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qualifying the d/4 rule

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: cdheller » Sat Oct 27, 2007 10:54 am

Just a couple of ramblings err um thoughts,
take them for all their worth.

I'm wondering if increasing the diameter of the piston/cylinder and using the increased area difference would help or hurt.
the increased area could hold the piston back or it could speed up the rearward travel and make more ?bounce?

I've kinda wondered about the venturi effect drawing the piston toward the escaping air and if it has much influence.

from some web page

"""4. Cut a 7 X 7 cm square of thin cardboard. Draw diagonals from each corner and put a pin through the card where the lines cross at the centre. Secure the head of the pin by covering it with adhesive tape. Put the pin in the hole of an empty thread spool and try to blow the card from the spool by blowing through the spool. Turn the spool and card upside down. Hold the card against the spool lightly with a finger. Blow firmly through the spool, then remove the finger. Air moving inside the spool is at a lower pressure than the air outside the spool. So, atmospheric pressure pushes the card against the end of the spool."""
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Last edited by cdheller on Sat Oct 27, 2007 11:01 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 27, 2007 10:59 am

I'm wondering if increasing the diameter of the piston/cylinder and using the increased area difference would help or hurt.
the increased area could hold the piston back or it could speed up the rearward travel and make more ?bounce?


From my results it can be deduced that more pressure in the chamber = more force on the piston and therefore less bounce. By increasing piston diameter, for a given pressure there will be more force on the piston, but you're introducing a lot more variables - the piston is now heavier, so with more momentum, though with the same pilot valve, your pilot volume is now bigger, so it will accelerate slower. It's quite a complex equation to balance.
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Unread postAuthor: clide » Sat Oct 27, 2007 11:02 am

To reduce bounce you would want to decrease the size of the piston relative to the barrel...Which also has the benefit of making the gun more efficient.
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Unread postAuthor: cdheller » Sat Oct 27, 2007 11:25 am

jackssmirkingrevenge wrote:
It's quite a complex equation to balance.


I see you're prone to understatements. :lol:

not exactly on topic and probably a moot point but
I've wondered about piston speed for a spell and weather the difference in volume in the projectile being at rest with a small dead space and end of barrel conditions with big dead space projectile moving close to max could be compared to different valve factors
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Unread postAuthor: jimmy101 » Sat Oct 27, 2007 11:45 am

So jack, what you are talking about is basically the same phenom as "honking" in a diaphram valve?

Once again dynamics raises it's ugly head. The "d/4 rule" is really a static view of the valve. In the real world things are moving and statics can be misleading.
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 27, 2007 11:53 am

jimmy101 wrote:So jack, what you are talking about is basically the same phenom as "honking" in a diaphram valve?


The same concept is true of "honking" as far as I can tell.
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Unread postAuthor: spanerman » Sat Oct 27, 2007 1:38 pm

yeah, the honk is the piston/diaphragm opening and closing rapidly....i normally get it when using a gun with excessive pilot volume or an inadequate pilot valve...not much to do with the piston iirc
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Unread postAuthor: psycix » Sun Oct 28, 2007 5:55 pm

Vacuum the pilot volume!

That will grant a faster opening time AND keep the piston sucked back more :)
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Unread postAuthor: ramses » Thu Jan 22, 2009 4:58 pm

that isn't a bad idea at all.

depending on your pilot volume with the valve all the way open, you could use a check valve to allow the air out, but not back in. This would interfere with filling, but on large cannons, filling could be done through a different hole. the smaller your pilot volume with the piston all the way back, the better.
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Unread postAuthor: Pilgrimman » Thu Jan 22, 2009 5:43 pm

Well here is my take on it... I wrote the part at the bottom. It obviously doesn't factor in advanced fluid mechanics or anything, but is reasonable enough for our purposes.

http://www.spudfiles.com/spud_wiki/index.php/Piston_valve
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