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Help With A Few Parameters In GDDT

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Help With A Few Parameters In GDDT

Unread postAuthor: n05gr0th » Sat Sep 13, 2014 7:38 pm

Hey guys, I'm in the middle of modelling a new pneumatic gun in GDDT but need help filling in a couple of parameters in the program.

1) I want my gun to blend aesthetics, ergonomics, and power (although the barrel will be a bit on the long side). While I haven't entirely ruled out the possibility of a QDV, I think I'm going to go with a 3/4 inch QEV actuated by another 1/2 inch QEV. Problem is, I don't know common opening times for this type of valve. What would you advise I input as the opening to represent the average commercial QEV? Also, is it possible to model the piloting valve in GDDT too?

2) I want to calculate the ballistic coefficient of my ammunition, so that I can get an idea of the range for my gun using GDDT's ballistics calculator. I've trawled the web for a while now but have had little success in finding a formula. I won't ask you to do it for me, but could someone please direct me to the right resource?

Many thanks!
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Re: Help With A Few Parameters In GDDT

Unread postAuthor: Technician1002 » Sat Sep 13, 2014 10:51 pm

Most small QEV valve have a small relatively lightweight piston that remains closed until there is a substantual drop in pressure on the pilot, so the time is in 2 parts.

First is time delay before the valve does anything. This is related to the input plumbing to the pilot. The bigger this is the longer the delay to reach the valve trip point.

The second is the valve operation time once it trips. This is quite fast and typically under 3 ms and varies with pressure. They are not as fast as a burst disk, but they are quick.
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Re: Help With A Few Parameters In GDDT

Unread postAuthor: n05gr0th » Sun Sep 14, 2014 2:12 pm

Thanks for helping. One more question - why does performance decline when setting valve opening speed below 6ms? Surely the faster a valve opens the more force can act on a projectile...
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Re: Help With A Few Parameters In GDDT

Unread postAuthor: Technician1002 » Sun Sep 14, 2014 9:15 pm

The performance suffers because much of the projectile launch happens when the valve is still mostly closed, so full valve open and full power does not show up until the projectile is most of the way launched. It's like pulling away from a stop sign by taking your foot off the brake.. moving slowly, then counting to 5 to while gradually and slowly steping on the gas. For a dragster, you want full power right away for the best launch times. Same physics apply to air cannons.

In my cannons, my design goals is to have my valve open 100% by the time the projectile has moved 2 inches in the barrel. The remainder of the launch is completed with the valve wide open. In my apple cannon, this was achieved by making the valve piston about the same weight as the apple it would launch, so the pressure that opens the valve is the same pressure that pushes the apple, so both move the first inch or two together until the valve is wide open.
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Re: Help With A Few Parameters In GDDT

Unread postAuthor: n05gr0th » Mon Sep 15, 2014 8:49 am

I think I understand you, and your QDV designs are truly impressive...but I think you might have misunderstood my question. What I don't understand is why I get higher velocity out of a GDDT simulated pneumatic with a 7ms valve opening speed (optimal OS for my design) when compared to one with identical specifications except for valve opening speeds of 6ms or faster. In practice, wouldn't a faster opening valve allow a projectile to take more advantage of the gases in the chamber and therefore accelerate faster down the barrel of the gun? What puzzles me is why GDDT doesn't predict this when valve opening times are 6ms or faster?

Maybe a GDDT screenshot will help. Would you like to see it?
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Re: Help With A Few Parameters In GDDT

Unread postAuthor: Technician1002 » Mon Sep 15, 2014 9:34 am

This has to do with projectile position in the barrel as the valve is opening. Look at GGDT for the graph of projectile travel. Note in the first 5ms or so, the projectile is still within a few inches of the breech, so faster valves don't change much for the remainder of the launch. A slower valve will start having the projectile much further down the barrel before the valve is fully open.

Where the projectile is located when the valve finishes opening is the key to the numbers you are seeing. First projectile movement is relatively at low speed compaired to projectle speed later in the shot.

This is one of the reasons for using a fast valve. With a projectile, the performance approaches that of a burst disk. For noise without a projectile, a burst disk is still very hard to beat. Same applies to very low mass projectiles such as marshmallowes or empty pop cans.

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Re: Help With A Few Parameters In GDDT

Unread postAuthor: n05gr0th » Tue Sep 16, 2014 7:39 am

Aha! That clarified things - thanks a bunch. Those damage pictures are sweet! What did you shoot those pop cans at?
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Re: Help With A Few Parameters In GDDT

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Re: Help With A Few Parameters In GDDT

Unread postAuthor: Technician1002 » Tue Sep 16, 2014 7:39 pm

The first picture with a bunch of cans isn't mine, the single can is my photo. It was shot about 6 feet from the cannon into a 1/4 inch steel plate. I should hunt up the thread later and post a pic of the setup.
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Last edited by Technician1002 on Tue Sep 16, 2014 11:28 pm, edited 1 time in total.
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Re: Help With A Few Parameters In GDDT

Unread postAuthor: n05gr0th » Tue Sep 16, 2014 7:49 pm

Definitely. Thanks for sharing.
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Re: Help With A Few Parameters In GDDT

Unread postAuthor: Technician1002 » Tue Sep 16, 2014 9:11 pm

Here is the link to the thread on high speed empty soda cans. Don't jump ahead. Start at the beginning and follow the progression of the thread to learn the most. Page two has the setup I used to make the flat can.

http://www.spudfiles.com/pneumatic-cannons/topic21132.html?hilit=steel%20plate%20can%20flat
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Re: Help With A Few Parameters In GDDT

Unread postAuthor: n05gr0th » Wed Sep 17, 2014 6:00 am

God damn it Tech, you've made me want to build a QDV cannon now! I think it's pretty cool that you can actuate it just by yanking a piece of string. It would be awesome firing a large cannon like an old Napoleon gun!
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Re: Help With A Few Parameters In GDDT

Unread postAuthor: jackssmirkingrevenge » Wed Sep 24, 2014 7:31 am

n05gr0th wrote:I want to calculate the ballistic coefficient of my ammunition, so that I can get an idea of the range for my gun using GDDT's ballistics calculator. I've trawled the web for a while now but have had little success in finding a formula. I won't ask you to do it for me, but could someone please direct me to the right resource?


You either need a wind tunnel or some pretty sophisticated software, but here are some "good enough" estimates for some typical shapes:

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Re: Help With A Few Parameters In GDDT

Unread postAuthor: n05gr0th » Wed Sep 24, 2014 1:24 pm

That's great, JSR - thanks!
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