26 replies to this topic

[buckbogey]

Ok, I saw some of the post about trouble with barrels. I've done some test and here is what I found. For the best possible combination of power and aim, the tank should be 5 time the size of the barrel. I know that that sounds like a lot, but it really isnt. Also you don,t need those huge barrels that I've been seeing, once they pass being 1/5 of the size of the tank, your power and control die. Please tell me if you dont think this is true. I tested with a couple of my homemades.

[cxwq]

Interesting results. You have to admit though that this is somewhat of an oversimplification since it depends a lot on what kind of darts you're shooting and what scale you're working with. Basically, dart/barrel friction becomes a limiting factor for the effectiveness of lengthening your barrel as velocity increases. Additionally, higher tank to barrel ratios introduce other problems like fishtailing that exacerbate the flight problems of imperfect darts.

[THIRST]

Although that may work well, we have no way of knowing the exact ratio of barrel to tank that works best. Besides, trial and error teaches the best.

Edited by THIRST, 16 January 2004 - 05:05 PM.

[buckbogey]

I understand your point, but i still defend my point. I'm an engineer, and this is just what i was tought. Also, with the fishtailing, the front of the dart should be heavier than the back. Even though air follows the dart out of the barrel, they should bot be moving at the same speed, so as long as your barrel isn't too long, fishtailing shouldn't be a problem

[cxwq]

Actually, given that your darts are properly weighted, fishtailing is more of a problem when your barrel is too short. The air and the dart are moving at the same speed until the moment the dart clears the barrel. At that point, if the air pressure is still too high, the dense air pocket will overtake the dart and cause the back of the dart to `wobble' during early flight and increase aerodynamic drag.

Your barrel volume (and therefore length) should be dependant on your tank size, pressure, dart length, and dart weight distribution. You want a balance where the dart never becomes friction limited in the barrel and the air pressure is reduced to a point that it doesn't interfere with dart flight. To some extent the latter problem can be reduced by porting the barrel.

This is a subject that I'm very much interested in and, having modded over a dozen guns, I have a significant empirical data set. Without arguing your point excessively I just want to note that 5:1 is not always the answer.

Oh yeah, welcome to NerfHaven!

[One Man Clan]

Let this be a message to all newbs. This is the kind of post that will NOT get you flamed and hated on these boards. This is how you show the community that you are serious about being a part of the boards. Like CXWQ said, Welcome to NH

[Ranger]

I dont think this guy should be called a newb just becuase he just started.

[IronRhino]

I dont think this guy should be called a newb just becuase he just started.

That's the definition of a newb.

I think that's oversimplifing it, but not horribly, like saying that Pi=3.1 . It's not exact, but it'll work.

[buckbogey]

Thanks Ranger. And for everyone else, I'm not a newb, they just screwed up my old account.

[FiveseveN]

5 to 1 ?! Man... Have you seen the episode in Scrapheap Challenge where they built huge air cannons with about 1.5 to 1 chamber to barrel volume ratio (the barrels were like one foot in diameter). And I've heard professionals talking about 1 to 1 ratio.
The thing is you can't just settle on a ratio. There are just too many variables, like you would wanna build up a lot of volume if you use an automatic gun (so you won't have to reload often) but if you have a single shot you might wanna use a small air chamber with lots of pressure (that way you don't have to work a sweat pumping).

PS: Don't anyone even think about calling me a newb, 'cause I've been in this field for like 4 or 5 years and I bet some of the guyz here don't even know Pascal's law.

[Black Wrath]

Pascal's Law huh?

Pascal's Law

In a static fluid, where there is no motion, the stress vector cannot be different for different orientations of the surface normal because there is no preferred direction in the fluid, which is isotropic in structure. At a point P in a static fluid, must therefore have the direction of and have the same magnitude for all directions of . This startling result is called Pascal's Law.

To prove that the magnitude of is independent of the direction of the surface normal , consider a force balance on the fluid element as pictured in figure 2.3.1. The stresses acting perpendicular to two faces of area and are designated by and , respectively. Since the fluid element is motionless, the sum of the forces in the x direction must add to zero:



As the fluid element shrinks to zero, the normal stresses and are colocated at the point P. However, the magnitude of must be the same for any direction since we could have chosen z or y for the direction of x in figure 2.3.1. Because we know that fluids can sustain only a compressive stress, or pressure, we identify the magnitude of with the pressure p and its direction as opposite to :

And if that isn't enough for you, Read this then.

[Spectre2689]

Wow...do you have ANY idea of what in the fuck you just said? I read it about 3 times, figures and all, and I still don't get it... And on another note, what does Pascal's law have to do with chamber:barrel ratios?

[EDIT] After another few readings, I'm SORT OF starting to understand it. But it still has nothing to do with Nerf that I can see. For one, it's talking about static fluids, not gases.

Edited by Spectre2689, 18 January 2004 - 04:33 PM.

[buckbogey]

hey, air or CO2, or most gases are fluids. Fluid means that a substance is severly plastic, or even simpler, anything that can flow between your fingers is a fluid. Liquid and fluid are not the same thing, so dont think that way.

[Black Wrath]

hey, air or CO2, or most gases are fluids. Fluid means that a substance is severly plastic, or even simpler, anything that can flow between your fingers is a fluid. Liquid and fluid are not the same thing, so dont think that way.

