question, because if the plenum is large enough, you might not get the advantage of regulation, unless you dump all of the air in the plenum with each shot. So too big a plenum could be a disadvantage, as far as air use goes.
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Disregard that, brain is some where else today.
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James, I thought it was spot on, but sometimes my two working brain cells disagree with each other. :o
Too much plenum and too much pressure and it's an air hog and efficiency goes out the door (barrel I mean).
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That precludes that PCP's operate as DUMP guns ... which they do not !
Dump being such as a POWDER CHARGE and all of its created energy / pressure expelling during said shot cycle.
The reason behind LARGE PLENUMS in a PCP is based on the Energy / Pressure used is just a Portion or Sip of whats available. With that sip you lose not only volume but pressure being the regulator simply Can Not supply air volume or pressure to replace whats used quickly enough. So .... MORE PLENUM volume has the "Average pressure maintained" during shot remaining higher due to the volume offered by the larger plenum :o
Simply put ...
Scott S
I KNEW I could count on Sir Motorhead! Thanks guys! I'm satiated for the day. ;D
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If you have a dump valve, then indeed there is a point of diminishing returns for the volume of air that is dumped.... Overall power is increased only fractionally, while air wasted increases drastically.... Roughly 50% of the barrel volume is about as far as it makes sense to go, for that reason.... Here is an example....
(https://hosting.photobucket.com/albums/oo221/rsterne/Millenium Pumper/.highres/DumpValves_zps8ff30e30.jpg?width=1920&height=1080&fit=bounds) (https://app.photobucket.com/u/rsterne/a/2b6b9a8f-bf4c-41f4-8426-985eaf5ed6aa/p/a0fd3d9a-96c7-4f4f-bfc0-9e346f70e1db)
Closing the valve at the instant the pellet leaves the muzzle, on a valve (or plenum) that has 100% or barrel volume, saves half the air, while not losing any power.... because air released from the valve after that does nothing except make noise and recoil.... PCPs generally close the valve even earlier, most when the pellet is only 25-33% of the way to the muzzle.... Keeping the valve open past 50% of the barrel length makes very little power while using a lot more air....
Using a plenum large than about 1 cc per FPE you want the gun to produce doesn't waste air, but gains little FPE.... So, all three situations have a "practical" limit....
Bob
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That's a great graph Bob! Thanks for posting and sharing.
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Bob, Lloyd and others have a much more detailed look at the question of a PCP's maximum achievable velocity, here: https://www.gatewaytoairguns.org/GTA/index.php?topic=94054.0 (https://www.gatewaytoairguns.org/GTA/index.php?topic=94054.0)