Here is a nice "dimpling calculator" it only works for spheres, and only gives forces. The drag coefficients calculated by this one give very slightly less of an edge to the dimpled shape than what I used above (and the previoudly linked article claims) - not enough to make a very big difference.

http://www.mame.syr.edu/simfluid/redder/dragforce/DragForce.html

You can answer the length/diameter question (like most questions actually) without using any math/engineering. Historical glandes are shaped that way for a reason, as are bird bodies and (american) footballs. All with different constraints, of course. If you take pure aerodynamic drag with a ton of assumptions, the trade off between form drag (what we're talking about) and skin friction, usually occurs somewhere around 30%




Turbulent flow (induced or otherwise) will flatten the curve, so that anywhere from 2:1 to about 4:1 should be just fine. By the time you take into acount the effects of a possible "off-axis" (even by a small amount) throw, 2:1ish is looking pretty attractive...



One thing to keep in mind with these graphs is that we are only looking at cd for a shape with no scaling. An ellipse of same mass as a sphere will have a smaller projected area, so total drag for matching projectiles will be as much as 50% lower again (1/4 sphere drag! why was it that we like these things so much ) The other nice thing is that as cd goes down, the advantage you get from lead is reduced as well, which gives some of us other guys a fighting chance!

Matthias

I see that the question has been revived again!

Here is the other nice toy, the Golf ball Trajectory Simulator:

http://www.ecs.syr.edu/centers/simfluid/green/traject.html

Matthias could compare his result with that one. Pitty it doesn't allow to change the density and ratio as parameters

Mgreenfield,

Quote:


I rolled the glande model of heat-hardened modelling paste for children, and next imprinted the dimples with the rounded ending of one of my ball pens That one seemed to have the suitable shape and size.

Here is the reposted picture, alas that one is from the flat scaner and its quality is poor.



Jurek

Yup, I guess we are going to have to keep dredging up old threads until we come up with a definive answer. I'll put some more effort in this weekend. You can tell that this week has been mind-numbing because my post freq has gone waay up... I've been doing sling calcs at work to spite my boss, who won't give me any time off... that guy is a loser (the joys of being self-employed)

I'd suggest that Yurek's dimples are much too large for the size of the glandes, to the point where they would start to harm performance. If you can achieve consistent point-forward flight, the ideal could be a single raised sharp edged ridge (or dashed ring) about 25-30% back from the forward edge.

If people want to play around with experimental glans design, I might suggest building large scale, light projectiles. A tennis ball sized series of wood projectiles would give pretty solid feedback (differences in drag are emphasised) and be easier to test and retrieve. Any advantages that turn up would be applicable with smaller/denser ammo...

Matthias

Matthias, .....this discussion of aerodynamics has gotta end up in the articles section, I say.    It's dynamite good info.     mgreenfield

I've cut up dozens of golf balls... They used to be (10-15 years ago) consistently rubber band wound around around (usually) a grape sized black rubber ball, which was often liquid filled. Lately, every ball I've cut up is a solid resin core with a hard/tough white cover. Some "range balls" are one piece.

A quick survey of an online golf ball shop (golfball.ch) reveals the following selection:

2 wound balls
36 2-piece (white cover around a solid core)
20 3-piece balls (two piece with an extra thin layer under the cover) _{**cough**-gimmick!-**cough**}
6 4-piece balls _{**cough**-expensive gimmick!-**cough**}

Matthias

Jurek
that golf projectile at the left side looks pretty cool   
Great job!

Thanks Zorrro!

English, I'm not a golf player but I think that just clubs create  the backspin. The dimples only streighten the Mangus' effect and reduce the air drag of golf balls.

Jurek

As several people have explained, the dimples do increase range in a few ways, including reducing drag.  This is also why tennis balls are furry. 

Although the density of a lead projectile is far greater than a golf ball, I think dimples would still be beneficial.  One of the things to remember is that sling projectiles have long flight times, perhaps similar to golf balls at their slower speeds, or even greater.  If you could reduce drag by a fraction for a 6 second flight, that could extend your range by a noticeable amount. 

Yurek, how long are your projectiles in flight?

Chris