Slinging.org Forum - Compound Sling

Aussie

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Re: Compound Sling
Reply #120 - Nov 14^th, 2008, 4:31pm

Quote from curious_aardvark on Nov 14^th, 2008, 6:34am:

I can partially answer that

1) the whole point is that the large weight does not need to be brought up to much speed - it's just there to store energy. Bear in mind that the original purpose of the sling (as envisaged by me lol) is to throw objects too small to throw easily by hand. it's much easier to put effort into throwing a heavy weight slowly than a smaLL weight very very fast.

the rest is just down to design

Agreed. All slings work on this principle; the pouch rotating on a larger radius moves much faster than the slinger's hand which rotates much closer to the centre of rotation. However conventional slings are not energy storage devices in the way that bows and crossbows are.

The compound sling is really quite different in this respect. The problem is to effectively recover the energy stored in the weight. Similarly to Thomas' crossbow with heavy steel limbs which have high levels of non-recoverable kinetic energy, the weight must be moving for the compound sling to be working. All the energy still in the weight at the instant of the projectiles release will not be transferred to it and in that sense will be wasted. That's not to say it won't work, just that ergonomically it will be inefficient. Steel crossbows from the middle ages were horribly inefficient but they were also very effective.

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Dale Interfector Viris Spurii Posts: 1581 Gender:	Re: Compound Sling Reply #121 - Nov 16^th, 2008, 12:41pm Quote: Dale and matthias reckon they got one working - but never took any pics Matthias does have pictures of his sling and a multiple-exposure shot of him using it (reply #61 in this topic).
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Yurek

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Re: Compound Sling
Reply #122 - Nov 22^nd, 2008, 11:11am

Quote:

...
Presumably the slinger holds the handle and the loaded pouch at the same time, similar to an atlatlist who hold the dart through finger loops on the actual atlatl. Then the weight is accelerated in the normal slinging manner as if it were the actual pouch and projectile.

Once the pouch/projectile is released the outward moving weight acts like a pulley and causes the pouch to move at twice its own speed until the weight strikes the release toggle. Now the pouch spins around weight much like a whip cracking and sends the projectile out at supersonic speed (hopefully). Theoretically it should work. Please let us know if it does.

Some observations:
1. The weight will probably have to be substantial, so may require several revolutions to get it up to speed.
2. The weight will slow down as it accelerates the pouch/projectile assembly but the energy remaining in it due to its residual velocity will be wasted, ie. ergonomically, the system will be fairly inefficient.
3. Timing the release will be fiendishly difficult; the whole system is rotating as the weight slides down the string, so accuracy will be virtually impossible.

...

The problem is to effectively recover the energy stored in the weight. Similarly to Thomas' crossbow with heavy steel limbs which have high levels of non-recoverable kinetic energy, the weight must be moving for the compound sling to be working. All the energy still in the weight at the instant of the projectiles release will not be transferred to it and in that sense will be wasted. That's not to say it won't work, just that ergonomically it will be inefficient...

Aussie

Ludek,

Looks like on the southern hemisphere things go in differnt way Wink

On the northen one the "perpetum mobile" actually works

There have been said a lot in this thread, so I'm not going to advance arguments for proving it again.

Today (we have first snow) I shot two short movies to ilustrate how sliding mas works in practice. A careful slow motion or frame-by-frame analysis is highly recommended. I wish my camera had a higher frame rate. Because of the frame rate is only 15fs/s the demonstrations were done very slowly. The files are big - 6, 11 Mb.

http://www.kajakowe.com.pl/sling/MVI_9684.AVI

The next moovie seems to be clearer but there was something wrong with the red trigger cord that pulled the retaining cord.

http://www.kajakowe.com.pl/sling/MVI_9685.AVI

Jurek

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Aussie

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Re: Compound Sling
Reply #123 - Nov 23^rd, 2008, 5:33am

Greetings Jurek,

Great to hear from you! It's been some time.

With only dial-up so large a file will take a while to download. I may be able to see it at the library.

I'm glad the project worked, if my reservations proved groundless then fine as well.

So the question remains, does the additional complication translate into enough improvement in release velocity and how hard is it to control?

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Thomas Descens Rocks? Posts: 285 Gender:	Re: Compound Sling Reply #124 - Nov 23^rd, 2008, 10:45am Jurek The full screen video was very clear and your setup made the demo easy to understand. The idea for an armless trebuchet was quickly visualized, but not its advantages. tom
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Yurek

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Re: Compound Sling
Reply #125 - Nov 23^rd, 2008, 5:06pm

Ludek, Thomas,

Ludek, your reservations were as "grounless" as my idea of sliding mass was. Though my limited experience with the rather rough compound sling seems to prove the idea, not mentioning Matthias' computer simulations and tests with a real sling.

