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Compound Sling (Read 52905 times)
Matthias
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Compound Sling
Mar 24th, 2007 at 5:43pm
 
...

Ok, this is an idea that I've been kicking around for a couple of years, but have never managed a chance to really dedicate myself to figuring it out. I'm releasing it to everyone here at slinging.org with one request - anyone who tries this please post here describing your results.

Slinging is limited by both the strength of the thrower and the speed that they can move their arm. Take away air friction, and it becomes a strength-only problem, with a coordination limit - the reason you can't just helicopter your way up through the speed of sound (speed of sound if there was air, of course Roll Eyes) You can trade strength against speed - a sling lets you move your arm slower with more force while moving the projectile quickly. A bow works by a different principal, storing energy, but this is also true of a sling with a longer windup - rather than just have the length of your arm to apply the accerating force over, we have the opportunity to add energy over a longer period of time.

The drawing above shows a sling-variant that might allow the thrower to "bank" energy that becomes available at the moment of release. The sling is compound - basically a sling attached to the end of a second sling. A mass heavier than the glans is attached to the end of the primary, retained sling, while the secondary sling holds the projectile. The pouch starts out in the slingers hand, and at the appropriate moment in the throw, it is released to swing through a larger path centered on the booster mass. The release is made in a conventional manner, but at the end of a significantly longer sling. The acceleration of the pouch and glans transfers kinetic energy from the booster mass, whose speed is reduced to near zero at the instant of release.

At least that's the way I picture it working... There are a couple major problems to be solved, as well as some tuning issues:
  • Controlling the cords - For the design to work, the release cord must either pass through a fairlead at the booster weight, or it needs to terminate there. If you think of the primary as a staff-sling, you can see the potential for a mechanical auto-release. This might be a good idea , as we'll see. Alternatively, the release could be manually triggered, as in a standard sling.
  • Dropping the pouch - this is both a timing issue as well as a physical problem. Once the pouch is released, the slinger has very little control over it's path and speed. It is also hard to drop the pouch without allowing slack to enter the cords and possibly dumping the ammo. This is only an issue for the first split second, but it might limit the throwing styles that can be used.
  • Ratios etc - the ratio of booster mass to glans should be an important variable. It should be possible to calculate an optimum relationship (while we're at it it might be nice to see if this idea even has wings) but you want the booster heavy enough that it can store and transfer enough energy to move the secondary around efficiently. Too light is definitely a problem, while too heavy, aside from strength/injury concerns, is less clear. At least double.
  • Timing - is *critical*. The secondary sling is supposed to be moving pretty quickly through it's sequence (the whole of which essentially takes place during the power-phase a regular sling throw). Coupled with that is the general lack of feedback, due to the booster mass. While it is definitely possible to release this sling normally, I suspect that something that times itself might be more reliable.


I've built a couple, but never had the space or the time to put them through their paces. I don't know if it can be made to work, but I'm pretty optimistic. The few test throws that I've managed without dumping the glans sound fast. The secondary cords whistle in a way that puts your neck hairs up. That's with a gentle lob. Given a big empty field, a weekend and a couple of interested slingers, I think there is a good chance of making this work. A release mechanism that seems like it might have potential is to make the booster relatively long, so that it is pulled in line with the retention cord. A pin and socket type release is added to the end (I made my booster around a piece of 20cm long pipe). If the hole goes all the way through as with a tube, you can use the same booster while trying a manual release.

A figure-8 seems the natural throw to use with this to me, as the motion fits the release sequence, and it is a relatively safe throw, with an easy to time windup.

CAUTION
: more than the usual disclaimers apply here. Until we have this thing figured out, you are working with a highly experimental, tempermental sling, with a heavier than normal retained mass, complex timing issues, and a difficult release. Did I mention that I'm expecting this to be very very fast? I don't know what the failure modes are. If the pouch doesn't release, where does it go? Right back at your head? Kneecaps? What happens to the booster if the glans falls out early? You now have a rapidly moving object tied to your pinky (please don't use a pinky-only grip with this one). How wide does your downrange safe-arc of fire cover (I highly suggest 360 degrees)

Apart from all that, I'd like to hear what you think.

Matthias
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riverwindflutes
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Re: Compound Sling
Reply #1 - Mar 24th, 2007 at 8:16pm
 
Wow!!
Thats a lot of info, I'm having trouble understanding exactly how to sling a rock in this way, maybe a picture or a video would help.
Don A.
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Re: Compound Sling
Reply #2 - Mar 24th, 2007 at 10:58pm
 
Very interesting.  It reminds me of something I can't quite dig out from the back of my mind, somethings to do with horology, maybe I've seen it in an old clock, can't remember sorry. 

