As I've mentioned elsewhere, it's been a long time since I did any hand slinging.  But my siege engine experience brought 'curve balls' to my attention and I wondered how you hand slingers delt with this effect.

For those who are un-aware of the situation:
When an object is cast (hurled, slung, shot, etc.) through the air, there are aerodynamic effects taking place.

In the case of a fairly uniform spherical object, which has spin, there is the "Magnus Effect" taking place.  In simple terms this is an unbalanced force that developes around the object, which can pull it of a purely ballistic path.

Think of a curve ball thrown in baseball, it's exactly the same thing taking place, for the same reasons.

For siege engines, there are many who attempted to maximize this effect for gaining greater distance. (This is done by increasing the back-spin rate of the object.)

Optimum launch angles for gaining maximum range is normally around 42 degrees above horizontal.  For pro's on the golf course though, the back-spin of the golf ball allows them to drive at much lower angles and the combination of the Magnus Effect combined with the low angle allows for a much greater range.

So, do any of you attempt to maximize your range through back-spin, or to control the amount of curve with side-spin?  Or do you just "let fly" with a lot of *oomph* behind it and cross your fingers?

I do see this effect quite often because most of the rocks I fire are oddly shaped. 

Because of their odd aerodynamics, some rocks will curve off unpredictably.  Also, as the irregular shape tumbles, it vibrates the air around it, causing sound waves.  It’s quite a high frequency because the projectile is quickly spinning.  You might have heard a buzzing or whizzing sound during your slinging adventures (?). 

As for taking advantage of back-spin, I think it’s highly unlikely with a sling.  Because the sling is such a variable weapon  (the pouch and cords could be at a different twist or the release angle could slightly off, etc), it would be hard to engineer a projectile or mechanism to achieve this effect consistently.   

I was chatting with my dad about his experience with throwing golfballs. He can get consistent backspin to "flatten" the parobolic flight path with a straight overhand release. This gives the same flight time but with a greater horizontal component and a smaller vertical component. This maximizes distance with golf balls, but doesn't come close to distances possible with a well thrown, football shaped stone spinning on its on its major axis.

I think nwmanitou has the backspin thing figured out.  Maybe look for posts by him.    LOTS! of other discussion of the magnus effect earlier on this forum.   mgreenfield

This is an old post but on an interesting subject so....Slinging with controled spin can be done but it's quite tricky.  You need a rough rather than smooth pocket, the suede side of leather should be fine. The one I used to use was flexable, flat though a little cupped from use,  it was  reversable, one side was a very rough and strong  canvas, the other was the plastic side of leather cloth ( which was for casting single stones without spin or shingle ) Thinking back I suppose the double layer may have made it relativly heavier, no idea if this has anything to do with it.  Next you need a a releace cord which is a few inches longer than normal and which doesn't have a knot at the end..   Normal cotten string is works for this but as there is no endstop whip the end with sewing thread to stop fraying and to give you something to grip.  In a normal throw one simply lets go of the releace cord, when throwing for spin the idea is to relax the fingers and thumb a little so the string passes through with a bit of resitance.  This holds the projectile in the pocket just a bit longer and gives it spin.  I didn't use a finger loop so my grip on the retention cord was based on a figure of eight around my little and ring fingers, a knot on the outside of the fist between them and the cord returning to the inside of it through the same gap..   The index and middle fingers  held the cord against the thumb.  One also emphasises the whip or pull away part of the cast  more. The spin you get will depend on the throwing style, under arm gives top spin, over arm gives backspin and side arm gives side.  When loading for spin don't center the stone as normal but put it nearer the retention cord so as much pocket gets to roll it in passing as posable.  It's not unlike spining a top with a whip in some ways.  You loose a bit of power and you don't get much spin in return, or I didn't, maybe one or two yards out of forty.  I found well shaped but rough stones worked better than well shaped smooth stones, logical really, bad shaped stones do their own thing anyway.  You'll know if you're getting it right by the sound: a loud buzz will become as normal when spining as a crack when not..  A warning though, your accuracy will suffer at first, it's almost another style, and it's harder work for the hand.

I only got back into slinging last year and hadn't even thought about spining untill I saw this thread.  When I was doing it I used my all purpose sling and didn't  practice the style for a high percentage of time  so all the above may be wrong.  Except that it definately works.  Hum.....a bespoke spinners sling.....well....long gutter shaped stone hugging pocket....hum.....well that parts easy.... high traction?.....paint with latex?....maybe ....or would it be overkill?......probably....must be some better way of making the releace than just string....whipped all the way?...certainly works a bit....a tiny braid?..  ..might be better.but with what?......drat.....yet another project.

Thanks cypher.  I've not used windups, except for very heavy rocks or long slings, so I'll try that.

I get spiral with my underhand wind up with a release that goes about 45° up and over my left shoulder (being right handed).  With my horrible rocks though, it's almost not worth bothering with any spin since their irregular shape throws them off course in unpredictable ways.  It's only when I spend a day or two collecting material from abroad that I get to enjoy the pleasure of controlled spin with casting.  I do enjoy the spiral a lot.  It looks like it would hurt, but I get better distance with the backspin.