9 replies to this topic

[Squishy]

So I have had a couple people ask me about how we can apply some basic principles of fluid dynamics to nerfing. I have mostly copied and pasted a PM that I thought might help anyone looking to mod or make homemades.

Firstly, avoid any abrupt changes in diameter (any sharp edges at all). A reasonable slope results in much less pressure loss.(at least 3-1 ratio of run to diameter increase) Just take sharp edges and make them smooth, and 10 diameters in length eliminates nearly all turbulence (and thus increases efficiency) of flow. Nerf is an inexact science. But for fluid dynamic applications, I say this:

No abrupt changes in diameter if possible, obstructions are not actually horrible if they obstruct less than 20%, and grids are better than a 20% solid blockage.

Make sure your barrel length volume (volume after the plunger to the end of the barrel) is at about half the volume of the plunger. Longer means less pressure, shorter means expanding gasses are wasted. Exception for Air powered, as they have a much faster expansion coefficient and thus don't need as long as a barrel.

Holes are bad, but we knew this already.

Any change in diameter is desired in this order when possible:
1. Very smooth change in diameter, no edges.
2. sloped change in diameter(change in diamter starts small, peaks near the middle, then goes back to small)
3. Smoothed edge, but abrupt change in diameter.
4. Abrupt change in diameter with sharp edges, like the straw mod. would have said something but it would have been a necro.
5. abrupt change in diameter with a lip, like so(flow from left to right):
______
.....__|___
.....______
_____|

Keep in mind these are general rules and sometimes things like the straw mod increase ranges depending on the properties of the dart and the delivery method of air pressure.

If you have anyone questions I will to the best of my ability answer them.

[CaptainSlug]

Turbulence isn't much of a concern in this application. Restriction and "dead space" on the other hand are.

It's taken several months of experimentation but none of the air passages inbetween the supply of air and the barrel should have an area below that of a 1/4" diameter hole. If the passages are any smaller than that a MUCH higher pressure has to be used to achieve a flow rate capable of firing a dart.

The removal of as much "dead space" inbetween the supply of air and the back of the dart being fired is also beneficial in both pneumatic and spring-plunger guns because the less volume the source is trying to pressurize initially will result in the pressure level climbing at a much faster rate. And that leads to a short acceleration cycle for the dart.

[Squishy]

Turbulence isn't much of a concern in this application. Restriction and "dead space" on the other hand are.

It's taken several months of experimentation but none of the air passages inbetween the supply of air and the barrel should have an area below that of a 1/4" diameter hole. If the passages are any smaller than that a MUCH higher pressure has to be used to achieve a flow rate capable of firing a dart.

The removal of as much "dead space" inbetween the supply of air and the back of the dart being fired is also beneficial in both pneumatic and spring-plunger guns because the less volume the source is trying to pressurize initially will result in the pressure level climbing at a much faster rate. And that leads to a short acceleration cycle for the dart.

Turbulence can be a concern, but not so much for distance. A more laminar flow means a more accurate dart. Also you are completely correct, I should have mentioned dead air spaces as they perform a dampening effect on the air pressure on the way to the dart. Smaller passages are not only more prone to breaking under higher pressure, a higher pressure is required to get the final velocity of the air in your tube by your dart (approx .5 inches) to a high velocity. This is because of two things: smaller diameters mean more shear stress forces due to friction, and thus more resistance to the air flow, and the expansion of the diameter at the end, no matter which diameter change pattern is adapted above. For more information see Bernoulli's Principle of Fluid Dynamics. Except for pressure losses due to friction ( the shear stresses) and losses created by turbulence (changes in diameter).

We can see through Bernoulli equations that what Captain Slug says here is very true, you know your stuff. In addition, we can easily find through an application of Bernoulli coupled with a change in pressure over time equation derived from the spring constant and diameter of the plunger, we can find that a larger diameter plunger is often better than a smaller diameter plunger with a longer chamber. This creates another increase in what Slug has designated 'a shorter acceleration cycle.' (because more volume of air is moved quicker.) This is part of the reason why guns like the Expand-a-Blast, though having a plunger with about the length change of a maverick, can get such nice ranges. Please excuse the rant, I just love the science behind the guns.

[shadowkid33]

we can find that a larger diameter plunger is often better than a smaller diameter plunger with a longer chamber. This creates another increase in what Slug has designated 'a shorter acceleration cycle.' (because more volume of air is moved quicker.) This is part of the reason why guns like the Expand-a-Blast, though having a plunger with about the length change of a maverick, can get such nice ranges. Please excuse the rant, I just love the science behind the guns.

But then why does a Crossbow shoot so much farther than an EaB? Is the total volumes of the plunger of the Crossbow so much more than an Eab that that principle doesn't apply to that situation?

Also, the basis of spring guns is more air moved slowly to build pressure behind the dart, so doesn't a gun which has a short, fat chamber(EaB) have a clear disasvantage over a gun with a long plunger(BBB, Crossbow)?

[CaptainSlug]

But then why does a Crossbow shoot so much farther than an EaB?

Because the EaB has a shorter plunger travel and a weaker spring. The plunger doesn't reach as high of a speed and doesn't move far enough to get the kind of performance a crossbow would out of the same volume.

[xNFx 37]

But the plunger is the size of someones fist.

I just got an idea, which I got from the Rebel Trooper Blaster mod.

THe Part in the eab where the barrel used to be and where you put the new barrel. If and only IF, the plunger tube of an NF and EAB barrel are the same, you could extend the NF plunger through where the EaB plunger is.

Basically you'd make one long-ass plunger. If anyone wants to try this, feel free, but give credit where credit is due

[keef]

The plungers ID isn't. A NF's plunger tube is pretty much the size of 2 of my fingers. A EAB is pretty big. By the way, the credit would go to bpso, then you, then whoever. Wait no- just bpso.

[xNFx 37]

I didn't say the credit would go to me, I said it would go to where it is due. But correct, I wouldn't have thought of that If i hadn't read his mod earlier today.

I wasn't talking about the plunger tube, but the tube that the barrel sits in. But I doubt they're not the same size anyways so whatever.

[keef]

Yeah, I made a BNA's plunger with PVC (replaced) because the plunger tube got smaller and let out a lot of air, so I just expanded it with PVC.

[CaptainSlug]

I have made a "long-ass plunger" with a fairly large diameter plunger head and a very strong spring and the results are phenomenal. But the EaB only has one of the three requirements for a really powerful spring-powered blaster. Trying to change the dimensions of a plunger chamber in a gun is about as much work as just making a whole gun from scratch.

Edited by CaptainSlug, 01 January 2008 - 04:48 PM.