Quote:

...The idea for an armless trebuchet was quickly visualized, but not its advantages...

All depends of what adventage is considered. I have no doubs that a compound sling would be much more awkward and impractical in everyday use, targets sports, hunting etc. Its complication needs more advanced technology for more efficient action. A good design will probably need a bearing pulley connected with the mass, sophsticated design of very precise and light trigger, puch (or something like that)... Better matherials should be used either, lenghts and masses adjusted. However it all isn't able to gurantee that the device will be practical for the mentioned activities. So considering more advaced designs, a bow, crosbow or even firearm seem to be a better choise. Well, I still prefere the coventional simple sling.

Anyway, the main question in this thread was about if additional mass gives adventage with regard to muzzle velocity and if the sliding mass is able to transfer energy to a lightweight projectile efficiently in order accelerate it much more. The discussion was rather about prnciples.

Thomas, please compare carefully (in very slow motion) the maximum velocty of the sliding mass just before the release of the pouch with the velocity of the released snowball, you probably will notice the difference.

Quote:

...So the question remains, does the additional complication translate into enough improvement in release velocity and how hard is it to control?

It is actually more difficult to control the projectaile release, but still possible if we are not talkig about target slinging. Timing is different than in convential slinging. It is mainly because the projectile release is a bit delayed - there are two stages of the lash, or rather two lashes. The first one is similar like in normal slinging, ended by the pouch relesae (from the palm), the socond one is not controlled by slinger, short and rapid like hell, ended by automatic release of the projectile. Precision of the first release depends of slinger, the second one depends on precision of adjustment of the trigger and (i suppose) on random orientation of the pouch.

Hence you have to change your habits (it isn't easy) and start the stroke a bit sooner. How it works in practice you can on the new movies I made today.

Consider that the used projectile is very soft and light (<<20 grams) ball, that is a toy for cats. Although it lose its velocity very quickly because of air drag an its small mass, the energy of impacts is still very impressive.

BTW the multiple rotations are done only for feeling.

http://www.kajakowe.com.pl/sling/MVI_9693.AVI

http://www.kajakowe.com.pl/sling/MVI_9695.AVI

Greetings,

Jurek

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In the shape, structure and position of each stone, there is recorded a small piece of history. So, slinging them, we add a bit of our history to them.

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wanderer Interfector Viris Spurii Posts: 1266 Gender:	Re: Compound Sling Reply #126 - Nov 23^rd, 2008, 7:27pm Good to see you back Jurek. I see that the ball left a mark on the wall in the second clip. Certainly looks pretty impressive. Do you have any estimate of the speed?
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Thomas

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Re: Compound Sling
Reply #127 - Nov 24^th, 2008, 12:50am

Jurek

I ran the frame by frame and slow motion the first time I saw the video and noticed the dramatic speed increase.

My comment about the trebuchet was not directed to your slinging demo per say but to the moving pulley mass as applied to a siege weapon application of this general principal. However, I can not imagine an armless trebuchet with a vertical falling weight and pulley mechanism being very efficient.

tom

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Yurek

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Re: Compound Sling
Reply #128 - Nov 25^th, 2008, 2:34pm

Wanderer,

Thank you, your feedback is much appreciated.

A precise estimation is difficult, because the final release (the last movie) was somewhere between the last frame (A) which is showing the ball in the palm and the next one (B) showing the ball in fly.

We can see that the time from the frame (A) to the impact is ca. "two frames", it means
2*1/15 s = 1.33 s. (15 frames/s)

The distance to the gate was at least 10 m, I think. At my gues time from the {A} to final release of the ball from pouch was ca. 0.3 s.

So the time from the release to the impact:

T = 0.133 - 0.03 = 0.1 s

What gives the average velocity:

V = 10/0.1 = 100 m/s = 360 km/h = 224 mile/h

I felt the shot as very vibrant indeed, but I can't really say how the result differs from the reality. I have also no idea what was the muzzle velocity. The used ball is ligher and softer than a stress ball (BTW excelent ammo for indoor practice) so its velocity was decreasing rapidly. I even imagine that it was somewhat flattened on the begining of its fly due to air drag.

Jurek

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In the shape, structure and position of each stone, there is recorded a small piece of history. So, slinging them, we add a bit of our history to them.

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Thomas

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Re: Compound Sling
Reply #129 - Nov 26^th, 2008, 2:06am

Jurek

I used a miniature digital sound recorder and the Audacity sound program to get an interval of .158 sec for the third video based on the snap of the sling versus the ball impact. Then I assumed a distance to the slinger of 5 m.
This was subtracted from a total of 15 m to the wall to allow for the speed of sound.

