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Improving darts - some thoughts

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Improving darts - some thoughts

Unread postAuthor: Ragnarok » Sat Mar 08, 2008 1:45 pm

Anywho, I’ve seen enough of coding for the moment, so I’m taking a break from that for a while.

So instead, I’ve been thinking about darts, and all sorts of different things associated with them. I’m still hoping to have a go at some long range shooting, and for that, I’ve been working on a specialised dart design. The exact design isn’t completely done yet, but I’m intending to use it for “long range armour piercing” - I’ll let you figure out what two old ideas my brain fused together the other day.

Anyway, I thought I’d share some of my thoughts (and hopefully get some interesting ones out of you at the same time...).
I’m not going to spend much time talking about drag stabilisation. We all know what that means – put the centre of pressure behind the centre of drag, and the dart will remain more stable.

Now, I was thinking about what drag coefficient actually means – why does streamlining reduce drag?
The natural explanation is that it’s easier for the air to move around the dart – it flows more easily. There is a certain truth to this, but thinking more scientifically is going to help.

What happens when anything sits in a fluid (in this case, the air), is particles in the air will hit it.
If it moves in that fluid, the “front” will hit more particles, and the rear will get hit less – much like if you stand in a corridor of people all moving in one direction. Move against that flow, you’ll bump into more of them, move with it, you get hit less.

Now, when they get hit, these particles exert a force on the object – (i.e. you bump into someone, you slow down). Because the force increases proportionally to the number of particles you hit, and you hit more particles with a larger projectile, you create a pressure on the front face, and lose pressure on the rear face.

Force from pressure always acts perpendicular to a surface. This means, that if the surface is angled relative to the flow, the force acts directly into this surface. With an pointed nose, you therefore direct a smaller component of the force against the projectiles motion – therefore have lower deceleration.

So, longer noses will reduce drag – not too surprising. Same goes for the next bit, but anyway...

In spite of this, the overall force is still the same – so, where does the rest go? It acts perpendicular to the projectile’s motion. In a perfectly stable and rotationally symmetrical dart in a fluid of constant density, this force is equally balanced against the force from the other side, and is therefore nullified.

In reality, the air is marginally less dense on the upper side of a projectile (if only by a less than thousandth of a percent), which will create a slight tendency for the projectile's nose to naturally lift up, which naturally prevents any projectile in flight from remaining eternally stable without the effect being forced.

Still, although this effect is in some ways annoying, it’s very much a cloud with a silver lining – it’s not all bad. Rather than trying to nullify this force, it can be converted to useful energy. With fins, the leading edge of these fins can be angled unevenly to deliberately create a force on the fin.

If you have multiple fins, these forces can be used to create spin. Not only does it reduce the drag force from the fins by having a more aerodynamic leading edge, and it converts the force that would have previously been a detriment into extra stability.
With the right fin shape, you can even get the projectile to a rate of spin that reduces drag further by reducing the relative incidence speed of the fins edges with incoming air particles.
This turns the old theories that drag-induced spin wastes energy on their head... when you think about it in depth, and if it's done in the right way - you can actually save energy, and add a beneficial effect at the same time.
However, angled fins, rather than an angled front edge WILL still increase drag force - increasing the sectional area exposed to the air flow is not a good move.

I helped a mate build some bolts for his homemade crossbow a few years back, and we were experimenting with angled edges on the fins. Even at the low velocity tests we did, the small rotational forces were still more than enough to create very noticeable spin rates (and very stable bolts as well, not one ever hit other than perfectly nose on to the target) - at higher velocities, the larger forces could create spin rates that should combine very favourably with drag stabilisation to make darts that should be capable of accuracy that, if not equal to firearms, would at least be a lot closer than we've ever got before.

So, I reckon that darts can be used to fire a lot further AND more accurately than people have been trying for before. Drag-induced spin is an effect that can be used to our advantage, and we should be grabbing it with both hands.

Anyway, now you’re all bored rigid – any thoughts?
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Unread postAuthor: MrCrowley » Sat Mar 08, 2008 2:42 pm

Interesting thoughts. I actually sat and read the whole thing a few times. I'm doing Projectiles in Physics at the moment, and seems like some good food for thought.

