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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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qualifying the d/4 rule

Unread postAuthor: jackssmirkingrevenge » Sat Oct 27, 2007 2:59 am

As most will know, I have been a long time enthusiast of the idea that in a coaxial pneumatic, as the piston only needs to move back 1/4 of the inner diameter of the barrel for maximum flow (you can find the math done here in defiance of the consequences of excess indulgence :roll: ), then this should be the limit of piston travel. The benefits or a smaller pilot volume are clear - for a given pilot valve, the pilot chamber will emtpyquicker with a correspondingly faster acceleration of the piston, and faster valve opening time -> more power.

However, this does not take into account the phenomenon of piston bounce. When the piston slams into the back of the pilot chamber, it will tend to bounce back, thereby once again reducing the flow. Therefore, there are circumstances where having the minimum piston travel might not be a good idea.

A case in point was my first attempt at a "velocity" themed 6mm launcher, the HVBB which was built with the "ideal" d/4 piston travel. I had a launcher with a similar valve and calibre but a 12" barrel as opposed to the HVBB's 36" that I could compare results with.

The data (using 0.12 gram airsoft BBs) was rather perplexing:

At 100 psi, I got 633, 637 and 635 fps for the 12" barrel, as opposed to 655, 657 and 661 fps for the 36" - a slight increase.

Increased to 400 psi, I got 994, 1002 and 998 fps for the 12" barrel and, rather annoyingly, 720, 711 qand 718 fps for the 36" barrel

To see the effects with heavier projectiles, I tried with 1 gram lead pellets for the following results:

At 400 psi, I got 515, 513 and 520 fps for the 12" barrel and 566, 567 and 561 fps for the 36" barrel.

I tried increasing the chamber size to double the original and got 652 feet per second for a 1 gram lead pellet, but the 0.12 gram BB however insists on being the anomaly. At 300 psi, I clocked it at 953 feet per second. Upping the pressure to around 375 psi, I got 888 fps.

I can contrast the results with those obtained with my 6mm burst disc pneumatic (similar barrel length to the HVBB and an equivalent chamber size to the original build) - 1104 feet per second for a 0.12 gram BB at less than 350 psi, at 400 psi they were off the scale. Of course you'd expect better velocites from a burst disc as opposed to a piston valve but what's important here is the trend. For the burst disc with the light BBs, more pressure = more power, which was not the case with the HVBB. The trend was not however replicated with heavier projectiles.

My interpretation of this data is that since light projectiles accelerate faster, the chamber pressure drops quicker meaning there's less pressure to mitigate piston bounce, which to some extent reseals the barrel causing a loss of performance.

I can therefore conclude that while in most cases it's a good idea, this rule cannot be universally applied.

...and now I'm strangely in the mood for some more high velocity testing, off to buy new o-rings for my shock pump :D
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Unread postAuthor: ALIHISGREAT » Sat Oct 27, 2007 3:29 am

sounds about right.

for stopping piston bounce maybe a latch or a sear to stop piston bounce which could be released before recharging the gun? i wonder what performance increase you would get, i would try it but i'm don't have one to build it into.
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Unread postAuthor: rixth » Sat Oct 27, 2007 5:31 am

Excuse my possible ignorance, but what IS a 'shock pump' not trying to thread jack ya, but I searched the forums and still didnt get a clear idea.
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Unread postAuthor: Novacastrian » Sat Oct 27, 2007 5:44 am

rixth wrote:Excuse my possible ignorance, but what IS a 'shock pump' not trying to thread jack ya, but I searched the forums and still didnt get a clear idea.


It is a device used for the pressurisation of a shock or a better description, a pressurised suspension device (the unit on dual suspension M/B's or the front forks* of a motorcycle).
Hope that was clear as mud!

*as opposed to the "rear forks", duh.

Edited to remove stupidity.

:lol:
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Unread postAuthor: jrrdw » Sat Oct 27, 2007 6:05 am

Dam Jack, your getten to be the 'Sherlock Homes' of spudguns, lol! You wanna let Watson out of the closet now, he deserves some time off for good behavior.
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 27, 2007 6:51 am

rixth wrote:Excuse my possible ignorance, but what IS a 'shock pump' not trying to thread jack ya, but I searched the forums and still didnt get a clear idea.


As Novacastrian said, they look something like this:

Image

They put out a much smaller volume than normal bicycle pumps but they go up to high pressures (officially 300 psi but if you're that abusive sort you can take them over 400 ;) ) and they have a special niche in the spudgunning world. They're next to useless for large chambers but if you're making a small calibre launcher, they will easily fill a small chamber ( a couple of cubic inches ) to the high pressures required for good performance.

Dam Jack, your getten to be the 'Sherlock Homes' of spudguns, lol!


hehe thanks, i believe that if you're willing to make certain statements then you must be prepared to back them up with hard data :)

You wanna let Watson out of the closet now, he deserves some time off for good behavior.


That could be so misinterpreted it's not even funny :D
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Unread postAuthor: Novacastrian » Sat Oct 27, 2007 6:59 am

Didn't "Watson" have some time off over your birthday weekend? :lol:
Poor "little" Watson, hehe :P
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 27, 2007 7:01 am

*facepalm* hehehhe
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Unread postAuthor: sandman » Sat Oct 27, 2007 8:53 am

back to topic, do you think using something softer as a bumper would reduce the bounce, or would that put the piston in danger of breaking?
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 27, 2007 10:01 am

Something that would absorb be impact would be ideal, and ceraintly easier to make than a latch that would retain the piston in place.
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Unread postAuthor: sandman » Sat Oct 27, 2007 10:12 am

magnet? like a neodymium one on the outside behind the piston, and to reset just pull it off
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 27, 2007 10:18 am

That would imply using a ferrous piston or piston insert, which would be heavy and therefore reduce the advantages of having a small pilot volume as well as increase stress on your launcher.

Maybe with a bit of modification, you could have a (weak) spring loaded pin that clicks into a groove on the piston to lock it in place, then use a magnet to pull down the pin while filling the chamber.
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Unread postAuthor: hyldgaard » Sat Oct 27, 2007 10:21 am

what about some kind of soft and sticky material, that held the piston back untill tthe pilot volume was filled with compressed air again? the only problem i can see about this, is if the "stickyness" of the material has to be so high, that for it too keep the piston back and not bouncing, its not able to blow it back into sealing position with filling the pilot. lol, just read that and realised how stupid it sounds, but i hope you understand what i mean :lol:
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 27, 2007 10:23 am

hyldgaard wrote:what about some kind of soft and sticky material


What, like velcro :D

You could have a chamber that reduces slightly in diameter towards the end of piston traved, that would lock it in place by friction, but this would be hard to achieve in practice.
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Unread postAuthor: hyldgaard » Sat Oct 27, 2007 10:27 am

yea, laugh all you want, but i still think the idea is valid :wink: when i was fidling around with the copper gun a week ago, i found some tape that was sticky on both sides, and about 2mm thick. i could see that working, but i dont have a clue how to tell if it did. well, except from deassembling the gun :idea:
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