A Question for the Pros - AIRSOFT DYNAMICS
 

Alright, here's my little deliema. I've been designing a custom marker, and I need to know something about the cylinder of a bolt action sniper rifle. I've been searching the Internet for the answer, with no prevail. I was hoping the professionals here might know.

How much of the cylinder's air is compressed when the piston slams forward? Or, how many centimeters of the cylinder is used to compress air?

Through various tests I have done with simple air compression, I have found that too little air can't push the BB to it's optimal velocity, and too much air slows down the spring. If the spring is a factor, it's a SystemA 400% PSG-1 spring.

Any help would be really appreciated.

//Tendril

Ask MadMax. He does all the calculations for volume vs. velocity. There is a previous thread in the "general" area I think on a similar subject about 3 months ago.

If you're designing it yourself, your best bet is probably to just test it empirically...

Alternatively, you can always try to fit existing parts into your design... Presumably they've already done the research you're proposing.

I thing the article (well, thread really) that you're talking about (mcguyver) had to do with porting the piston heads, noting the issue where too much compression in the cylinder too fast creates that buffer of air that slows the spring down...

I suppose you could always start with a solid face piston, and port it one hole at a time until you came out with a sweet-spot result. You'd probably wind up going through a few testers first...

Also, I'm pretty sure a 400% spring is going to force the air down the barrel pretty damn fast, and any slowdown is going to be negligible. It's probably not the optimum situation though. You might even find that the compressed air leaks backwards past the piston head...

Then again, take apart a classic spring-powered air rifle and look at the piston in those. Those are heavy springs, and I'm not sure about how much of the air gets compressed, and how much doesn't actually do any work on the pellet... Those guns depend on the extra air pressure to buffer the piston slamming into the front of the cylinder. I get the impression they're over-designed in terms of necessary compression to fire the pellet, just for that reason (but I have no clue, really)

Anyways, you'll probably only find out for sure with testing, if you're designing from scratch. What kind of tools do you have access to?

Thanks for the response, both of you. I was checking over the article you mentioned, and I see that they were talking about an automatic fire, and barrel to volume ratios. I see that that does not apply to me, because this will be bolt action.

I took up this project, as to find the most economic, and reliable way to make a very powerful rifle. I'm not going to go into the details, but I can say that I am creating the cylinder, piston, spring-guide, and other things from high quality hydrolic pipe. I'm quite adventurous, and the only things I plan on purchasing will be the barrel, and the spring.

It's a very experimental thing, which was the point initially. And, I love the challenge. Here's what I have so far:

The spring reaches maximum velocity on equilibrium. Meaning, it's neutral point, but I have to compensate for cylinder friction. So, I want to have the actual cylinder long enough to match. And, I know, that displacement from equilibrium affects velocity, too [Ep=1/2kx^2]. So, the optimal piston starts with the spring fully compressed, and hits the end of the cylinder at top speed.

Now, the conflict I have, is where in the piston's journey does it start to compress air? What I'm wondering is, what is standard? If nobody knows the answer, does anyone have any idea how to calculate it?

As for tools, I am pretty limited. For testing, I would need a means to meause velocity, which I don't have. So, tinker-testing is sort of a last resort.

//Tendril

The gist of my posts that mcguyver is referring to is that rapid fire does not affect the relationship between cylinder cross sectional area, piston travel and spring force profile so the calculations I use are applicable to your case (if my assumptions are valid).

Unfortunately if you don't want to get into differential equations you're going to have to a tough time optimizing your bolt action rifle. The difficulty is that the situation is mathematically pretty complicated. More complicated than just plugging gr. 12 physics energy equations. My previous analysis was simplified because I was plugging actual numbers from diameter, force, and velocity measurements. In a sense, reality did all the calculation and all I did was close the theoretical loop by plugging all the numbers into simple equations.

Figuring out the results in the other direction is much more difficult. At any given point in the piston travel, you can calculate the instantaneous spring force which gives you an idea of how much energy has been transferred to the piston, spring (it's centre of gravity moves about half the speed of the piston at any point), and pellet.

However, you are faced with not knowing the breech pressure at that point. It is definitely higher than atmospheric. You can't assume zero compression and therefore proportional speeds of piston and pellet based on cross sectional areas.

Optimizing your system to maximize spring and piston velocity is not optimal for energy transfer to a pellet. The faster the heavy things move (piston/spring) the more energy they absorb in accellerating to speed.

It's really not worth going the theoretical route. It's interesting to see if reality can be codified, but designing products soley from a theoretical perspective is pretty prone to forgetting about important things which are subtle, but actually manifest themselves in important ways in materials.

