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Piston in a Cross cannon

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: jhalek90 » Sun Sep 13, 2009 6:35 pm

Tht makes sence, but, is is not the same for a valve in a T??

Are goes from the chamber, 90 degrees, into the valve, and then 90 degrees into the barrel...

Am i correct?? or am i missing something...???
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Unread postAuthor: dewey-1 » Sun Sep 13, 2009 7:02 pm

MrCrowley wrote:
So, MrC Why would the air chambers be more efficient if i used an elbow... like most non-coaxial piston cannon, instead of the T i used now?


When you open the valve, air has to travel both down and up from your 2 side chambers, then the air has to turn 90 degrees through that tee and then another 90 degrees through the cross in to the barrel, just means there's a lot of air turbulence and the air wont escape from the chamber and in to the barrel as fast.


I think what MrC is trying to state is that you have a total of 6 places that are creating air flow turbulence.
Each tee creates two and the cross has two compared to two of a typical tee setup.

Thats three time more points of turbulence.

The less direction changes the more efficient!
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Unread postAuthor: MrCrowley » Sun Sep 13, 2009 7:07 pm

Dewey explained it pretty well, took the words right outta my mouth :)
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Unread postAuthor: c11man » Sun Sep 13, 2009 7:22 pm

there may be more places for air to change dirrection causing air to slow down, but there are two inlets for the valve! im not sure how much it slows downt he air but 2 fast inlets might be better than 1 very fast air inlet
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Unread postAuthor: Technician1002 » Sun Sep 13, 2009 8:01 pm

Ideally the maximum air speed through a pipe transition or orifice is Mach 1. I learned this studying a NASA site on the design of rocket nozzles. Some transitions provide a much lower rhetorical maximum.

At lower pressure the volume that gets through is less. The opposite is true, with higher pressure, at the same speed, higher mass goes through as the compressed gas is at a higher pressure.

With a series of bends and joints, the pressure drops at each joint with the final maximum speed through the valve would be at best mach 1. With back pressure and projectile, barrel and such, the maximum quickly falls off. Larger diameter chambers even if shorter for the same volume can deliver higher pressure flow into the T than long chambers with bends, small diameter and such that drops the pressure into the T. I've stated earlier that I am a fan of short fat chambers for this reason and have gotten excellent results using a valve with as little internal restrictions and turns as possible.
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