We're making stupid bottle rockets, and we're supposed to come up with an effective fin design.
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Unread postAuthor: Sticky_Tape » Sat Mar 08, 2008 3:00 pm

Don't dis bottle rockets man they're awesome and an effective fin design is sort of like this.
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Unread postAuthor: MrCrowley » Sat Mar 08, 2008 3:23 pm

They're bogan as. Two bottles taped together and they're pumped to 100psi. :roll:

I offered to bring in my sprinkler valved golfball gun, and he said it would be good if I did because we could get far more reliable and consistant results.

So we'd just go on the field, he would supervise, which makes it 100% legal, i'd charge it to only about 30psi, and I would be the only one who fires it.
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Unread postAuthor: chisox21 » Sat Mar 08, 2008 3:56 pm

why not curve the fins in a way that they would all curve a certain way
like the way a hockey stick is curved or like a "c" to increase spin faster spin = more aerodynamic?

just a thought
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Unread postAuthor: boilingleadbath » Sat Mar 08, 2008 4:41 pm

Drag is caused, generally, when the projectile has to impart energy to move the air, for instance:

1) Out of the way of the nose cone
2) Into turbulent eddies behind the projectile
3) To generate lift off a wing

1: Because the diameter and velocity of the projectile are constants in our optimization, it is best to move the air out of the way <i>slowly</i> because E= 1/2MV<sup>2</sup>. (stagnant air pockets streamline really inefficient shapes, like flat faces, so that the air doesn't have to gain infinite velocity to get out of the way)

2: It takes energy to generate turbulent eddies! Avoid creating turbulent eddies! Avoid sharp corners, they'll cause flow separation = turbulent eddies!

3) See "1" - the less velocity you give to the air, the more thrust (lift) you get per unit of energy used to accelerate the air... this means that large wings at low angles of attack are good.
**********

A spinning projectile, of course, has to accelerate some airflow to get up to speed (taking energy), and then has to constantly accelerate in a rotary direction the air in it's boundary layer... taking energy. It may be a reasonable compromise for accuracy, but it does increase drag.

Bent tips on your fins will do nothing other than mimic an airfoil with a large, eh, camber I believe... you still need to direct the air in another direction to generate lift, and a large flat wing (angled fin) would probably be more efficient... IDK, check it with an airfoil simulator.
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Unread postAuthor: nz_cannons » Sat Mar 08, 2008 5:16 pm

Bro, I used to make water rockets, those things I crap you not would go to about 180-200 meters. Straight up. Reduced nozzle size and circular fins are the way to go.
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Unread postAuthor: Ragnarok » Sat Mar 08, 2008 9:02 pm

boilingleadbath wrote:Bent tips on your fins will do nothing other than mimic an airfoil with a large, eh, camber I believe...

Ah, I thought I might be misunderstood - Not bent, using a file (or similar) to put an angle on the front edge of the fins (and the back one too, if you want), rather than a squarer edge.
You should really streamline these edges anyway, but by moving the leading edge towards one side of the fin, you can create an unbalanced force instead of trying for an equilibrium.
In this way, you can use energy that would have been lost anyway. You do then lose energy to accelerate the boundary layer of air - but still, this is the same energy that would have already been lost.

This won't produce extra drag-losses either - this really is a rare case of something for nothing. No free energy here, just recycled energy.

It may not seem like a lot of area to generate a turning force with, but as I said, the crossbow bolts I helped make turned very well, and we tested those at very low velocities (only a few dozen fps at most), and those picked up more than enough spin to be perfectly stable. The darts have a small enough moment of rotational inertia that quite small forces will be converted to fairly large rotational accelerations.

Up the velocity and the forces generated will be more than enough to induce enough spin to achieve decent stability.

I can tell you, all my darts in future are going to have fins designed to convert the drag into spin. Don't know exactly how accurate the results would be, but I reckon that with a small pile of consistent darts, you could shoot groups that wouldn't completely shame a moderately decent rifle.

Myself, I hope to be able to accurately hit and penetrate steel plates at ranges into hundreds of yards.
The ultimate aim would be to hit a 5" square 1/4" steel plate at 500 yards and penetrate it... :D (That's the two ideas my mind fused)

Because as was once said by BC Pneumatics (even if Zen made it infamous):
BC Pneumatics wrote:You aren't allowed to call this a 'sniper' until you hit a 5"x5" target at 500 yards, and even that is being generous.


Certainly, being able to hit and penetrate a plate steel target that size at those ranges would be a pretty major achievement.

The main challenge is hitting, because I reckon that even at those ranges, if I could get a stable dart to hit, I could still penetrate 6mm of steel - drag losses shouldn't be more than about 20 or 30% of the dart's original energy (depending on the exact design), and with enough energy at the start, the velocity could still be sufficient at the other end.