I like the idea, I had a similar but far less efficient one for a mounted sling ( part of the physics of which your idea resembles, the leverage part ) The principle was to put a weight on the end of the releace cord and use it to jerk the pouch forward on casting.  So starting at the retention the sling components would be, stick with loop in end, flexable part of the retention, pocket, releace cord to stick with weight at stick end, second part of the releace cord fed through loop to hand.  The weight would be like a mace, with the folded sling on a wider orbit but trailing slightly behind.  At releace the weight would be releaced to move into it's larger orbit and the sling would rapidly unfold.  The glands would have what it normally does plus a bit of kick.  The concept being to turn some of the centripetal saved energy in the weight into forward motion of the pocket, the weight being releaced just prior to casting.  I only fiddled with it breifly and never got it to work.   You seem very close.  One suggestion I have is to go the full monty and go for a long mounted sling, the pole part being longer than both retention cord stages together.  This would give even more leverage and power but the main reason is safety. keeps the weight out of your way after the cast.  Running cords up tubes does seem best, though a fiddle to load, as it keeps the cords together. you may find however that the cords always stay to the same side of the tube when you throw which would allow you to split the back of the tube all the way along, or use hooks on the outside of a pole, which would be easier to load. 

If you did this though you would end up with a three stage sling, I'm not sure if the pole would compensate with regards to contollability.  If not then not such a good suggestion as I'm fairly sure from what you have said of testing with the prototype, the glands would exceed the speed of sound, a standard sling is only a hundred meters a second off or so and whip ends do it already.  Don't want to read "Fluke metiorite hit kills man, astronomers baffled by earth like rock.  Passers by heard a loud bang and found the man in a feild.  Unconfirmed reports say police removed a mysterious device from the scene of the tradgedy."
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Re: Compound Sling
Reply #3 - Mar 25th, 2007 at 2:17am
 
Hats off, Matthias.

Sounds amazing!  I'd love to try that design out.  Did you ever talk to Techstuf about it?  I got a feeling he would have something valuable to say.  

I can see the design work very well as a staff sling.  As a sling though, it would be very hard to control.
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Re: Compound Sling
Reply #4 - Mar 25th, 2007 at 4:02am
 
Matthias, great idea.

Is your diagram the result of a simulation? Reminds me of compound pendulums. If the release could be made to work it would surely scare a lot of people - not just the slinger Grin.
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Re: Compound Sling
Reply #5 - Mar 25th, 2007 at 7:18am
 
makes sense - pretty sure techstuff had a smiliar idea a while back.
Love the picture, though to be honest the written explanation made a lot more sense to me :-)

Well if you get a spare rig - I have plenty of big fields to test it in :-) Also going to be doing a video shoot after easter with an etruscan sling.

The main problem I envisage is that you'll have to hold both the pouch and relesae cord in the same hand, otherwise it'll just tangle you.

Certainly like to either try one or see one n action :-)
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Re: Compound Sling
Reply #6 - Mar 25th, 2007 at 8:19am
 
This sounds a bit like a Staff Sling, which boosts performance by giving more leverage.

The compound sling uses a booster mass to get more power, but I fear that the energy required to move the booster mass will slow down the entire sling, effecitvely eating up all extra power. The ideal sling should be light so that no energy is wasted for getting the sling itself moving.

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Re: Compound Sling
Reply #7 - Mar 25th, 2007 at 8:30am
 
Not so adaxl the enrgy you put into the booster mass is converted directly into the missile via the longer cord.
What it should allow you to do (if correctly made and working) is to put a lot more strength into the throw itself.
The heavier the missie in a normal sling the harder you can pull against it without straining your own muscles (it's like throwing a punch and missing, the hader you punch the more you hurt yourself when you miss).
The problem with a conventional sling is that sure you can use heavier ammo and up to a point you will get more distance. But at a certain weight your own strength will start to tell against you and the velocity will drop - even if you are putting more effort into the throw.
With this method you can put a lot of effort into a fairly slow throw which is then directly converted into a much faster throw with a lighter missile.

If it helps, think of it as gearing. High gearing on a bike lets you push really hard and pedal slowly but the wheels go round faster than low gearing where you pedal faster but with less effort and the wheels go round proportionally slower.
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Re: Compound Sling
Reply #8 - Mar 25th, 2007 at 12:19pm
 
Matthias

It is clear that your compound sling uses a slack cord to supplant a rigid or semi-rigid rod akin to fishing tackle. Your concept might be developed and tested using a smallish treb setup, this can be a  much safer, consistent and rewarding way to experiment.

