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GGDT help please?

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 please?

Unread postAuthor: vng21092 » Fri Jan 09, 2015 2:30 pm

So I just downloaded GGDT because I wanted a general idea of the performance I'd get with a project I had in mind. So to learn how to use the program I tried to replicate the results of the gun I made in this thread. http://www.spudfiles.com/pneumatic-cannon-database/topic26107.html. Only problem is I can't. I'm pretty sure all the fields in the Chamber Data are correct. Valve data, I think its all there, except for "Flow Coef", no clue on that one, wasn't in the details on Amazon, someone give me an estimate? Link just in case, http://www.amazon.com/Inlet-Outlet-Ports-Quick-Exhaust/dp/B007Q82QTM/ref=sr_1_1?ie=UTF8&qid=1420831352&sr=8-1&keywords=qev. Projectile and Barrel Data, I'm fairly certain it's correct. Here's what I get.

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I tampered with the flow coefficient percentage and BB mass but I can't jump pass 200fps. Based on poor mans chrono I should be well in the 450+ fps range. I'm able to shoot through deodorant cans, penetrate one side and severely dent the other. Put a small tuna can on it's side? Clean through both sides. And these shots aren't from point blank, they're from 10 yards away. Help please? Where are my errors?

EDIT: changed chamber data, that was all wrong, I thought that referred to my tank at first. After change I ended up with 130 fps -.-

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Re: GGDT help please?

Unread postAuthor: DYI » Fri Jan 09, 2015 2:47 pm

Unless this gun is a coaxial design, the "inner diameter" field should be set to zero - that's the outer diameter of an inner pipe running through the chamber (and thus the inner diameter of the air volume in the chamber).

Five grams seems a high mass for a "BB" of 0.173" diameter (not actually achievable with any known material, and if it's not a ball, it's not a BB). What did you use to measure the mass of the BB? If you calculated it, your calculation is probably wrong.

You also have a very loose fitting barrel there, but that shouldn't be throwing the performance calculation off by as much as you describe.
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Re: GGDT help please?

Unread postAuthor: vng21092 » Fri Jan 09, 2015 3:19 pm

Oh... I assumed outer and inner diameter referred to the walls of the chamber, but ok, ill fix that. As for the mass, I just went on google and searched "mass of .177 bb" results came back around 5 grains, I thought GGDT wanted grains, woops =\. What does a loose fitting barrel mean? Also, thoughts on flow coefficient, results seem constant at all percentages. Thanks

Now were getting places:
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Re: GGDT help please?

Unread postAuthor: DYI » Fri Jan 09, 2015 4:19 pm

What does a loose fitting barrel mean? Also, thoughts on flow coefficient, results seem constant at all percentages. Thanks


The projectile is loose in the barrel. In this case, about 7.5% of the barrel cross section is unoccupied by the projectile, resulting in significant blowby, thereby reducing performance. Also, it's obviously very bad for accuracy when the projectile is bouncing around off the barrel walls...

The flow coefficient is just a correction factor for the mass flow rate through the valve as a function of the orifice size, gas, temperature, and pressure drop. Insensitivity of the performance to the flow coefficient generally means that the valve is bigger than it "needs" to be be, given the other parameters of the gun, which is obviously the case here, given how the valve orifice is much larger than the barrel diameter.
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