Airgun calculations: Pressure
Hi all,
I want to build my own airgun but am running in a problem: How can I calculate the pressure needed to propel the pellet (size and weight known) down the barrel (dimensions known) to result at a defined muzzle velocity (V0)?
I searched on Google an Wikipedia, but can’t seem to find the information.
Thank you for your help!
Christian
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New avatar…just for you. 😆
Better to be a smart ass than a dumb one. 😉
Al
“Whoa look at the big brain on Brad”….sorry Pulp Fiction just popped into my head 😯 … I say + 10 on the variables for best use of 😀 faces.. LOL that was M.I.T. ninja stuff there man 🙂
Alrighty then…I can some figuring and calculating now, draw prints,…
I was actually thinking of buying an airgun ad take it appart..I might just do that.
Of course you can come along. We can probably help each other out on certain questions and issues.
That will be a hobby project for me though. It can take a while until I am actually starting to build it. It’s going to take me some time to finish the calculations and the prints.
Do you know to carve rifles into the barrel? The Barrel wil be one of the last things to finish, but I always like to know how to accomplish something before I start. I will do some research on clips and magazines to and a mechanism to automatically recock and reload the gun after every shot.
Christian
Not piston…striker. I was making an assumption that you were wanting to build pcp/co2 powered.
I believe that piston mass will have an effect on both the performance of your springer, and the severity of the shot cycle….but I’ll admit right now…I don’t know jack about springers….and I’ve a lot to learn about pcp/co2 as well. 🙂 Built a couple…still learning to tune them…but having fun with the process.
As to a guess at how much higher the starting pressure should be, with a springer, you have compression heating going on during the shot cycle. I’m thinking that with a higher initial temperature, the cooling and associated pressure drop will be less. I suppose 10% greater sounds reasonable…if it shoots too hard, you can always shave some weight from the piston or shorten the spring a bit.
Odds of it shooting hard “right off the bat” are slim IMO…I believe there’s going to be a bit of a learning curve here.
If you’ve no objection, I’d like very much to stumble along and learn while you’re doing this.
Might be a productive thing to tear down a springer, and measure the crap out of it….use it as a starting point. Research into various mods for tuning springer performance may also yield some knowledge.
Al
Sweet!
Thank you so much. The formula to calculate the spring I knew already. I though you said it had to do with the piston weight (1st answer) or did I get that wrong?
One more thing: How much higher would you estimate the pressure? I though 10-15% should cut it…
I want to build a spring piston airgun, because I can adjust the pressure to my pleasing (considering the strenght of the material of course). With a CO2 airgun I would always depend on the tanks that are sold.
Next time I turn my computer on I will search for the law. It’s just a pain on my phone.
I’ll keep you up to date!
Christian
Well hold the bloody phone! 😯
Are you wanting to build a spring piston airgun? That’s somewhat unusual…most want to build pcp or co2 powered.
New territory for me, but we’ll take a stab at it. :12:
Might want to do some googling on “Hookes law”. 🙂
F = -kx
F = Force
k = spring constant (spring rate)
x = compression distance
Use the required pressure (1286psi) in conjunction with the area of the piston bore to calculate the required force on the piston. Use this force and the piston travel to determine the required spring rate.
Should get you in the ball park, but understand it WILL likely need to be tweaked. Do some figuring and get back.
Al
Hi Al,
Thank you for your help!
From the book I mentioned earlier I know how to achieve that compression and how to calculate the energy needed to compress the piston.
But how do I calculate the strength of the spring?
Christian
All righty then! 🙂
Calculating the pressure required to achieve a given velocity with some known variables:
MV = 900fps
Pellet = .218″ dia with a mass of 15.9gr
Barrel length =23″
Some derivations/conversions from the given information:
Bore area =.0373 in2
Pellet mass = .00227 lbs
Barrel length = 1.917 ft
Average velocity will be 1/2 of muzzle velocity…450fps
Time to travel the length of the barrel will be 1.917ft/450fps… .00426 seconds.
Acceleration = change in velocity divided by time ….. 900fps/.00426 seconds ….. 21126.761 fps2
Force = mass * acceleration …. .00227 lbs * 21126.761 fps2 ….47.96 lbs of force.
Pressure = lbs force divided by area upon which it acts ….. 47.96 lbs / .0373 in2 ….1285.791 psi ….lets just call it 1286 psi eh. 😉
So IF we could maintain 1286 psi in the bore behind the pellet, we could theoretically achieve 900fps with a 15.9gr pellet in .22cal. We’re ignoring bore friction, but this is a “rough idea” only..so I’m not gonna sweat that.
So HOW to maintain 1286psi while the valve closes and the pellet is moving….volume is increasing…gases are cooling…pressure is dropping. 😕
💡 Start with a higher pressure! Enter those variables… 😆
Have some work to do this am….if you’ve another question, I’m sure someone around here can help ya….I’ll check back later.
Al
Hi Gippeto,
Thank you for your answer. I will try the program later – I am on my cell right now. The screen shot looks promising though!
I know it’s not an easy question. I spent about three weeks on searching google, forums and so on finding about nothing useful…
What I did find is the scan of a book called ‘The Airgun From Trigger To Muzzle’ with quite a bit of information and a few formulas.
I do not want to build many prototypes ending up trashing them. I have a CAD program to draw the prints and build a 3D model of the gun which can also ‘test’ the draft for weaknesses.
If you have more information or some other links, please don’t hesitate to let me know.
Christian
It’s not that simple Christian….how I wish it were. 😆
You could use a few physics formula to derive a theoretical velocity, but it won’t hold up in the “real world”. Valve flow, spring strengths, striker mass and more all play a role.
A better description of what you want may be in order.
Or you could install ggdt and play around with variables.
http://www.thehalls-in-bfe.com/GGDT/
Help with ggdt can be obtained by posting a question on Spudfiles. Dave Hall (who wrote ggdt) hangs out there.
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😎 NICE i like it.. always room for naked women, she can leave the sword though 😯 i already got one… 🙂 ..and yes Al i agree much better to be a smartass than a dumbass…but we all know being an ass is just more fun… 🙂 be cool Al i really enjoyed the post
and the dueling smilies…funny shit man, got to find a website with some downloadable smiles…c ya, Terry