That would actually mean brushing up on my skills a little, because my best at the Army Cadet ranges was around 2 MOA (5 round group), and if I don't want to be firing a large number of darts, I'll need to improve my aim a little. Still, I do have a bipod and scope for HEAL (not to mention the custom grips!), and having spent a while using them, I know it's much easier to hold a steady aim with than it ever was with the .22 rifles - even in spite of it's higher weight.

But then again, I might just have flipped. It's still a task I'd very much like to try and beat. We can all have our dreams.
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Unread postAuthor: Skywalker » Sat Mar 08, 2008 9:33 pm

Shucks, I saw an article several months ago about a fin-stabilized round for a smoothbore sniper 'rifle'. It was a tungsten dart with a discarding sabot, and the blood thing could be shot something like a km, yet the dart's trajectory would only peak like 20cm above a straight line to the target! It was really impressive. So I think you may be on to something.

Also, I've tried shooting finish nails (the ones with almost no head) out of a 6mm bb coax I made, and they seem to self-stabilize; I shot one a good 60 ft and it stuck in a peice of wood. I don't think you need much in terms of fin to stabilize.

As far as the most aerodynamic fin design, you probably just want to make them as thin as you can w/o them breaking off. Your idea of putting the bevel on the same side of the find probably would induce some spin, but I'm betting you wouldn't see much improvement in accuracy from the spin. Especially if the projectile is slightly asymmetric; then it would cork-screw.

Speaking of which, what are you think about in terms of materials and manufacturing techniques?

(Off-topic: I did some experimental rocketry during highschool, so I know a few things about rocket aerodynamics, although I was really more into the "let's mix up some flammable chemicals" part. And I've made my share of water rockets too: even one with an air-speed activated parachute deployment. Those suckers are lots of fun, and will go pretty dang high too.)
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Unread postAuthor: POLAND_SPUD » Sat Mar 08, 2008 9:44 pm

homemade ammo has one major disadvantage... it takes a lot of time to build... sometimes I use nails as projectiles (nailheads stabilise them; center of mass is more to the front becase their 'tails' are grinded)... suprisingly they don't tumble... I don't really know how good they are at greater ranges becuase their velocity is relatively low (since they are relatively heavy and I fire them through 6mm aluminium barrel... )

ok I know it doesn't have much to do with this topic but I am just curious has anyone done something like this before ?

EDIT
ok someone has done something like this :D
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Unread postAuthor: Hawkeye » Sun Mar 09, 2008 12:16 am

Have you seen the fletches they use when shooting Olympic style? Tiny little plastic things with a kind of curl to them. They are expensive but probably worth a try. They would lie perfectly inside a barrel.
I'm pretty sure you could just make a crossbow type projectile with a teflon washer on the back end to make it airtight.
One of these piston guns tends to shoot well over 700 fps. An arrow can be shot over 500 yards with nowhere near that velocity.
I don't think you're going to have much trouble on a calm day. Just getting the arc right will be the main challenge.
It has been proven that a regular bow placed in a shooting machine will shoot practically rifle groups at 100 yards so this type projectile is very good.
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Unread postAuthor: Ragnarok » Sun Mar 09, 2008 3:31 pm

Skywalker wrote:Your idea of putting the bevel on the same side of the find probably would induce some spin, but I'm betting you wouldn't see much improvement in accuracy from the spin. Especially if the projectile is slightly asymmetric; then it would cork-screw.

Speaking of which, what are you think about in terms of materials and manufacturing techniques?

I found that the crossbow bolts worked very well with the spin, and if I invest enough care into the darts, I don't see why they couldn't share in the success.

The materials... I was thinking steel for the main core of the dart, with the fins made out of PVC (or something similar) to keep the COG forwards, as well as keep the weight down a bit, so I can get a slightly higher muzzle velocity to shorten time to target.

The other reason for PVC fins, is that it keeps the moment of inertia low, and if the fins are slightly asymmetric, the higher weight of the dart's core would keep the COG near the main axis, and thus minimise the amplitude of any oscillations.

Finally, the fins are being designed to break loose on target impact (to increase the dart's penetrative capabilities), and the lighter the fins are, the less of the dart's kinetic energy is contained in them - thus, less is lost when the fins break off.
It's also likely easier to get PVC fins to break off for minimal energy costs.
I think I might even have a way to get them to give up part of their energy into the core, then break themselves off using what remains of their energy, giving the dart optimal (armour) penetration on impact.

