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Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: inonickname » Fri Jul 31, 2009 9:50 am

Joerg's slingshots are directly producing 50 fpe.


That's because it's using the rubber energy directly with an extremely heavy projectile, you could not propel the same projectile at 50 fpe using the piston I'd say, but a lighter one would easily reach much higher velocity.

Looking forward to it anyway.
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Unread postAuthor: boyntonstu » Fri Jul 31, 2009 10:25 am

inonickname wrote:
Joerg's slingshots are directly producing 50 fpe.


That's because it's using the rubber energy directly with an extremely heavy projectile, you could not propel the same projectile at 50 fpe using the piston I'd say, but a lighter one would easily reach much higher velocity.

Looking forward to it anyway.


The slingshot pouch speed becomes the speed of the projectile.

The projectile, light, or heavy, is the speed of the slingshot pouch.

The weight of a 14gr .pellet or a 200 gr ball does not appreciably affect the retraction speed of a 35 pull rubber.

I believe that the gains in speed comes from decompressing the chamber compressed air in a smaller diameter barrel, or simply the speed gained by the Venturi Effect. ( same thing?)


WIKI:

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is balanced by a drop in pressure or a pressure gradient force. An equation for the drop in pressure due to the Venturi effect may be derived from a combination of Bernoulli's principle and the equation of continuity.


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Unread postAuthor: inonickname » Fri Jul 31, 2009 10:30 am

You realize that a big thing with springers is that the rapid motion of the piston adiabatically heats the air, causing a higher particle speed in the gas, and therefore a higher velocity?

Only reason some springers will happily go mach. It's also the cause of them being destroyed after using the wrong lubricant (i.e. diesel).
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Unread postAuthor: Technician1002 » Fri Jul 31, 2009 10:44 am

Ragnarok wrote:
Technician1002 wrote:This why I like the air as a spring.. Less kinetic energy lost to the spring, bow, string, arm, etc, and more transferred to the projectile.

Unfortunately, the numbers disagree with you.

Springers are about 30% efficient in converting spring energy to kinetic energy. Crossbows - over 90% efficient.
This energy is all wasted in the compression of the air.

So, with a straight mechanical energy transfer like a crossbow or similar, you can stand to lose quite a lot of that efficiency into the mechanism (provided of course you don't damage it in the process) and you'll still be winning out.

...Actually, I've just been helpfully reminded of a design I did last year for a high velocity crossbow that used mechanical advantages to propel projectiles to much higher speeds than normal (at surprisingly little efficiency loss) but the problem is although I can remember the basic principles of the design, I can't immediately remember the finer points of its operation.


Absolutely true.. for the much heavier projectile of the arrow. When working with pellets that have a mass close to the mass of the bowstring, the efficiency changes and the energy transfer through an air spring is more efficient at the higher velocities.

Often the energy transfer is able to launch small light projectiles at higher rates of speed than the crossbow string is able to reach.

The projectile, light, or heavy, is the speed of the slingshot pouch.

The weight of a 14gr .pellet or a 200 gr ball does not appreciably affect the retraction speed of a 35 pull rubber.


First part is very true.. The speed is of the slingshot pouch.
For the second part.. I would have to do the acceleration math to see if 14 gr pellet or 200 gr ball makes an appreciable change. Since I don't know the mass of the pouch and other parts that accelerate, knowing how much the total accelerating mass changes is unknown at this time. It is true, both will be fast and most likely the difference in velocity will be less then 30%.

The kinetic energy in the heavier projectile will be significantly higher as the ball is a larger portion of the accelerating mass.
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Unread postAuthor: jackssmirkingrevenge » Fri Jul 31, 2009 11:13 am

inonickname wrote:You realize that a big thing with springers is that the rapid motion of the piston adiabatically heats the air, causing a higher particle speed in the gas, and therefore a higher velocity?

Only reason some springers will happily go mach. It's also the cause of them being destroyed after using the wrong lubricant (i.e. diesel).


It's also worth pointing out that the detonation of lubricant in many springers is actually providing some of the power, adding chemical energy to the mix. Springers are considered a "black art", even by today's engineering standards, it's always a question of trial and error and getting it right is tricky.

