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51mm Anti-Tank

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Ragnarok » Fri Jul 31, 2009 4:15 am

thedeathofall wrote:I didn't even have the correct drag coefficient typed in.

So how could you even expect it was going to give you the right figure?

I don't want to kill anyone on the field if this thing misfires.

On the note of definitions again, no-one should be harmed as the result of a misfire. Misfiring is failure to fire, not unintentional* firing.
*I prefer not to say accidental, because that implies there's nobody to blame.

Firing unintentionally is known as "Being a total plank."

Of course, why it misfires might be a different matter. If it's some part failing explosively, that might well cause injury.

POLAND_SPUD wrote:check the data you've entered into GGDT... AFAIK 0.5oz is rougly 28g.. I doubt that the piston is so heavy as the one on my 'european' 1/2" QEV is less than 5g

A whole ounce is 28 grams. Half an ounce will be ... I'm sure you can work that one out.

most guys model QEVs as generic valves with 5ms opening time and dwell time of 1000...

Not actually true, as far as I know. I certainly don't model them as Generic valves, I model them as barrel sealers, because... well, that's what they are, in all but name.

Also, they open in closer to 1 or 2 ms, not 5 ms.

also 60% for flow coef seems too optimistic as 65% is the max flow you can get (enter something like 45%)

Partly true. 60% is optimistic, and 45% is more typical for a QEV.

65% is however not the maximum you can get. By definition, the flow coefficient is the orifice's flow divided by the maximum possible flow possible for the orifice size.

65% is therefore 65% of the maximum possible. 65% is what happens when hard edges exist in the flow. In other circumstances, efficiencies of approaching, or even theoretically a full 100% are achievable.

also I suspect that a nerf dart has a better cd value than 0.5... (ask btrettel IIRC he has done some research)

It will do. Unfortunately, I don't have it listed anywhere, nor have I the data I would need to calculate it, so I can't tell you what it is.
There are lots of things I could tell you, but Nerf vortexes are not among them.

However, I do need to find out myself, because I have no doubt that when I'm done with the LRC (and yes, I am still working on it, and getting close), people will start asking me.
So, at some point, I'll need to enlist the help of patient people with chronographs to start determining some of the Cd values I don't know.

Also, I should point out before you bother him, Btrettel's research is on nerf darts (homemade ones, I believe), not this form of thing.
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Unread postAuthor: dewey-1 » Fri Jul 31, 2009 10:51 am

[quote="Ragnarok
There are lots of things I could tell you, but Nerf vortexes are not among them.

However, I do need to find out myself, because I have no doubt that when I'm done with the LRC (and yes, I am still working on it, and getting close), people will start asking me.
So, at some point, I'll need to enlist the help of patient people with chronographs to start determining some of the Cd values I don't know.



I could send you some 2D and/or 3D CAD drawings if that helps Ragnarok!
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Unread postAuthor: thedeathofall » Fri Jul 31, 2009 12:18 pm

Okay rag... so I may not have the best choice of words :roll:

Most people don't know how much drag most projectiles have. I am included in this group. I assume that the default drag coefficient in GGDT is that of a spherical object. (please correct me if i am wrong. I really need to know these things) I know for a fact however, that a nerf pocket vortex has much more air drag than a spherical object of the same weight. So i can assume that GGDT would predict much more power and range than it would really have. This way, i can be safe when im designing guns that are meant to be used for milsim.

However, this is just the opposite. GGDT predicted far less power than I got. I am trying to figure out why. Do you ave any ideas?


@Randompkguy,

A bought QEV such as the one i am using, is really simple. There are three parts; the housing, the piston and the seal. The piston is what will most likely break. It is simply a piece of rubber. There are people here that have taken QEVs as far as 700 psi and above. I wouldn't recommend it but do as you wish.
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Unread postAuthor: Ragnarok » Fri Jul 31, 2009 1:24 pm

thedeathofall wrote:Most people don't know how much drag most projectiles have. I am included in this group. I assume that the default drag coefficient in GGDT is that of a spherical object.

Close. Spheres are about 0.46 normally. You can get less, but it entirely depends on surface texture and smoothness - golf balls for example are 0.3

I know for a fact however, that a nerf pocket vortex has much more air drag than a spherical object of the same weight.

What's the source on that?
My first instinct would be that its shape had a lower drag coefficient than a sphere.

dewey-1 wrote:I could send you some 2D and/or 3D CAD drawings if that helps Ragnarok!

Unfortunately, drag coefficients are one of those things that should only really be determined experimentally. While I can find approximations with computer modelling, it's far better to determine these things from actual data.

That means I'll need to find people with both the interest and the equipment to get involved.
Of course, anyone who did help out with would then get (in addition to due credit) the drag coefficient for their projectile of choice.

I do eventually plan for the LRC to have a package dedicated to finding drag coefficients from chronograph data - including from sound rec chronies - but that will still be a lengthy task. It's obviously preferable to do it once, then just put the figures in the package.
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Unread postAuthor: POLAND_SPUD » Sat Aug 01, 2009 2:50 am

@rag
yeah I know that 1oz is 28g..
What's the source on that?
My first instinct would be that its shape had a lower drag coefficient than a sphere.

that was my first guess too...
AFAIK nerf ammo was designed to be safe... to accomplish this it must be able to travel relatively far without the need to achieve high muzzle energies, which would be dangerous at close range


thedeathofall check 'ballistic coefficient' on wikipedia
(...) ballistic coefficient (BC) of a body is a measure of its ability to overcome air resistance in flight. It is inversely proportional to the deceleration—a high number indicates a low deceleration. BC is a function of mass, diameter, and drag coefficient


what we are referring to here is drag coef the rest is calculated by the ballistic calculator in GGDT

so even if nerf ammo is a poor projectile (as it has low BC due to low mass and relatively large diameter) cd value of its shape might be 'better' (lower) than that of a sphere...

so if you enter a higher cd value than it has the projectile will perform even worse... you can check it in GGDT and see how different values change the max range

you can also use a program I mentioned here -> http://www.spudfiles.com/forums/chairgun-t18813.html
it's useless for your needs with this project because the max mass of the projectile you can enter is 5gram but it produces very informative graphs that can help you to understand better extrernal ballistics

this should give you an idea how mass, diameter of ammo and drag coef affects max range, velocity etc. at give ranges
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Unread postAuthor: Ragnarok » Sat Aug 01, 2009 3:57 am

POLAND_SPUD wrote:AFAIK nerf ammo was designed to be safe... to accomplish this it must be able to travel relatively far without the need to achieve high muzzle energies, which would be dangerous at close range.

Well, not so much muzzle energies, because they weren't designed with being fired in mind.
But they are designed to fly a long way when thrown, and given the natural limits on how fast something can be thrown, they do have to try and keep a lot of that velocity to go any real distance.

The fact is, to be safe, they have to be reasonably lightweight, and to then have a decent ballistic coefficient (to allow them to fly a long distance), they need a low drag coefficient.
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