For a combination ballast weight/release, a battery pack and solenoid can be triggered by a relax thumb button. Of course the testing treb needs some sort of adjustable auto release. Compare the results from this new treb setup with same treb using the usual release cord.

Tom   
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Matthias
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Re: Compound Sling
Reply #9 - Mar 25th, 2007 at 1:24pm
 
Bravo aardvark! You're officially promoted to associate question-fielding guy on this one.

I know it looks like a staff sling, or a treb, in that it has two parts and the motions are similar, but it's a quite different beast. The goal is to store energy in the booster (as much as possible) that can be released into the projectile in the final phase of throw. The booster itself essentially comes to a complete stop as the pouch is released - no loss of kinetic energy. Let's say I can throw a 300g rock just about as far as a 100g one, or a 50g (I'm speed limited, not strength). My speed at release is around 50m/s. The energies transferred to the rocks are 375J, 125J and 62.5J respectively. IF we could manage an efficient transfer of energy stored in a 300g booster, it might be possible to put those 375 Joules into the 50g projectile - 122m/s. _That_ is a very very fast throw. A golfball that normally goes 150m could hit 330...

...

Tech and I talked about it privately a bit a few years ago. There are a limited number of ways to get more power into a sling - he already has the length and aerodynamics covered Smiley

A couple of points raised:

re. the tube. This sling cries out for captive cords. If trying for a conventional release I'd allow maybe 6 inches of slack in the secondary before it caught itself. a knot or bead on the primary retention cord should be enough to handle this.

Rigid links. It's important that the "joint" at the wrist be flexible. Unlike a staff sling, we aren't levering this one into position, and the rotation about the hand is actually slowing during the final phase of launch. It is also desirable to concentrate as much mass as possible in the booster (and projectile) - this is the conventional sling argument that ADAXL raised. I think you could probably use a rigid link in there though, maybe witha  short piece of cord attached to a handle to give some more freedom of motion. Stick? Carbon tubing? A rigid or semi rigid link should simplify the release a little if using an automatic system. I think It's doable without going all high-tech though...

Two handedness. Yessss... It is a little tricky, but it seems to work ok. If using a "staff sling" secondary release you are back to only one thing to drop, and one timing problem (apart from intial setup)

I think that some of the more advanced treb simulators might be able to make a decent go of this, but the tip path isn't "standard" treb, nor is the weight distribution. Unless someone knows of one that is really flexible, it's probably easier to sort it out from scratch. The other option is to use one of the dynamics packages that are out there. Any engineering students should have access, and trials are available on the web. One day I might get to it, but it's a bit too much like my normal work...

Thanks for the comments on the drawing. I really like the sort of crazy nested epicyclic look to it - a lot like an orrery. Interesting the difference between the visual and verbal understanding!

Matthias
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Re: Compound Sling
Reply #10 - Mar 25th, 2007 at 3:23pm
 
TechStuf does indeed have a technique where he holds the cords and pouch in the same hand.  This is his around-the-elbow technique.  You need a sling long enough to loop around your elbow and back up to your hand.  Start your cast with your arm bent (so the sling cords are secure around your elbow), hand at about face level.  Flip the pouch out of your palm and forward, and draw your hand back to where it started.  When the pouch reaches the bottom of its swing, flip your hand out again and release.  The sling will be accelerated around your arm and back up, for an overhand release.

Matthias, I've been looking at your drawing and trying to make sense of it.  The spiral curves for the pouch and for the booster mass, are fairly clear.  The curve coming in from the left, looping around itself and passing off to the right -- that has me puzzled.  Does that line represent your hand position, as you do a figure-8 with a step forward?

And where can I find that Slinging.org projectile motion calculator?
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Re: Compound Sling
Reply #11 - Mar 25th, 2007 at 3:56pm
 
Right - hand motion. Only the last part of the path (from about the bottom of the little loop onward) relates to the steps shown, the incoming paths are just for show (although they should be pretty accurate). The paths and positions aren't taken from a simulation, or traced from video or anything, so there is a bit of artistic license. We'll have to put leds or reflectors on Tint one day and see if we can do some long-exposure-time photography to see how close I got!

The sim is another project started but not finished... Undecided I never got enough data to validate the results, and the interface is a little obtuse as it stands, but it is fun to play around with. It allows for spherical or ellipsoid ammo, and interpolates the Achenbach data to give an approximation of boudary-layer effects. Drag for ellipses asumes that they magically orient themselves tangent to the flightpath, instead of maintaining attitude.

Using it is pretty straightforward. Clicking on the display area clears and resets it (and draws the grid). Checking "show analysis" displays speed, drag and drag coefficient in addition to the tragectory. The curves implemented in the compiled copy are step-dicontinuous at transition, and constant at low Re. The drag values read from the top down.

