Good job Hondero!

I'll throw in my vote that we add a (substantial ) "physics of the sling chapter" to "Project Goliath". I've been working on a couple of projects that evolved out of the last thread, so I hope to have some interesting info to contribute in the near future...

(unfortunately, my most recent "experimental" sling session came to a very abrupt end when my "aero" projectile disappeared into the woods. I'd like to believe that it was due to the incredible range but the truth is that the cords wrapped... can you "shank" a sling throw? - we need some better terminology)

Looking forward to reading your next post!

Matthias

Matthias, of course the physics of the sling can be a good chapter in the megaproject of the sling, the Goliath (good name Chris!), and in addition to be accessible separately in the site like a tool of work and orientacion for the slingers.

Yurek, it seems interesting to separately develop the physics of the throw as you say, although all the subjects are so related that there will be considered in different places. A correct and optimized style is an essential aspect in all the sports, that besides usually is performed the same way in each sport (serve in tennis, swing in golf, pitch in baseball). Here it would be necessary to speak of the different uses of the sling (distance, accjuracy, impact, penetration, etc) and the most effective styles in each case. Also it seems interesting to make an introduction to the subject, to detail an index, since as we are going to work in team it is the way of not losing in the great diversity of involved aspects. So I will delay the first point relative to the ballistic simulator and will try that introduction or exposition of the problem to be discussed.

1- INTRODUCTION
The throw begins with the development of a muscular effort that is translated in the acceleration of the sling. This muscular effort is a chained serie of efforts that are made with the hips, torso, shoulder, arm, forearm and wrist.  At the end the projectile is released with a kinetic energy that depend of its mass and the initial speed. Each slinger has a maximum force to develop, and therefore the obtained speed will be greater whichever minor is the mass of the projectile. The length of the sling influences the speed of launching, but not as much as it could seem, since after all the kinetic energy of the projectile comes from the muscular effort, which is limited, and as longer is the sling as much is the energy required to accelerate the projectile.
The projectile is affected in its flight by the air drag, which exerts a force that is proportional to the projectile frontal surface and to the square of its speed. It seems that the air drag it will affect more to the small projectiles that to the great ones, since its greater speed is more detrimental that its reduced section. For a same mass projectile, and therefore equal speed of launching, the use of dense materials, as the lead, provides small projectiles, of little frontal section and therefore the air resistance is smaller. To equality of size and different weight, the air resistance due to the frontal section will be equal, although the speed of the heaviest projectile will be smaller and the total resistance will be smaller too. All these factors are related in a complex way, and the best thing to calculate results is to have a balistic simulator. It avoids to enter complicated mathematical formulations, being enough the definition of the parameters to input.

Other factors to consider are the form of the projectile and its aerodynamic behavior. The fusiform projectiles present  less frontal section that the spherical ones for the same weight, supposing a flight according to the greater axis, or with little inclination. But the projectile must be thrown point-first and spinning on its axis to conserve the stability, which takes us to the study of the dynamics of the projectile within the pouch, or " interior ballistic".

The projectile surface can be important, offering smaller air drag the rough surfaces that the smooth ones, due to creation of turbulences that diminish the air drag Also this rugosity increases the Magnus effect, what can be used to get greater elevation of the projectile in the flight and prolongation of the range. But this effect will depend much on the density of the projectile and will be despicable for lead projectiles that are those that has better behavior for distance.

It is also important the sizing of the sling and its relation with the weight of the projectile. The cord and pouch air drag must be controlled, as well as the design of the pouch. It  could be said  that the sling must be designed for the optimal projectile and not contrariwise.

Well, I´m exhaust and have the feeling that all this is not much useful ??? ...and maybe I´ve forgotten some points to consider  for making the detailed index:.

You guys should make a formal comitment about working on the "Physics of The Sling" section.  I'll pencil you in and it will be all yours.  I think you two should co-author it.  Maybe rope Matthias in aswell, who has some great ideas. 

Think about all the different topics within this section you want to cover. 


• PHYSICS OF THE SLING
- releasing the retention cord causes the pouch to open, and the projectile to fly out tangentially.
- mechanical advantage of the sling.
- ratio of length to potential speed?
- drag, air resistance? 
- pouch design, and it's physics ramifications
- projectile design (tons of stuff to consider here, although don't get into historical evidence, as thats covered in other sections.) 

etc.

Develop this among yourselves (by email perhaps), present it to me, and We'll make an official update to the outline. 

Chris

RANGE WITH LEAD GLANDES

Let us see now the range that would have got Larry using a lead projectile of similar shape to the one of stone that he used and the same weight of 32 gr, that is to say, a small sort of glans not very perfected. I suppose the same CD = 0,3 and the value that we obtain for c/m is 0.00077. The speed of launching would be the same 396 Km/h that we had considered previously for the stone projectile. The obtained range would have been of 747 ms, a 70% longer than with the stone projectile. We see that now the lost of range with respect to the shot without air is only of 40%, whereas with the stone projectile it was of 63 %.

If my calculations are right this is astonishing!! With a lead glande even I could beat the present record!
Yurek, congratulations for your coming new record of distance with sling!!  

P.D  Well... let´s be a little humble...  , the most likely is that the little glans can´t fly perfectly point-first oriented and so its cd and the frontal section are greater. I guess cd =0.4 and c/m = 0.00125. The range would be then about  616 m. Also very good.

Mathias, your experiments with the ballistic pendulum, throwing glandes at very close range  , will be definitive. I think is the only way to know some critical thinks like the actual ratio weight-speed of the projectile. This will allow to conect the physic of the throw with the physics of the projectil fly and thus find the optimal weight of the projectil. This could be calculated theoretically, and Yurek (and myself too) is working on the subjet, but in my opinion the intrincacy of it is great and the results might be only rough.
I know you are a clever craftsman and can accomplish the task. I look forward to your experiments 

Hondero,

You are the most exemplary partner among us You one and only are doing the declared job as yet. I'm just trying to gather all my thoughts, but my mind is a bit lazy due to the hot and sunny summer we have Besides, I spend quite a lot time outdoors with my familly and also try to practice a bit, we need to use some our short summer. So, please be patient and give me some time.

That simulator really seems to be the best for our use among others we have, because it considers the drag coefficient and mass of projectiles. Your estimations looks likely.

Matthias,

I was been thinking about the ballistic pendulum too, but I haven't got the idea of the soft glandes (made of plasticine). Great idea! BTW, lead glandes during hiting concrete slabs with a big velocity, act very similar to plasticine Plasticine glandes would be also good for testing different its shapes, dimples etc. Looks like they are perfect, easy to changing test amo, but not during hot days, because they could become deformed during releases.

I also thought about stroboscopic photography, but I haven't a suitable lamp. I wonder, if there is a simple domestic way of doing it. Any ideas? Even useing a usual and cheap analog camcoder should be good too. It can be set at "high-speed-shutter mode". In this case we get the frames with clear pictures of the sling and projectile. If we know the distance between stone in following frames and the frequency of the frames, we are able to calculate the velocity of the projectile easily. I think it is a cheap and pretty accurate way. I already desrcibed that one on the forum sometime.

Quote:


I suppose, it would be enough to fix the led on the pouch and thin wires could be drawn along the retention cord. It would be cheaper and easier.

Jurek

George Alsatian just submitted a pretty comprehensive review of the physics involved with slinging.  You should check it out asap in the articles section, and perhaps work with him developing the section.  I was pretty impressed with my quick glance through it.  I might print it out and read it thoroughly soon (just too busy now).  

Let me know what you think.
Chris