Did he ever hit that on the nose.

Spectre, of course I know what I'm saying. The man thought he was all big saying he thought no one but him knew Pascal's law, and I'm just showing him that he is wrong in thinking that. If you still don't understand, read the whole site that I have provided.

Yeah, so there's Pascal's Law, which can be applied for gases and liquids.

[cxwq]

Lets see, we have penis waving, penis waving, and... yup, more penis waving.

Having suffered through a year of physics, two years of calculus, and various and sundry other exotic forms of torture to get my CS degree, I'll just avoid that link.

Can we have a reasonable discussion here without getting into who's a newb, who's an engineer, and who's watched the discovery channel? Your credentials here are the sum of your contributions to the online Nerf community.

[Zero Talent]

I was working on a little equation for optimum barrel length a while back [before I just stopped caring ]. These are the variables, with notably large simplification, that I took into account:

Normal Tank Volume (measured)
Normal Tank Pressure (measured)
Barrel Length (just maybe?!)
Barrel/dart pressure (empirically tested)
Barrel area (measured)

Regrettably, anything exact in the way of temperature changes (I was using the Ideal Gas law, of all things!), localized pressure increases directly ahead of the projected dart, or fluid flow friction within the pipe, were kind of botched. In most cases, ignored altogether...

So any of you kids able to enlighten me on any equations or relationships for these, perhaps upgrade my estimation of the decreasing force exerted on the dart as volume increases and local pressure decreases? Again, I can't remember any way to factor in temperature between the two steps, so I've just lamely been assuming an isothermal process, so any help there would be appreciated.

For anyone that wants the quick summary:

If we know the above mentioned variables, we can estimate where the force exerted by the expanding gas equals that of opposing friction, in terms of a distance variable "l" that we can use with the area of the barrel for the final volume into which the air effuses. If this model provides an accurate model of the whole firing process (minus some obvious other important details, such as the above mentioned, as well as the behaviour of the dart as it leaves the barrel, and the air effuses into an essentially infinite volume at an essentially constant pressure), we could perhaps rate dart types and barrel/dart combinations by friction coefficients, and Nerf blasters by energy levels determined by the accepted optimal pressure multiplied by the total measured tank volume. Joule notation sounds like a fun determination, though that would still be ruled by practical factors such as prime time (or number of pumps to optimum pressure), reliability, and other unique properties.

So, now that we all know Pascal's law, for whatever use that is in a dynamic situation, and are all a little more enlightened; Is anyone interested in actually investigating this, in all it's empirical tedium? If it's really important to anyone, I'll make time to do it, but otherwise I couldn't care less, especially when the results seem like they would prefer grossly overextended barrels for the sake of only a couple meters' effective range.

Edited by Zero Talent, 18 January 2004 - 07:30 PM.

[buckbogey]

Whats up with all of the waving. Forget all, Forgive all.

[Langley]

After having nerfed in near zero temperature, as well as temperatures in the mid-nineteys, I think figureing in environmental temperatures could be important. Has anyone done anything relating to optimum barrel length for spring guns?

[buckbogey]

I like the idea of the air temp. Ill try that out. Sry, but i dont own a spring gun, so I cant help you there.

[FiveseveN]

I do believe I know my own words and I believe I said "I bet some of the guyz here don't even know Pascal's law. "

And it's an elementary law, one that all of us take for granted, one without which nerfing, hidraulics, and God knows what else wouldn't exist.

But 'nuff of that. I just don't see what the big deal is. I mean... the pressure delta 'tween zero and 90 degrees F can be overlooked, it can be compensated by one or two more pumps. It's not like we're building Feinwerkbau grade airguns... it's freakin' foam...
Don't mind me, I guess I'm just too Eastern-European, doin' it the Russian way: build first and calculate later. ( And I wonder why there are more AK47s in the world than M4s ).

PS: I'm not a big fan of Automatik Kalashnikov.

[buckbogey]

Here is what i found:

t=tank volume
b=barrel volume
a=airloss ratio
o=output pressure

The equation si true if normal pressure=1

(t*B)a-:-a*b=o

or even simpler:

t-(a+B)=o

to find optimum barrel lenght ajust the tank and barrel volumes until o=1.5. The first equation is more accurate, but the second one is better for barrel lenght. I will post the equation i got this from if anyone would like, but it's a eye crosser.

sry, the smillies are b's

Edited by buckbogey, 24 January 2004 - 03:48 PM.

[xedice]

Edit your post and de-select "Enable emoticons" below the big white text box if you don't want those insidious smilies interfering with your math.

[Cadmond]

For anyone who cares
http://www28.brinkst...re-equation.png
Not really exact, but it's useful if you're just playing around.

[xedice]

Attach a lighter/heater to under your airtank, with a switch by the trigger to turn it on. When you're about to make a long range shot, and your pump can't handle any more pressure, switch on the lighter/heater for a few seconds. Instant range increase , like the NOS of nerf, at least in theory...

[buckbogey]

Tried it. Long time ago. I dont think it worked. I'm going to try it again, but what worries me is that to get enough heat to make a noiceable distance improvement, it might start to melt the tank.