Jungle, take a look back through the archives at our ongoing discussions on projectile shape and aerodynamics. We still need to transition some of those posts into the wiki.

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I didn't say you couldn't do it, just that you had to be careful . You can't simply scale down an object and test it without carefully adjusting the conditions to more accurately (and never perfectly, in most wind-tunnels) model the flow. For the purposes of estimating drag, or comparing drag of similar shapes, you usually want the reynold's numbers to match. For sling glandes, this is particularily important as we happen to be throwing at just about the right speed to cause all sorts of havoc with laminar/turbulent flow transitions. The bomb is *effectively* travelling much faster.

In terms of density, the material is much less dense than lead to be sure, but the object isn't, again thinking of scaling. If we assume that the bomb material is half as dense, and it is 20 times longer than our glans, when we shrink it down we get the following:

length = 20 times smaller
frontal area = 400 times smaller (!)
volume = 8000  times smaller (!!)
mass = 4000 times smaller

If we're talking about form drag, the important value is the projected area. The bomb is effectively 10 times more "dense" than the glans...

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From a previous post, we figured the performance of a 140g glans thrown at ~65 m/s:

Stone sphere - 280m
Lead sphere - 340m
Lead ellipsoid - 370m
Zero drag - 410m

You can see a pretty big jump between stone and lead, and a smaller one for the ellipse. The total possible improvement over the lead glans is about 10%. If you could get half of that, it would add about 20m, which is nothing to sneer at!

Interestingly, if you can manage to throw heavier projectiles, you pick up nice gains as well. A 200g lead sphere is not very much bigger than a 100g sphere, and the aerodynamics work out in your favour.

Fun stuff!

Matthias

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You are implying that 45deg. is no longer the standard?


I'll sling what he's slinging!







TS

Siguy,

Apologies for your long wait, I've been busy the last few days.  I finally dug out that chunk of steel rebar ("reinforcing bar") and slung it.  Three times, three styles: Greenfield underhand, Nwmanitou (or Spencer, or Weaver, or Ballard) figure-8, and Forsyth Apache.  Same results every time.  Based on watching my usual ammo, it was probably spinning at several hundred RPM (at least five revs per second).  But it was also precessing, at two or three complete cycles per second, at an angle of 30 or 40 degrees to the flight path.  The only thing I could have done to make it fly worse, would be to attach a parachute.

The sling I used has a flat (not cupped) pouch about an inch-and-a-quarter (a bit over 3 cm) wide, so the three-inch rebar stuck out both sides of the pouch.  I was careful to balance the rebar in the pouch, but it made no difference.  I do not think anything of these proportions can be slung effectively using an ordinary sling.  I am going back to my tennis balls, golf balls, and rocks with about a 2:1 length:width ratio.

Dale, Without fins to keep the bar point on, it isn't going to work. Unless the nose can be kept pointing into the relative wind you may as well stick with rocks.

I've already learned quite a bit about slings here. I hope that I can convince you to take a closer look at ballistics. This stuff is largely academic, but for someone wanting to break records it could be important.
45 degrees is never optimum for elevation of departure  of a ballistic object in the earth's atmosphere.
A 6:1 fineness ratio is near optimum for shapes in the range we are talking about.
If you take these two facts together with construction of the shape from dense material you will be closer to throwing as far as you are able.
You may have some other ideas and I'd be most interested in hearing them, but I think we can disregard those that run counter to known facts about ballistics.

So what shape is best in a slingable object? I've got a few charts listing shapes with CD values if you need help.
There is a lot to learn if you are so inclined. Thanks for the warm welcome.

No problem, I enjoy a good dose of sarcasm coming and going. I just thought a discussion of ballistics might bear fruit for those searching for answers.
I think flight archery has some interesting projectiles, although the practicioners can't often explain just why they get the results they do. I also find it interesting that ancient glandes use about a 2:1 fineness ratio for a very real improvement in drag reduction over more sphere-like objects(no dimples needed).
I surely don't know it all, but it is obvious where real gains can be made without too much effort.
I don't really care about who is right, only what is right. Enjoyed the conversation, and thanks for your many excellent posts on slinging.