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GGDT? help from the pros?

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? help from the pros?

Unread postAuthor: Mr.michael.2468 » Sun Jan 09, 2011 6:51 pm

it might just be that i haven't had higher level math yet but i find GGDT rather confusing and difficult to use. i've done a lot of looking for help with it found a little bit for it but not much :? my giant spud gun was said to have a range of about 300 ft it definitely shoots 3x that at least... any help? especially with the valve configuration... i think my input values in this category may be screwing with my results. also i can just keep increasing the diameter and size of barrel and it infinitely increases muzzle velocity and energy which makes no sense an air tank with like 50 in capacity at 50psi couldn't even move an object the length of a 300in barrel let alone create super sonic muzzle velocities. any help would be great
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Unread postAuthor: Slugfoot » Sun Jan 09, 2011 7:58 pm

Thread noted.

You're not the only one confused by it. :oops:
I too would love to be able to use GGDT but have no idea what values to enter into the valve area for my intended configuration. :scratch:
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Unread postAuthor: saefroch » Sun Jan 09, 2011 8:28 pm

GGDT has been verified many times with actual testing and has been found to be incredibly accurate, but like all programs, GIGO (probably why you're getting wild predictions). If you want specific help, post a screenshot of it with the values entered.

Which specific input values are you having trouble with?
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Unread postAuthor: Technician1002 » Sun Jan 09, 2011 8:32 pm

Remember air expands. The pressure accelerating an object can be much higher than the force decelerating an object once the supply air runs out.

For example when a valve opens the pressure may apply forces exceeding 300 lbs in a 2 inch barrel. When the supply runs out, the projectile has kinetic energy and will continue in motion. It may slow some as the pressure drops behind the projectile to less than one Atm, but that does not mean it won't exit the barrel at high speed.
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Unread postAuthor: Crna Legija » Sun Jan 09, 2011 8:57 pm

on his page there is a tab called usage read it tell you what to write in each of the things

post a screen shot of ggdt and you gun spec then we could help
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Unread postAuthor: Slugfoot » Sun Jan 09, 2011 9:09 pm

I didn't want to hijack Mr.michael.2468's thread but what the hey... sorry. :wink:

What I'm confused about is what to enter for the values 'Valve Type', 'Flow coef' , 'Dead Volume' and 'Piston Mass' (I don't have any scales).
The setup I intend to use would be a "European" style 3/4" QEV piloted by another, 1/4" QEV. If anyone could give me a hint at what figures I should be entering here, I would be grateful. Thanks.


Edit -
on his page there is a tab called usage read it tell you what to write in each of the things

post a screen shot of ggdt and you gun spec then we could help


That's the first thing I tried. The valve in question doesn't seem to be catered for (I think. I could be missing something obvious... :scratch:).

There is no spec (as such) for the gun yet; - That's why I require the data for the qev - so I can design the gun around it. :?
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Unread postAuthor: Technician1002 » Sun Jan 09, 2011 9:26 pm

Ignore the second QEV and use only the values for the main valve. There is some information on valves in the wiki. The second valve may provide a slight improvement on the opening speed of the main valve. QEV's are fast by design, so a second one only provides marginal improvement in speed.

It is true that trying to model a valve without all the values is difficult. A QEV is a variation on a barrel sealing piston valve with a very low mass piston.

The Flow Coef is how much resistance to flow the valve provides once the valve has opened. It is related to the amount of air that can pass through the valve in a given time with a specified pressure differential on the valve. As example with the chamber at higher pressure than the barrel and the valve open, just how much air will pass through.

Some manufactures list the CV value and some list the SCFM flow value, so even the specs are not very standardized. Then to complicate things, the CV in GGDT is a % of CV, not the actual CV value you will find on a valve manufacture site.

GGDT works up to a lossless value of 100% of a valve CV value, where in real life the CV is not a percent but is based on GPM for liquid or SCFM with a gas.