In product design I do use mathematical physical relations to arrive to some understanding, but it's very important for me to get physical fast and make proof of concepts in materials. Usually my first model shows me how I am so wrong I am on paper. Product design is more about developing not "know how" but "know how NOT" which is a bitch.

Thank you for the reply, MadMax. I really enjoy reading over some of your older concepts, and only hope I can do the same, as I am still learning. This kinda stuff is what gets me interested in the game. Maybe you can relate with me.

I want to support my ideas and predictions with facts, and values. However, lacking supplies is the biggest issue. I can't really measure anything, so this is a project of estimation, and reason.

I caught an idea while I was reading the post. I'm still a little novice to the workings of the airsoft gun, so correct me, if I'm wrong. You say that you can't know the breech pressure, at any given point. But, what if we did this:

http://img.photobucket.com/albums/v448/jdupl/idea-1.jpg

You see, if you put air holes, allowing the air to escape in the cylinder, the acceleration, and velocity would remain precise, because we would know that the pressure would be atmospheric. So, it would be high school physics, until the piston head passed the holes. In which case, the air would be cut off, and compression would occur. From that second, we could find the velocity, and take it from there.

Couldn't we? Please fix any holes in my quite disorganized reasoning.

//Tendril

Just an FYI, your idea is actually exactly how AEGs with shorter barrels work. Barrels shorter than 500mm usually use cylinders with holes, essentially exactly what you propose. To get optimal performance, one must match the volume of the barrel to the volume of the cylinder, past the aforementioned holes. Check this out: http://www.21stcenturyairsoft.com/cylinderguide.php

Someone who knows more will probably have to confirm/deny this, but I don't think acceleration of the piston is something that you should be concerned about taking a long time... It's my impression that what you're getting at with the holes in the cylinder is that it takes that much time for the piston to accelerate to terminal velocity. Even if it's pushing a BB, a piston starting from rest will accelerate extremely quickly.

Tendril:

I think you really have to re read that post more thoroughly. We're running over a lot of territory covered in that old topic.

http://www.airsoftcanada.com/showthread.php?t=20534

AEG cylinders for rifles with barrels under 500ish mm are ported for exactly the reasons you're considering as Harley points out.

I'm trying to figure out how some kind of custom jerry rigged manometer type setup might help him gauge relative pressures...

That, combined with some denser liquid, might be able to at least give rough estimates of output pressure...

I just forsee the quick acceleration of the air forcing jets out of the tubes...


Any ideas?

Also, Madmax, that not "know how"... "know how NOT", is one of the funnier (and more reassuring) things I've heard in a while. School's getting a bit tougher... :banghead:

If you are considering manufacturing an entire airsoft gun, I hope you have checked the rather severe legal aspects of that project...
400% pushes an airsoft BB well above sane safety levels.

If it comes above the 500fps/whatever Joules, you are manufacturing a firearm without a legal permit to do so. That means serious jail time.

Wait until you leave engineering school thinking you can actually design a car.

Manometers aren't really quick acting enough. They're good for measuring static pressures, but if you have to rapidly accelerate a slug of liquid to reach an equibrium height, the density of the liquid gets in the way of your measurement. You also have flow resistance issues in the tubes.

I'm not sure how industry measures very high and rapidly changing impulse pressure. I would guess that it's easier to use a strain gauge or piezo transducer in the cylinder head to measure how hard it's being pushed out of the cylinder before the piston slams into it to determine pressure. I would guess that the force exerted by air pressure is pretty small compared to the impact force when the piston hits. It may be difficult to prevent a gauge sensitive enough to measure the air pressure from being damaged when the cylinder head is struck.

Still, all this discussion is just jibba jabba until you get into a proof of concept or start making direct measurements on your AEG. All the king's paper and all the king's pens couldn't get their theoretical masturbations to do anything useful again.

Thanks for your responses. I re-read the thread you linked me, and it's all coming together. I should have read it thoroughly, the first time. I feel so stupid.

And, now I see that my idea isn't revolutionary, nor is it even unheard of. And that's good, because I know it has been tested, and it works.

Basically, the norm is to match the volume of the barrel, to the volume of air being compressed in the cylinder, it'll be alright? That way, the piston head will most likely hit, right as the BB is leaving the barrel, thus maximizing the velocity?

If that's all I need to do, then consider me a happy man. Thanks for your time, everyone. My question has been answered.

Agreed, but I want to test with the absolute most power the weapon can handle. As to test the parts, and endurance of the design. I'll bump it down to legal, when I play. 250% - 300% of the same model; however, remember that this is a sniper rifle, so the fps is staying at about 500.

//Tendril

Kinda figured it wasn't sensible (the manometer). Didn't hurt to ask though!
Do people really come out of engineering school feeling so 'empowered' ?