On a small footnote, I had forgotten about it, cheers :)
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Unread postAuthor: Ragnarok » Fri Jul 31, 2009 11:16 am

Technician1002 wrote:Absolutely true.. for the much heavier projectile of the arrow.

Bear in mind, 30-40% is the limit of springer efficiency. (30% being typical). Regardless of projectile weight, you won't see more than that.

The truth is, springers tend to get higher muzzle energy with lighter pellets (around 8 grains in .177", as opposed to heavy 10-11 grain pellets).

So, ~30% for springers with projectiles "of the ideal mass".
However, bows can hit 90%+ with projectiles "of the ideal mass".

When working with pellets that have a mass close to the mass of the bowstring, the efficiency changes and the energy transfer through an air spring is more efficient at the higher velocities.

Very approximately, there's about 0.05-0.1 grams of air in the compression chamber of my air rifle (I'm not about to fart around measuring the chamber volume). Pellets are 0.5 grams, so that's about an order of magnitude more than the air.
We'll say 10% of the mass is in the air.

Typically, an arrow is about 25 grams (~400 grains), for either bows or crossbows. A reasonably modern bowstring (for a bow, rather than a crossbow) is ~7 grams.
That means that that about 20% of the mass is in the string.

That means that MORE of mass proportionally is in the bowstring than in the air. The air rifle has a more favourable mass split, but it still has less than half of the efficiency.
So, it's nothing to do with the fact that the arrow is heavier. As a proportion of the accelerated mass, the arrow is lighter!

You can get faster with springers, but it's done much less efficiently.
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Unread postAuthor: boyntonstu » Fri Jul 31, 2009 11:59 am

jackssmirkingrevenge wrote:
inonickname wrote:You realize that a big thing with springers is that the rapid motion of the piston adiabatically heats the air, causing a higher particle speed in the gas, and therefore a higher velocity?

Only reason some springers will happily go mach. It's also the cause of them being destroyed after using the wrong lubricant (i.e. diesel).


It's also worth pointing out that the detonation of lubricant in many springers is actually providing some of the power, adding chemical energy to the mix. Springers are considered a "black art", even by today's engineering standards, it's always a question of trial and error and getting it right is tricky.

On a small footnote, I had forgotten about it, cheers :)


Fair.

Remember the lubricated piston is rapidly sliding down a cylinder and compressing an heating the air.

I can conceive of a burst valve that should give the slingairgun the same performance as any other pneumatic gun with the same chamber volume and pressure that it sees at the end of the stroke.

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Unread postAuthor: Technician1002 » Fri Jul 31, 2009 12:03 pm

boyntonstu wrote:

I can conceive of a burst valve that should give the slingairgun the same performance as any other pneumatic gun with the same chamber volume and pressure that it sees at the end of the stroke.

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Very true, but can you provide the same gas temperature? The light gas from compression heating is part of the gain and helps near SOS in the sling gun.

Part of my next cannon is to intentionally include dead space between the valve and projectile in the venturi just to create compression heating of this pocket of air. Only the mechanical piston is omitted. The compression and heating remains the same.
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Unread postAuthor: boyntonstu » Sun Aug 02, 2009 6:46 am

Ragnarok wrote:
Technician1002 wrote:Absolutely true.. for the much heavier projectile of the arrow.

Bear in mind, 30-40% is the limit of springer efficiency. (30% being typical). Regardless of projectile weight, you won't see more than that.

The truth is, springers tend to get higher muzzle energy with lighter pellets (around 8 grains in .177", as opposed to heavy 10-11 grain pellets).

So, ~30% for springers with projectiles "of the ideal mass".
However, bows can hit 90%+ with projectiles "of the ideal mass".

When working with pellets that have a mass close to the mass of the bowstring, the efficiency changes and the energy transfer through an air spring is more efficient at the higher velocities.

Very approximately, there's about 0.05-0.1 grams of air in the compression chamber of my air rifle (I'm not about to fart around measuring the chamber volume). Pellets are 0.5 grams, so that's about an order of magnitude more than the air.
We'll say 10% of the mass is in the air.

