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Pilot Volume vs Piston Weight

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: MrCrowley » Sun Oct 28, 2007 12:07 am

Yeah I see what you mean, maybe D_Hall (creator) needs to have a 'piston friction' variable, which conflicts with the pressure being used, so when the piston has a certain amount of friction it can stop a certain amount of air pressure from getting through.

Also there needs to be a whole valve table for the pilot valve, which is just labeled as vent diameter. A 1" Sprinkler valve will pilot the pilot volume faster then say a 1/2" ball valve, so there should be an increase in performance.
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Unread postAuthor: spudgunnerwryyyyy » Sun Oct 28, 2007 12:10 am

Well yeah but he seems to have not updated his site or done any air cannon research lately. but ggdt is just a estimate tool and is great for checking c:b ratios.

btw the piston friction is the exact terminology i was looking for.
He has a email link on his site so maybe we should bring it up just for kicks to see if the math for that is feasible.
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Unread postAuthor: MrCrowley » Sun Oct 28, 2007 12:13 am

Yeah, well it's worth a try.

He could be busy with his awesome job but he may be willing to update it a bit, seeing as members like joannaardway don't find it reliable enough, though her program(I think it's hers) has a lot more variables but is a spread sheet.
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Unread postAuthor: ammosmoke » Sun Oct 28, 2007 4:37 pm

Joannaardway has her own program? If she made it, how can she be sure it is accurate? Oh well, I'll leave that up to her. Man, she hasn't been on here for a while... When did she say she would be back again?
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Unread postAuthor: clide » Sun Oct 28, 2007 6:35 pm

jackssmirkingrevenge wrote:I disagree there. The heavier a piston, the slower it will accelerate and you can't deny the link between valve opening time and performance. Also, the lower the pilot volume, the quicket the pilot chamber will depressurise and the quicker the piston will start to move.


Unfortunately it is not that simple, although a lighter piston will accelerate faster, it will also decelerate faster. Pistons often encounter deceleration before they have moved all the way back which means in some cases a heavier piston can actually increase valve opening time.

As for the pilot volume it is true that the piston will start to move faster, but it's actual opening speed can be slowed down by a small pilot area because the moving piston will repressurize the pilot; this is also what causes the deceleration of the piston mentioned above.

As for the link between opening speed and performance it is not as drastic as many people think if you are confining yourself to the range of piston valves. On a typical gun, providing the piston wasn't bouncing closed or not opening completely (and assuming a perfect piston seal), you could change the piston weight or pilot volume to virtually anything you want and not see a noticeable change in muzzle velocity. Even though one setup may open 5 times as fast as another, the difference between a valve that opens in 10 ms and 2 ms is not huge unless you are using a light projectile or short barrel.

As for the GGDT stuff I suggested that he add a piston friction input in there awhile ago back on Spudtech. He didn't seem to interested in adding it.

Also, I believe Joanna made her own external ballistics calculator because GGDT's is about as basic as you can get without neglecting air drag, but I've never heard anything about her making her own internal ballistics calculator.

GGDT's internal ballistics and valve modeling calculations have always seemed very accurate to me. Often I measure withing a few percent of what GGDT predicts when I am using standard valves.
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Unread postAuthor: jackssmirkingrevenge » Sun Oct 28, 2007 8:49 pm

As for the pilot volume it is true that the piston will start to move faster, but it's actual opening speed can be slowed down by a small pilot area because the moving piston will repressurize the pilot; this is also what causes the deceleration of the piston mentioned above.


So, in a nutshell, couple my minimal pilot volume fascism with a high flow pilot valve and you're sorted :)
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Unread postAuthor: spudgunnerwryyyyy » Sun Oct 28, 2007 8:56 pm

Yeah if the pilot valve remains open the pressurization of the pilot volume by the piston will not occur,

Mr. Crowley can you post or PM me a link to get that spreadsheet, i love them they seem so much more scientific, so yeah.
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Unread postAuthor: MrCrowley » Sun Oct 28, 2007 9:47 pm

I have no idea where it is sorry.
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Unread postAuthor: clide » Mon Oct 29, 2007 7:43 pm

spudgunnerwryyyyy wrote:Yeah if the pilot valve remains open the pressurization of the pilot volume by the piston will not occur


It depends on the size of your pilot valve. Even if it is open the pilot can still repressurize if the valve is not big enough to keep up with the moving piston, as it often isn't. you also have to remember that the lighter the piston, the harder it is for the pilot valve to keep up. You also have to consider the opening speed of your pilot valve.

I think if you are truly going for the fastest valve possible then the optimization of piston size, seat size, and piston seal (o-ring and check valve) are more important than optimization of pilot volume and piston weight, but like I said before piston valves rarely need to be optimized because they are just so darn fast in nearly all cases.
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