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GGDT Problems

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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Do you think GGDT is a good accurate guide when building cannons?

Yes
3
60%
No
1
20%
With a little patchwork and fixing of errors
1
20%
 
Total votes : 5
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GGDT Problems

Unread postAuthor: Jolly Roger » Fri Jun 29, 2007 10:22 pm

I don't know how many others out their have noticed this as well but when using the GGDT program to find out some figures on my cannons, I find it very confusing and I question how accurate it is. Alot of the features on it do not work at all, for example when setting the diameters of the resevoir, if you change the outside diameter the results come out different. Then when I make the inside diameter bigger the shot performance lowers. And the bigger I make the projectile, even if it is larger than the barrel, alot of the time it performs better. Also when I "optimize" the gun, say if I try keep the barrel the same and see what sort of chamber dimensions perform best with it, it just randomly changes the values. I don't think it has any performance optimizing in mind at all, because every time I press it, the results are different. Even from these few obvious errors, I feel uncomfortable using the program when making my cannon. Does anyone have any info on this, or know of an update. It would be greatly appreciated because I really need a program like this to help me build my cannons as efficiently as I can.
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Last edited by Jolly Roger on Mon May 04, 2009 10:01 pm, edited 1 time in total.
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Unread postAuthor: iPaintball » Fri Jun 29, 2007 10:53 pm

Why would you have to model the inside diameter of your chamber? The only time you would have to do this is if you had a coaxial setup. When you change th OD of your chamber, you aren't changing the OD of your pipe, just the actual chamber. The ID only needs to be used if your barrel is running through your chamber, whick I'm guessing it's not. And it most sertainly does not "randomly chang the values." It calculates them based an complex equations, not by random. All of the errors stated are on your part, not GGDT's. GGDT is an excellnt program, if you know how to use it correctly. (which, obviously, you don't)
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Unread postAuthor: Killjoy » Fri Jun 29, 2007 10:55 pm

well the inside diameter setting is for if the cannon was a coaxial so it subtracts the volume of the barrel whose diamter is the inside diameter which it assumes is the barrel. If the cannon is not a coaxial then the inside diameter should be zero. Thats why the perfomance lowers cause it subtracts the barrel volume from the chamber volume. The errors seem to be on your part, not the programs.
And personnally i feel the program is quite accurate in predicting the perfomance of a cannon, and for me is generally able to predict the velocity and range within a 10 or 15 percent margin of the actually results from physical test. But thats just me.

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Unread postAuthor: iPaintball » Fri Jun 29, 2007 11:02 pm

Well you don't have cry about it! :wink: I actually type very slow too...
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Unread postAuthor: Jolly Roger » Fri Jun 29, 2007 11:16 pm

Cool okay we got the diameter thing out of the road then. So whats the go with the projectile diameter and the barrel diamter ratio. And when it optimizers, on what basis does it optimize. I would have thought it would be referring to performance, yet when nothing is changed on the data and optimize is pressed, whats with all the different answers. Do they all refer to different apsects of the gun? And new guy, I say random because I see no patern or consistency in relation to the results.

These are some examples of the projectile and barrel problem I was having. The first where the barrel is 0.5 inches and the projectile is 3 inches. The rest as follows.
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Unread postAuthor: boilingleadbath » Sat Jun 30, 2007 5:06 am

Honestly, all these things are covered in the text of <a href="http://www.thehalls-in-bfe.com/GGDT/usage_menu.html">DHall's website</a>.

I think it allows one to use a projectile diameter bigger than the barrel's diameter so that one can model guns in which the projectile projectile slides over the barrel... but I'm not certain. Either way, those are rather exotic scenarios - in most of our use, you will be using a projectile diameter equal to or smaller than the diameter of the barrel.

The optimizers are also covered in his text... but I will just say that, yes, they optimize by changing different dimensions; one changes the barrel length until performance is maximized (for a fixed chamber - like the latke tests), one changes the chamber size until acceleration is zero at the muzzle, and one changes both lengths (keeping the total length of the barrel+chamber equal to the original sum barrel+chamber)

Note also that the optimizing search pattern can be buggy at times... but you can do the same thing manually if that program feature is not working for you.
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Re: GGDT Problems

Unread postAuthor: joannaardway » Sat Jun 30, 2007 9:02 am

Jolly Roger wrote:I don't know how many others out their have noticed this as well but when using the GGDT program to find out some figures on my cannons, I find it very confusing and I question how accurate it is.

It's very accurate, most of the time, anyway - give it the right numbers, and you'll probably get the right result.

Alot of the features on it do not work at all, for example when setting the diameters of the resevoir, if you change the outside diameter the results come out different. Why? Then when I make the inside diameter bigger the shot performance lowers. Why?

This has largely been covered. Outer diameter is a bit ambigous - really it should be "The inner diameter of the outer pipe on the chamber." But that's too long.
And inner diameter should be "The outer diameter of any pipe running inside the chamber". Again too long.
Bigger "Outer diameter" = bigger chamber volume, so more potential power.
Bigger "Inner diameter" = smaller chamber volume, so less potential power.

And the bigger I make the projectile, even if it is larger than the barrel, alot of the time it performs better. Why?

If you have blow-by on the projectile, that represents lost power. Less blow-by = more power. Simple.
GGDT has an automatic feature that if you assign the projectile as bigger than the barrel, it just treats it as being the same size as the barrel.

And when I "optimize" the gun, say if I try keep the barrel the same and see what sort of chamber dimensions perform best with it, it just randomly changes the values. I don't think it has any performance optimizing in mind at all, because every time I press it, the results are different.

That's how the optimiser works. It fiddles with the values in a rather ad hoc way, to find at what point the performance is "optimal".
But as BLB said, these are a little buggy - I often find that using them results in the program crashing.

Even from these few obvious errors, I feel uncomfortable using the program when making my cannon.

You shouldn't! The writer, D_Hall is a fully qualified engineer, so he does know what he was on about when he wrote it.
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Re: GGDT Problems

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Re: GGDT Problems

Unread postAuthor: Gepard » Sat Jun 30, 2007 10:04 am

joannaardway wrote:
Even from these few obvious errors, I feel uncomfortable using the program when making my cannon.

You shouldn't! The writer, D_Hall is a fully qualified engineer, so he does know what he was on about when he wrote it.


I think he was referring to the program's GUI rather than the data it produces....

Michael
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Unread postAuthor: Jolly Roger » Sat Jun 30, 2007 10:05 am

Cool well that explains heaps. Cheers guys. Just using it for the first time. Seems to be pretty handy by the sounds of things. Again thanks guys.
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