Typically, an arrow is about 25 grams (~400 grains), for either bows or crossbows. A reasonably modern bowstring (for a bow, rather than a crossbow) is ~7 grams.
That means that that about 20% of the mass is in the string.

That means that MORE of mass proportionally is in the bowstring than in the air. The air rifle has a more favourable mass split, but it still has less than half of the efficiency.
So, it's nothing to do with the fact that the arrow is heavier. As a proportion of the accelerated mass, the arrow is lighter!

You can get faster with springers, but it's done much less efficiently.


"
You can get faster with springers, but it's done much less efficiently."


Does efficiency count in a slingshot?


http://www.youtube.com/watch?v=kwYej4hngEw

Four different weight projectiles are shot with each slingshot.


FPS and energy results posted.


9,5 mm steel is 3 gm, 48 grains. 249 fps

14 mm steel is 10 gm, 160 grains. 232 fps

20 mm steel is 30 gm 480 grains. 180 fps

19 mm lead is 36 gm, 576 grains. 164 fps ~120 MPH OUCH!


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Unread postAuthor: inonickname » Sun Aug 02, 2009 7:10 am

I'm going to have to say the efficiency of a slingshot will be closer to that of a bow (though probably not as high) than a springer. You lose a lot of energy and efficiency by doing this, but gain a motherload of velocity.
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Unread postAuthor: JoergS » Tue Aug 04, 2009 6:37 am

Hello group,

I'd like to introduce myself. My name is Jörg, I am the big, bald and beefy guy who does those crazy slingshot videos.

I noticed (through youtube's insight features) that my stuff is in discussion here, and therefore I joined. Good to see that Stu is here, he is making very good progress with his construction!

I'd like to say that one of my biggest motivations to work with slingshots is the very strict gun law in Germany. Everything is regulated, with severe punishments for those who do not obey. Without a license that noone ever gets, you can really only build slingshots and bows. "Springers" are a no-no, and even wrist braces for slingshots are illegal.

Having said that, there is a lot of potential in modern rubber. You can build fine weapons with it, inexpensive, silent, efficient and compact.

Sorry for the lengthy statement, looking forward to interesting discussions here!

Jörg
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 04, 2009 7:26 am

Hi Jörg,

good to see you on the forum, I have to say it's good that you are being so thorough with your investigation of slingshot performance an your videos are very impressive.

JoergS wrote:Having said that, there is a lot of potential in modern rubber. You can build fine weapons with it, inexpensive, silent, efficient and compact.


This is true, but except at relatively point blank range I don't think you can ever include "accuracy" among those attributes, unless you create some sort of trigger mechanism. Still, if you have no other legal options when it comes to firing projectiles, it's certainly not a bad route to take :)
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Unread postAuthor: JoergS » Tue Aug 04, 2009 7:33 am

I am very accurate with my slingshots. I can hit a shoebox size target at 20 Meters easily and reliably.

Noone ever attacked me, but that kind of accuracy is probably all you ever need.

And I can shoot very fast, check out

http://www.youtube.com/watch?v=k5h0sGu5POY

It does take much practicing, though. I started at age 9, but I made a break for a few decades before I picked slingshooting up again...
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Unread postAuthor: jackssmirkingrevenge » Tue Aug 04, 2009 7:39 am

JoergS wrote:I am very accurate with my slingshots. I can hit a shoebox size target at 20 Meters easily and reliably.

Noone ever attacked me, but that kind of accuracy is probably all you ever need.


I guess you can say you are as accurate as your average smoothbore shotgun :)



The fastest slingshot in the West :) as I pointed out earlier in this thread, you are lucky to have the sort of upper body strength which can take full advantage of the slingshot's potential and give a lot more energy to the projectile than the average person.
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Unread postAuthor: JoergS » Tue Aug 04, 2009 7:42 am

jackssmirkingrevenge wrote:you are lucky to have the sort of upper body strength which can take full advantage of the slingshot's potential


Luck? It was hard work, countless hours in the Gym.

But you are right, I was lucky to have more than enough to eat :P
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