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GGDT strange behaviour

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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GGDT strange behaviour

Unread postAuthor: Demon » Tue May 03, 2011 11:20 am

Some of you may know, but I want to change the caliber of my M82A1 project to avoid lawsuits, to convert it to cal .69 (paintball) instead of cal .545 (cartridges and wadding).

But, as I haven't expected at all, I got much worse performances with the paintballs, altought they weight less and have a bigger surface area.

Fixed parameters:
Chamber volume, air pressure, barrel length, valve size.

Variables :
Barrel diameter.

I barely get 400 FPS with a 3.2 gram paintball in .69 barrel diameter, while the 4 gram .545 projectile was going way ofer 500 FPS.

What could cause that ?
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Unread postAuthor: Crna Legija » Tue May 03, 2011 11:31 am

how big is the chamber, you should go for 1:1 if your using under 250psi

screen shot ggdt would be nice.
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Unread postAuthor: warhead052 » Tue May 03, 2011 11:31 am

Weight. And possibly shape? Maybe because the paint inside the ball is moving around as it flies, causing it to mess with the speed? Try making a rubber round, the same size.
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Unread postAuthor: Technician1002 » Tue May 03, 2011 11:34 am

Limited volume of air, and limited rate air can fill the new larger volume of the barrel.

I would vote valve first, and chamber second.
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Re: GGDT strange behaviour

Unread postAuthor: jackssmirkingrevenge » Tue May 03, 2011 11:38 am

Demon wrote:What could cause that ?


The valve is feeding x amount of air into the barrel.

If you increase the barrel diameter, yes you've increased the cross-sectional area of the projectile for the pressure to act on, however you've also proportionally increased barrel volume, meaning pressure will rise at a lower rate.

Also, heavier projectiles compensate for lower valve speeds and flows, so muzzle energy tends to be higher, though not necessarily muzzle velocity.

warhead052 wrote:Weight. And possibly shape? Maybe because the paint inside the ball is moving around as it flies, causing it to mess with the speed? Try making a rubber round, the same size.


step 1) think

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step 4) read post to self

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step 7) delete/submit post

;)
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Unread postAuthor: Demon » Tue May 03, 2011 4:10 pm

Jack and technician, it seems you are right.

Simply making the valve diameter bigger of one millimeter gives tens of m/s more.

(And I can't make the valve seat any bigger)

So that means that the efficiency of this gun would go way down if I made the barrel bigger ? Where does all the pumped in energy goes ?
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Unread postAuthor: jackssmirkingrevenge » Tue May 03, 2011 4:33 pm

Demon wrote:So that means that the efficiency of this gun would go way down if I made the barrel bigger ? Where does all the pumped in energy goes ?


It's wasted if you use a light projectile, try modelling some heavier rounds with the larger barrel and you should see a rise in efficiency and muzzle energy.
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Unread postAuthor: Technician1002 » Tue May 03, 2011 7:11 pm

Demon wrote: Where does all the pumped in energy goes ?


Same place the high HP from a shop air compressor goes when spray painting. The energy goes into making very turbulent air. You are spraying air into the barrel, and the valve is limiting the volume delivered.

A larger valve permits delivery of a higher volume of air into the barrel with less pressure drop on the valve and more pressure on the projectile instead.

I try to build valves to just get out of the way of the flow and do it quickly.

For high speed and light projectiles, the valve is the big limiter.
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