See the Wiki for more information.
http://www.spudfiles.com/spud_wiki/index.php?title=Flow_coefficient

This link from the Wikepedia article is excellent.
http://www.swagelok.com/downloads/webcatalogs/EN/MS-06-84.PDF
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Unread postAuthor: saefroch » Sun Jan 09, 2011 9:41 pm

Dead volume is the volume between where the projectile touches the barrel walls and the sealing face on the valve. On most piston valves it's close to zero, and is usually negligible unless you have a section of pipe between the valve and initial position of the projectile (like an elbow or ball valve breech).
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Unread postAuthor: Mr.michael.2468 » Sun Jan 09, 2011 9:46 pm

well unfortunately im working from two separate computers, but i'll use my potato launcher for specifics, it uses 2in throughout the gun i used a caliper and measured the exacts and here they are if anyone on this thread has ever wanted to know, mind this is all sch40 pipe,

the barrel is 53in long with a 2.05in ID

the chamber is 78in long with a 2.05in ID

for the valve its a pretty easy opening ball valve thats also 2in,
i'd like to know what i should put in for all the places in the valve data section because im definitely not dealing with a piston valve or anything that complicated, i do have a modified sprinkler valve but i'll worry about that later i found some data to put in for that,

also what would an good estimated weight for a potato be? another problem that i've had is that its like doesn't matter how heavy the projectile is it always gains velocity and distance which makes no sense, just from testing i know that little air tanks don't make big heavy things like potatoes go very far or fast, it seems like it should be able to know that a 100000 grain projectile wont hardly go any where with a 50 in cubed air tank let alone 400ft at 700fps, any help or suggestions would be great, i'll try to get that screen shot.
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Unread postAuthor: saefroch » Sun Jan 09, 2011 9:55 pm

First off, what pressure are you using? Nvm, found it.

I'm not sure if you can model a ball valve with GGDT, due to the unknown opening time, but we can certainly try.

The interwebz says 650gm is about the average weight of a potato.

Using a 'Generic' valve with a flow coef of 45%, seat diameter of 1.59" and an opening time of 15ms, no dwell time, and 2 in^3 of dead volume, I get 103fps muzzle velocity, with an unnecessarily long barrel warning.
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Last edited by saefroch on Sun Jan 09, 2011 10:01 pm, edited 1 time in total.
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Unread postAuthor: Lockednloaded » Sun Jan 09, 2011 9:59 pm

you could film yourself opening the BV a few times and slow down the frame rate to see a pretty accurate opening time, and get the average and plug that in

EDIT
or...
the spudfiles wiki wrote:Ball valves: about 100 ms hand-actuated, maybe 20-30 ms spring-actuated.
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Unread postAuthor: saefroch » Sun Jan 09, 2011 10:03 pm

So ignore that last post by me, but oddly enough GGDT gives a higher muzzle velocity (127fps) when I increase opening time to 100ms. :?
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Unread postAuthor: Slugfoot » Sun Jan 09, 2011 10:12 pm

Some manufactures list the CV value and some list the SCFM flow value, so even the specs are not very standardized. Then to complicate things, the CV in GGDT is a % of CV, not the actual CV value you will find on a valve manufacture site.

I don't understand. What would I enter for a given CV of 7.5, say? :scratch:

Dead volume is the volume between where the projectile touches the barrel walls and the sealing face on the valve. On most piston valves it's close to zero, and is usually negligible unless you have a section of pipe between the valve and initial position of the projectile (like an elbow or ball valve breech).

'K...Thanks. :thumbleft:
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Unread postAuthor: Lockednloaded » Sun Jan 09, 2011 10:14 pm

the only reason I can see for that is that with ball valves, you usually need a longer barrel than faster valves in order to have an efficient launcher. So when you had such a low opening time you got an unnecessarily long barrel warning, but with a slower valve, the barrel is a good length to allow the valve to open all the way.
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Unread postAuthor: Mr.michael.2468 » Sun Jan 09, 2011 10:15 pm

Image sorry getting use to this upload pictures thing easiest way i could think of was post the url to a photobucket page just copy and paste it inand check it out hope it works :/ any better way to upload to spudfiles?

sorry that link doesn't work, well it seems to be making a little more sense now though, i think the generic valve setting is better for what im dealing with
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Last edited by Mr.michael.2468 on Sun Jan 09, 2011 10:30 pm, edited 2 times in total.
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