I'm bouncing numbers around between GGDT and my half complete range calculator to try and get the best results.
I'm hoping that if I can reduce drag to a minimum and save on the energy needed to break fins loose, the core of the dart could keep above 75% of the original muzzle energy at a 1/4 mile.

I think I can achieve above 5000 ft-lbs/in<sup>2</sup> on the right type of target at those distances. That's a respectable figure - not quite in the same region as DYI managed on his 1/4" steel plate, (I think he managed about ~12000 ft-lbs/in<sup>2</sup>) but considering the secondary damage he achieved, I think possibly still enough to pierce 6mm steel plate at a 1/4" mile, which wouldn't shame many proper rifles.
What other terminal effects the darts will be able to display is another matter.
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Unread postAuthor: Darkerweb12 » Tue Apr 01, 2008 10:05 am

I must agree with you all here, I was always a fanatic about firearms in general, so when I built a pnuemie in 7th grade, about 5 years ago, I did make a discarding sabot that used tilted fins for stability, it was amazing! Just seeing the sabot discard as it left the barrel and then seeing the dart pick up rotation was wonderful, then the perfect parabolic path it traveled was amazing!!!

Has anyone ever thought of using playing darts as projectiles, I think they might make a good penatrating ammunition, they're already perfectly balanced for flight making the time consuming and precise manufacturing obsolete.

The idea of rifled darts isn't new either, it's been around probably since the invention of crossbows, probably earlier. I know they were used on the Chinese's repeating crossbow. The darts were actually rather ingenious, instead of using the standard short rather stocky fins, they used long thin fins that didn't raise far up above the shaft of the arrow, and followed in a spiral pattern along 80% of the shaft, so that they would fit easier and neater into the magazine.

-Chinese Repeating Crossbow-
Image


What's actually kinda cool about this crossbow, is it is still used to this day by chinese soldiers in the very remote parts of china...

Either way, check this old crossbow out, it was the minigun of it's day.
http://www.atarn.org/chinese/rept_xbow.htm
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Unread postAuthor: Thunderbug » Tue Apr 01, 2008 8:16 pm

Sorry to state this, but I think you have a couple of concepts confused (well, to some point).

Stability (1) is just a question of keeping the orientation of the projectile at impact as it was on firing. Front end first, duhh ;)

Stability (2) is a question of having the projetile going the way we want.

Stability (1) is dead easy to achieve, just add some drag aft of the center of gravity. Fletch your arrows, put some yarn on your darts, MAKE A PAPER TUBE EXTENSION FOR YOUR SLUG LOADS!

Stability (2) is a bit of a female doggy.
Considering everything else is perfect, we can blame the projectile, and one way to chastise it is to make it spin.
That will (sort of) make up for any inconsistencies we may have built into it during construction. Such as an almost-crooked arrow shaft, that hits the bullseye anyway, because of slightly angled fletching.

Figure this: The projectile has a tendency to veer off to the side so much per so many metres, which can add up.
But if it spins, it will veer in a circle returning to aim point once per revolution.

Phew, my beer looks like motor oil, and I have to get up for work tomorrow.

I'll just cut here, but any rants, rectifications or questions will be welcome.
I WILL be back :)
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Unread postAuthor: Jared Haehnel » Thu Apr 03, 2008 7:23 am

As I've stated before I really believe that hitting a target at 500 hundred yards with a home made dart from a spud gun is a pretty lofty goal. There is not doubt that the dart will retain the energy necessary to penetrate the steel. And I am confident that your design will be able to travel over 500 yards but what it comes down to is accuracy.

I doubt HEAT will fire darts much past 1000fps. At that speed if it remained a constant...(and I know it won't) the dart would take (give or take) 1.5 seconds to reach the target at 500 yards. Given that gravity is 32.2ft/s2 let me give you a rough estimate of the drop... well at one second the round would have dropped 32.2 feet and we can tack on at least half that to make 48.3 feet of drop before it reaches the target. Given my math is not going to be accurate so take it with a grain of salt.

This doesn't included wind age or incorporate drag which will also adversely effect accuracy. (Wind age will be a huge factor as well at that range) Now I know you and others here can figure out much more accurate figures and know exactly how much to raise the barrel to make it all work out. But still its no easy feat... and nothing that a scope or common iron sights will fix...
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