I'm attaching a copy, zipped for anyone who wants to try. Windows only I'm afraid.

Matthias
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SlingSim.zip (13 KB | )
 
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Re: Compound Sling
Reply #12 - Mar 25th, 2007 at 5:21pm
 
Some years ago Matthias mentoned he was working on a compound sling. He interested me in it and I was thinking about it a while. I had a few ideas but never tried them out in practice. Anyway the question is so much interesting.

Matthias, your idea is great (not to say about the drawing) but I have some doubts regarding your solution. They are just my guesses and I might be wrong.

First one I'm not convinced about a superiority of the mentioned compoud sling over a good staff sling. I mean it made of a light springy carbon rode or so (let say with the "booster" mass on its end - it is another question for discussion). I'm rather prone to agree with ADAXL in this point. I would even say that the staff sling should work much better because it additionally can be controlled by the palm (either accelerated or slowed down) and the springy rod is able to accumulate an exess energy that can be released/given back in the last stage of the throw, when the body is not able to accelerate the hand more.

The second, in my opinion, big problem is the unprofitable angle between the cords just when the pouch is released from the palm. As a result the tension of the cords between the masses will be lost. Too bad for efficient slinging. It was mentioned though.

I'm also not sure if the booster mass stops completely and as a result transer the energy to the projectile. I'm not saying it is impossible, but it isn't very likely and predictable, because it depends on a lot of factors, similary like in mentioned by wanderer compound pendulum. That one made of two masses is enough crazy.

Matthias, sorry for my scepticism. I will try to add some of my ideas to yours, but first have to make some sketches.

Jurek

Please guys, tell me if I'm not clear.
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Re: Compound Sling
Reply #13 - Mar 25th, 2007 at 6:50pm
 
Matthais,

I really like your idea, and frankly the innovative side of slinging is what I find more interesting. I see what you mean about it not being similar to a staff sling, in that most of the force on the projectile comes from the energy stored in the booster mass. From what I understood this means that if the mass of the booster is large enough, relative to the projectile, even a fairly slow swing will still send out the ammo at an impressive rate.

What if your booster mass, was actually a staff with a large weight at the end. From the picture below what you see then is….A small retention loop and cord attached to one end of the staff…The release cord and red pouch attached to the weighted end of the staff

So essentially it would look like you were swinging a triangle around. My guess is the more sections you have like this the more of a whip like action that you can get, am I right?

Shihab

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Re: Compound Sling
Reply #14 - Mar 25th, 2007 at 10:03pm
 
Great to have your input Jurek! Always highly valued. Welcome shabundi as well. I'm a born skeptic - maybe we should start a new order: Sphendonetes Skeptikoi. I'm not sure that it can be made to work either, but I do think that the principles are sound.

I'm sure that a well made staff sling could produce a significant ammount of power, but the machanism is different, unless we start talking about very lightweight materials. Likewise, it turns out that yu can't use most treb simulators to play around with as they work off of the momentum of the arm and counterweight. The moving mass with a human slinger is quite low - alomst all of the energy input is force. With an elastic staff, you're adding another energy storage mechanism, and liek this one it would have to be tuned to the mechanics of the throw. I have no doubt that it would work either.

The angle problem is a very real one. I have managed throws like this though - in one of my earlier posts I think I mentioned that a captive pouch like Techstuf's star might be a big benefit. The problem can be lessened with a handle, or "mini staff" to give some space between the primary and secondary cords at the hand. The paths taken are relatively complex, but there seems to be a window in a fig-8 where the booster is moving fast enough and the hand is reversing directions. No one said this would be easy! Grin

The booster ends up moving pretty slowly at the time of release. Length of cords, weight ratio etc all factor in, but with no mass in the cords you can come very close. Here is a bodged simulation (and I said I wasn't going to do this... Angry) showing a simple setup. This one is not optimised for energy transfer - with different initial conditions it was possible to stop the booster. I have no idea if that is an efficient thing to do with a human thrower. A double pendulum becomes chaotic very quickly, but the first swing or two are very predictable - just like a trebuchet?

...

This was done with a program call "Working Model 2D" for which a free trial (no saves) is available. Slings aren't easy to simulate though - the hand motion is very much not a simple arc, as here, and the initial conditions are tricky to set up. A halfways ok start is to only simulate the final part of the throw, and give the components initial velocities to compensate. Even doing that, a four-bar linkage or similar would allow a better representation of the cast. Maybe cams? If you were really keen, you could do a pretty nice job, even releasing the projectile...

Matthias
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