.158 sec /10 m = .0158 preliminary interval for 1m
1/.0158 sec = 63.29 m/sec
15-5 =10m = distance traversed by sound to be deducted
335 m/sec = speed of sound
10/335= .0299 sec = interval to be deducted

.158-.0299 = .125 sec = corrected interval
.125 sec /10m = .0125 = interval for 1m
1/ .0125= 80 m/sec

tom

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David Morningstar Slinging.org Moderator Forum Moderator Slinging Rocks! Posts: 2886	Re: Compound Sling Reply #130 - Nov 26^th, 2008, 2:23am Hells teeth, that's fast!
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wanderer

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Re: Compound Sling
Reply #131 - Nov 26^th, 2008, 9:27am

Jurek and Thomas,

That's nice work! Given the uncertainties the two estimates seem pretty close - and frighteningly fast. If I had anywhere I could sling with a 360 degree safety zone (which I would need) I'd be inspired - again.

Hmm... just thought that if you can pick up the echo of the whip crack from the wall you could measure that distance as well.

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Dr.Q Tiro sometimes, the sharpest blade is not enough Posts: 35	Re: Compound Sling Reply #132 - Nov 26^th, 2008, 3:44pm The idea that you guys turn such a primitive device into a totally mathematical and graphic, is just plain exelent.
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Aussie

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Re: Compound Sling
Reply #133 - Nov 26^th, 2008, 8:46pm

Some approximations on energy with typical projectile masses and velocities from compound and conventional slings:

(Formula: KE = 1/2 m x v^2)

Compound sling:
Mass of projecile = 20 g Velocity = 100 m/s KE = 100.0 J

Conventional sling:
Mass of projectile = 75 g Velocity = 52 m/s KE = 101.4 J

The smaller projectile travelling at almost twice the speed has very close to the same energy as the conventionally powered larger but slower moving one.

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Yurek

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Re: Compound Sling
Reply #134 - Nov 27^th, 2008, 2:15pm

Quote from Yurek on Nov 25^th, 2008, 2:34pm:

...

We can see that the time from the frame (A) to the impact is ca. "two frames", it means
2*1/15 s = 1.33 s. (15 frames/s)

The distance to the gate was at least 10 m, I think. At my gues time from the {A} to final release of the ball from pouch was ca. 0.3 s.

So the time from the release to the impact:

T = 0.133 - 0.03 = 0.1 s

What gives the average velocity:

V = 10/0.1 = 100 m/s = 360 km/h = 224 mile/h

...

I just relezed that I had made a mistake in the calculations. Alas, the true is less impressive. I took wrong frame rate for them. My camera catches the 15 fms/s only for resolution 320 x 240, but I used the 640 x 480. In this case it gives only 10 fms/s, really poor rate.

Then the calculations (for the last video) should look like this:

We can see that the time from the frame (A) to the impact is ca. "two frames", it means
2*1/10 s = 0.2 s. (10 frames/s)

The distance to the gate was at least 10 m, I think. At my gues time from the {A} to final release of the ball from pouch was ca. 0.045 s (I increassed it from the 0.03, because before I estimated it as a part of the frame interval 1/15 s.).

So the time from the release to the impact:

T = 0.2 - 0.045 = 0.155 s

What gives the average velocity:

V = 10/0.155 = 64.5 m/s = 232 km/h = 144 miles/h (10 m ???)

Anyway, still impressive result considering that the ball was ultra light and soft. I'm pretty sure the result is impossible with a common sling and that kind of projectile.

Tom,

Thank you for the neat calculations. Yesterday I used VirtualDub to separate the soundtrack from the last video. Then, after removing noises I measured the time between the snap and the impact on the gate using Audacity. The time was 0.19 s.

http://www.kajakowe.com.pl/sling/audacity.jpg

Sound lag from the release point to the camera: 5 m/335 m/s = 0.015 s
Sound lag from the impact point to the camera: 15 m/335 m/s = 0.045 s

Then the corrected interval is: 0.19 s + 0.015 s - 0.045 s = 0.16 s

Then the average velocity is:

V = 10 m / 0.16 s = 63 m/s = 227 km/h = 141 miles/h

The result is very similar to the above one. However they both are still disputable because of the estimated distances. I'm going to verify them as soon I get an occasion.

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« Last Edit: Nov 28^th, 2008, 12:04pm by Yurek »

In the shape, structure and position of each stone, there is recorded a small piece of history. So, slinging them, we add a bit of our history to them.

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