Anyways, Good luck Tendril. Um... Keep us posted?

You idea wasn't dumb. It just wasn't experienced.

You'd have hooked up your apparatus and realized that something wasn't working right. If you carefully thought it through after seeing the practical issues, you might have come to similar conclusions. I just have the luxury of taken a lot of wild stabs at mechanical problems so I can speak from a high ground of having learned from more mistakes than most.

Some principles can be easily taught and demonstrated in class. All the wierd stuff and complicated effects are actually mostly class room principles which are compounded in unexpected ways. Because they're unexpected, you don't bother to calculate for them and they kick you in the face when you make a practical model.

For instance, in Gr. 12 physics, you can calculation the acceleration of an object down a ramp. If you're good with HS calculus you can also determine the position the object would be at any given time.

In real life, if you rolled a solid sphere, a disc, and a tube down the ramp, you'd realize that they all accelerated differently despite your simpler calculations. In HS, you didn't realize that energy is also stored in rotation so the object with more mass concentrated at it's centre accelerated linearly faster because less potential energy was converted to rotational kinetic energy spinning it. The calculation to determine rotational energy is not difficult. It's actually similar to the 1/2mv^2 for linear energy. You just substitute m for a rotational inertia number (defined by the objects mass distribution) and v is substituted for the rotation rate in radians/sec.

With the circumfrence of the object, you can relate linear and rotational velocity to get rid of one of the variables and replug your equations back in HS math again.

Sounds complicated, but it's just an extension of the basic principles.

Are you planning on modelling this project off any real steel sniper rifles? Or is it going to look like a paintball gun?

It'll be a custom design. Realism is an option, but coolness is a fact. I borrow elements from all my favorite "real steel" models, and mix them together. It won't be a paintball marker, that's for sure.

//Tendril

I dont think you got it... just building it at all is against a whole LOT of nasty laws. If you dont have a manufacturing permit, it's Illegal. But, that's your life, right?

Hmm. I wasn't aware of that. Do you have any articles I could read, on the subject?

//Tendril

No he doesn't.

Your not making a real gun.

You're making something that falls under the Replica laws, or the firearm laws at that power level.
Read up on what it takes to make those legally. That will also tell you the consequences if you decide to do it illegally.
Now if you have at least your gunsmithing license and machining business running, disregard what I said.
I said what I had to say, feel free to ignore it.

Well, since the gun is of custom design, Replica Laws need not apply to me. No name brands, models, or resemblance to a "real steel" weapon.

And, even if I decided to make the weapon with the 600 fps spring I originally intended, I wouldn't be selling it, or giving it away. If I registered it, and kept it to myself, according to those laws, it would be legal ... but I'll just use a 300% spring and keep the fps at 500. To stay on the safe side, and avoid the conflict.

Plus, I emailed the Canadian Firearms Center, with request to the laws you speak of. And, I asked for their professional opinion on the legality on the issue.

I wouldn't ignore your advice. Afterall, getting shot down by you is just another way I can keep learning things about this sport.

//Tendril

The question is, do you really think a person who could make their own rifle would be stupid enough to get caught firing it before bringing it down? I realize that doing something illegally without getting caught is still illegal, but as he said, he plans to lower the FPS right after he finds out if he can hit that mark. It's the challenge, and once you set a goal like that you can lose sleep over not meeting it.

*sigh*

There are 2 separate problems; it's speed, and what it will be seen as (legally or by interpretation).

If it has the power of a firearm, and you manufacture even ONE, you risk jail.
If it even looks like a firearm AND has the power of one on top of it all, it's illegal; you just built a weapon without a permit. It's not because it can be toned down that's the problem, but it's based on what it CAN do by your design.

The CFC has no say in this, we ALL know how bad they are at interpreting even the simplest concepts.

So let me tell you a scenario that will happen: you will probably build this anyway with little research or after the first 'ok' you hear from a bureaucrat.
It will, by your specs, quite probably fall into the firearm level of power. That just by itself is illegal.
When you test it, bad luck is sure to put someone nearby who will see it and freak because they dont know crap.
Police WILL be called (happened before to others, for much stranger things like a replica musket used at a reenactment that was called in as a 'shotgun' to the local 911 service).
Police will arrive with a whole lot of other police and you will have to convince them that THEIR "interpretation" of the laws is wrong no matter what the CFC told you.

Have fun. This happened before.

After being a Chief Range Officer for a bloody long time, I'm killing myself giving you free advice about things I sure as hell would not do myself.

Now go live your dream. I hope it does not bite you in the ass. Criminal records have a habit of sticking and ruining your life.

Want a fancy airsoft? It will cost you far less, and be legal, to get something rare instead.