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Need help with school project

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: Labtecpower » Tue Apr 12, 2011 11:34 am

380 meters is a very nice distance for a gun like yours, I think.
At 15 psi i'm getting about 100 meters with a less-than-optimal projectile.
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Unread postAuthor: Crna Legija » Tue Apr 12, 2011 11:37 am

i get over 9000centimeters(yes i know its under 100m) at only 500psi :lol:
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Unread postAuthor: Technician1002 » Tue Apr 12, 2011 12:43 pm

warhead052 wrote:I have used my own paper shotgun shell looks sabots for paintballs and bbs, and they work perfectly because when I release the pressure, they dont create a tight enough seal to hold everything in place, just enough of a seal to let it all go flying out of the barrel to 100-150 feet away at only 20 psi... You tell ME if thats a good seal.


The difficulty in a large efficient launcher is to get precise low pressure shots. You don't want to know what all I had to do to get an accurate 20 foot shot into a wheelbarrow. The band between no launch and too far is very small. :)

It is much easier to do slow speed shots with a highly restrictive valve.
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Unread postAuthor: mafaka » Tue Apr 12, 2011 1:25 pm

Technician1002 wrote:It is quite plasuable you overshot the field. I get almost 400 yards with a golf ball on only 50 PSI. I always lose full pressure shots for distance.


Yay! Glad to hear that, I thought we might just had lost them in the dirt. Ok, I can imagine we are not the only ones with problems finding the projectile.

Labtecpower wrote:380 meters is a very nice distance for a gun like yours, I think.
At 15 psi i'm getting about 100 meters with a less-than-optimal projectile.


What kind of projectile is it?
I think that sounds quite nice anyway. With optimal gas mixture (which we didn't manage to get as you have read in the earlier pages) we calculated the pressure to about 10 bar. And today my friend found a fault in our excel program; we had put the diameter for the golf ball where the radius should have been when calculating the drag. Now we got a distance of 500-550 m instead of the 120 we got earlier, it seems closer to reality.
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Unread postAuthor: Labtecpower » Tue Apr 12, 2011 1:31 pm

What kind of projectile is it?

A golfball with a cardboard tube strapped to the back
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Unread postAuthor: mafaka » Tue Apr 12, 2011 2:33 pm

Labtecpower wrote:
What kind of projectile is it?

A golfball with a cardboard tube strapped to the back


I see, out of curiosity, why don't you skip the cardboard tube?
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Unread postAuthor: Labtecpower » Tue Apr 12, 2011 2:48 pm

I tought it would make the projectile more aerodynamic. I was wrong. According to the sound it made, I think it had a lot of drag.
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Unread postAuthor: mafaka » Tue Apr 12, 2011 3:16 pm

Labtecpower wrote:I tought it would make the projectile more aerodynamic. I was wrong. According to the sound it made, I think it had a lot of drag.


If you could make it more streamlined that would probably help, otherwise golfballs have quite low Cd as they are anyway. My friend found a graph that shows that the Cd drops a lot after a certain speed, can try to find it if you want.
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Unread postAuthor: saefroch » Tue Apr 12, 2011 3:45 pm

The design of a golf ball attempts to create a small region of turbulence towards the front of the ball. This creates an area of a slower-moving fluid, which by Bernoulli's principle exerts less pressure on the golf ball and reduces drag, by attempting to balance that force with the force created by the turbulence in the wake of the ball. By adding cardboard, you've probably disrupted this effect.
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Unread postAuthor: mafaka » Thu May 26, 2011 5:30 pm

Hello again guys!
Sorry for the late update, the report for the project was handed in 2nd May and we got the best grade possible for doing it. (MVG as it is called in Swedish)

And by the way, we corrected an error in our excel program. We had accidentally used the diameter instead of the radius when calculating the drag, and now it is measured to go about 520 m instead of 125 m. Makes more sense actually. The excel calculation is the Euler Method.
If someone is interested I can maybe share the file with you, if I log in here any time soon.

Anyway, I just wanted to thank the very polite and pedagogical members of spudfiles for helping us. It wouldn't have been possible without you.
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