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Question on Distance i Should Expect, GGDT seems unreasanabl

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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Question on Distance i Should Expect, GGDT seems unreasanabl

Unread postAuthor: bruno6491 » Fri Oct 19, 2007 3:35 pm

I have a a air cannon that i want to change a barrel from a 2" to 3" piece. I currently have a 7ft piece and would make the 3" 8-9ft long as well.

My chamber is a 4" diameter with a distance of 6 feet and using a 2" ball valve

i want to launch little pumpkins that weigh about 120-130 grams

At about 80 psi or 100 psi, what should i expect for a range, GGDT says only 380 feet, which seems really low

Anyone??

Thanks again Bruno
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Unread postAuthor: potatoflinger » Fri Oct 19, 2007 4:07 pm

Well, that actually seems about right, since the valve you are using is a ball valve, and it is also quite a bit smaller than your barrel, but I could be wrong.
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Unread postAuthor: bruno6491 » Fri Oct 19, 2007 4:09 pm

thanks, i was hoping for like 500 feet or more

any other opinions??

how can i make it reach about 600+ feet??

Thanks Bruno
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Unread postAuthor: MrCrowley » Fri Oct 19, 2007 4:13 pm

Piston valve, probably 3" or a 4" piston valve should do.

Don't ask what it is, just search it, there are hundreds of topics on piston valves here, so don't even think about posting a topic about one unless you have done a decent search. Trust me if you have a question, we already have the answer.
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Unread postAuthor: Fnord » Fri Oct 19, 2007 4:24 pm

Assuming you have the ball valve spring loaded, I got this result.

A pumpkin won't have a drag coef of .5, unfortunately. Are these pumpkins round or kinda flat oval shaped?
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Unread postAuthor: bruno6491 » Fri Oct 19, 2007 6:25 pm

they are almost perfectly round, like a baseball, and i would add some slight wadding for a best possible seal.

The piston valves look really complicated to make, so i might stay with a ball valve, since i already have everything cemented together, maybe for my next cannon

Would a piston valve really add that much more distance vs a ball valve?

Thanks for the help

Bruno
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Unread postAuthor: MrCrowley » Fri Oct 19, 2007 6:32 pm

Yes, it should add a lot of power if you build it well.
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Unread postAuthor: Hubb » Fri Oct 19, 2007 6:35 pm

MrCrowley wrote:Yes, it should add a lot of power if you build it well.


and they're only as complicated as you want them to be.
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Unread postAuthor: Marco321 » Fri Oct 19, 2007 7:29 pm

bruno6491 wrote:they are almost perfectly round, like a baseball, and i would add some slight wadding for a best possible seal.

The piston valves look really complicated to make, so i might stay with a ball valve, since i already have everything cemented together, maybe for my next cannon

Would a piston valve really add that much more distance vs a ball valve?

Thanks for the help

Bruno


A piston would alot alot more, because it opens much faster, having more flow going to the projectile.

In regards to the complexity, they work on a simple principal of differential pressure, meaning they move because there is more force acting on one side. Look at the piston valve animations, go through them frame by frame and think about what is happening, break each part down into their core aspects. You will quickly get the idea, also try drawing a design of one and pm or msn or aim it to a member who understands them and see if you went wrong.

If you want, add me to msn or something.
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Unread postAuthor: bruno6491 » Fri Oct 19, 2007 7:41 pm

okay thank you

i dunno if i will have time to build a new one but for future reference i might design one and see what you think of my design

i think i may just spray paint an apple orange, that way when they see me quickly stick it in they will never know what it was, plus it goes so fast, no one will know

I never thought that going to a 3" barrel would reduce performance and distance in half

Thanks Bruno
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Unread postAuthor: Marco321 » Fri Oct 19, 2007 8:16 pm

bruno6491 wrote:okay thank you
I never thought that going to a 3" barrel would reduce performance and distance in half
Thanks Bruno


Well, the projectiles speed comes from air pressure. A larger diameter barrel will cause the air pressure to drop faster per unit of length down the barrel. This creates a slower projectile.
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 19, 2007 11:02 pm

Marco321 wrote:Well, the projectiles speed comes from air pressure. A larger diameter barrel will cause the air pressure to drop faster per unit of length down the barrel. This creates a slower projectile.


It's not as simple as that, a larger diameter barrel also increases the base area of the projectile and since (force = pressure x area), your pressure (for the same valve) would be reduced but there would be more surface for the air to act on.

The piston valves look really complicated to make, so i might stay with a ball valve, since i already have everything cemented together, maybe for my next cannon

Would a piston valve really add that much more distance vs a ball valve?


Ball valves are really the lower end of valves, you'll be amazed what a difference a fast opening valve will make. If you're not willing to go through the effort of making a piston valve, why not try and get your hands on a 2" or so sprinkler valve? Modding it is childs play and believe me, you *will* see the difference ;)
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Unread postAuthor: Marco321 » Fri Oct 19, 2007 11:22 pm

jackssmirkingrevenge wrote:
Marco321 wrote:Well, the projectiles speed comes from air pressure. A larger diameter barrel will cause the air pressure to drop faster per unit of length down the barrel. This creates a slower projectile.


It's not as simple as that, a larger diameter barrel also increases the base area of the projectile and since (force = pressure x area), your pressure (for the same valve) would be reduced but there would be more surface for the air to act on.


I know.... I was making an edit to say that and how increasing chamber volume but keeping pressure the same would also increase speed, then my computer crashed for some reason........ and if forgot about it........

I have been meaning to make a post about this for a long time so here it goes. Since the more area exposed to the air pressure means more force, but with increased surface area weight increases, making the projectile slower. But slower doesn't necessarily mean less energy, it will have the same kinetic energy (since kinetic energy is dependant on speed and mass ). Does that mean there is a theoretical optimum C:B and pressure for a projectile of a certain surface area and mass?
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 19, 2007 11:30 pm

Since the more area exposed to the air pressure means more force, but with increased surface area weight increases, making the projectile slower.


This is exactly why sabots are used - you're increasing the surface area, but at the same time keeping the weight low in comparison by using lightweight materials to support your sub-projectile.

Does that mean there is a theoretical optimum C:B and pressure for a projectile of a certain surface area and mass?


As far as I'm concerned, for pneumatics the optimum is as much as you can afford. In practical terms, the more pressure then the more power you're going to get - same goes for chamber volume, though this tapers off to a point where the increase is barely noticeable.
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Unread postAuthor: bruno6491 » Sat Oct 20, 2007 10:46 am

thanks so much, i am learning why this is not as powerful and also the pumpkins are fairly light, like a 120 grams, so they should be capable of going about a 100 yards, a 2" sprinkler valve is quite expensive, so i am going to pass, but i will make a piston valve on my net building, probably after winter.
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