Brain trust question about lead projectiles

[lane912]

casting round shot for black powder rifle, has anyone tried a dimpled sphere like a golf ball?
do you think there would be any performance gain v.s. pressure slipping by the dimple?

discuss-

please show your math

[Mo_Gearhead]

QUOTE: "...any performance gain"
_________________

You need to define "performance".

Are you chasing more velocity (fps) ...or enhanced accuracy?

[John Rogers]

On a website named castboolits.gunloads.com which has a lot of info on casting bullets, this was mentioned and one or two actually tried it.....did not work. The consensus was the dimples made the round ball act like a golf ball and they they temp to go where they want to it seems. Smooth round balls and a nice tight fitting pillow ticking patch generally works best.

[lane912]

Originally Posted by Mo_Gearhead

QUOTE: "...any performance gain"
_________________

You need to define "performance".

Are you chasing more velocity (fps) ...or enhanced accuracy?

really just looking for info on a discussion had over scotch one night-

if...... then Why not.....

[jyl]

Golf ball dimples generate lift from spin. Golf balls travel with spin, specifically underspin (a ball moving from right to left will be spinning clockwise). The dimples increase air pressure below the ball and decrease air pressure above the ball, creating lift.

Black powder guns are rifled (usually) so the projectiles spin, but it is a spin around an axis that is the same as the projectile's direction of travel, like a football. Dimples on such a "ball" would not create lift in the same way as a dimpled golf ball has lift. It might create some other force, especially if fired in a cross-wind, and I'd guess that force is likely to be undesirable.

http://www.leaderboard.com/WHYDIMPS.htm

[island911]

That's not quite right, John.

Golf ball dimples generate turbulence, which knocks back the 'lift' (usually drag)

Any spinning sphere with forward velocity will produce the lift you speak of.

Because Golf ball dimples generate turbulence, they help reduce forward velocity drag within a only a certain range of speed. --I'm going off of decades old memory here. But, it follows, that at higher speeds, (out of a gun) the sphere is already producing big turbulence. (no dimples needed to foil the otherwise nicely formed trailing air-pocket of 'lift'.

[jyl]

Originally Posted by island911

That's not quite right, John.

Golf ball dimples generate turbulence, which knocks back the 'lift' (usually drag)

Any spinning sphere with forward velocity will produce the lift you speak of.

Because Golf ball dimples generate turbulence, they help reduce forward velocity drag within a only a certain range of speed. --I'm going off of decades old memory here. But, it follows, that at higher speeds, (out of a gun) the sphere is already producing big turbulence. (no dimples needed to foil the otherwise nicely formed trailing air-pocket of 'lift'.

I think there are two different effects that dimples can have.

One is to reduce drag by creating a turbulent boundary layer. The object doesn't need to be spinning for that effect. This treatment is used on things that don't spin - bike frames, submarines, etc. I don't know much about when this effect is and isn't worthwhile - not every fast moving object is dimpled, so there must be some situations and locations when you don't want to use it.

The other is to change the trajectory by increasing air pressure on one side of a spinning object, the side that is spinning "forward" (in the direction of travel) and reduce it on the other side, the side that is spinning "backward" (away from the direction of travel). If the "forward" spinning side is on the bottom - backspin - the effect is to push the object's trajectory up, i.e. lift. If the forward spinning side is on the top - topspin - the effect is to push the object's trajectory down. If the forward spinning side is on the side - slice - the effect is to curve the object's trajectory to the right or left.

The second, spin-trajectory effect, is seen with golf balls and, perhaps more obviously, with tennis balls. It depends on friction between the surface of the ball and the air. Tennis balls are fuzzy so there is a lot of friction, and you can get more pronounced spin effect with a fresh fuzzy tennis ball than with an old bald one. Golf balls use dimples to increase friction.

But for this spin-trajectory effect, the ball has to be spinning on an axis different from the direction of travel. So I don't see it working for a rifle ball, which spins on the same axis as it is traveling on, like a football.

You raise an interesting point about speed. It makes sense that, if the ball is moving very fast relative to its spin (angular) velocity, the spin-trajectory effect would be reduced. And at some even higher speed, the shockwave might stymie the spin effect. But as long as the ball is spinning like a football, I don't see the spin-trajectory effect being useful.

[island911]

Originally Posted by jyl

I think there are two different effects that dimples can have. ...

Yeah, you can see it that way, but that would be like seeing gravity as acting sideways - as in; a soapy bathtub makes gravity pull your feet sideways.--as a second gravity that acts only on feet, and only sideways.

Anyway, my point was about looking at the varying velocity, and that the physics of dimples (surface roughness) are non-linear wrt velocity.

SOmewhere, decades ago, I saw the chart specifically for golfballs, but this will do.




And on that two thing...



Edit: diagrams credit to Roberson /Crowe (Crowe was one of my fluid dynamics profs)

[otto_kretschmer]

engineers arguing about a golf ball..



BMW put dimples/vortex generators on their System helmets in the 80s and it probably worked but I have not seen any helmet on the market with them since

[island911]

Cool picture. Helmets now often have a little spoiler.

I don't expect that JYL is an engineer. Just an inquisitive guy.

[tabs]

The dimples would allow gas to escape around them as the ball travels down the barrel. Also the dimples would not allow the rifling in the barrel to fully grab the ball.

[lane912]

and--
all of the responses are figuring the barrel is rifeled, what about a smooth bore?
from the responses it seems that the ball would "slice"

[jyl]

Originally Posted by island911

I don't expect that JYL is an engineer.

God no! But my couple years as a physics major comes in handy on occasion.

I wonder if there is a way for black powder enthusiasts to make a black powder rifle that fires the ball with backspin. Perhaps you'd get a flatter trajectory out to medium range. But at that point, why not simply make a black powder flechette launcher. Or a black powder Gyrojet? Or a black powder cruise missile.

Here is an interesting article on how tennis balls behave when fired from a tennis ball launcher with topspin, nospin, and backspin. I believe the same qualitative principles would affect the trajectory of a golf ball hit with backspin; AFAIK it is impossible to hit a golf ball with topspin.

Tennis Ball Trajectories — The Role of Aerodynamic Drag and Lift in Tennis Shots"

[cockerpunk]

casting such a ball would not work. you could not keep both the dimples even and consistent, and get the ball out of the mold.

thats why they don't work in paintball. in order to get them to demold, the dimples near the seam have to shallower, and so it creates an unequal surface and boundary layer, which basically means, it amplifies the effect of the seam imperfection.

accuracy was substantially reduced.

[island911]

Originally Posted by jyl

God no! ...

Ha!

Physics, eh? So what's your excuse for being so weak on the subject? Theoretical physics?

Originally Posted by cockerpunk

casting such a ball would not work. .

Clearly you know little about molding. Hint: not all molds are two halves.

You're welcome.

[jyl]

I got the concepts fine but couldn't stand grinding out the solutions and tracking the units. Tedious. I switched majors after sophomore year to mathematics. More elegant. Less work. More time to party.

I think everyone should study some physics. Even if you don't grind out the numerical answer, having some sense of how things behave provides a reality check for all the BS that people try to feed you later in life.

Nowadays, I find the biological sciences way more interesting. People are making fundamental new discoveries all the time, in small labs, with increasingly cheap DNA sequencing hardware. Don't need to be European and affiliated with a multi billion dollar accelerator to have a chance of making major new discoveries. Unfort, it is way too late for me to learn much about genetics and molecular biology. Too old, too dull. My daughter may study it in college, fingers crossed.

Quote de jyl

God no! ...

Ha!



Physics, eh? So what's your excuse for being so weak on the subject? Theoretical physics?

Quote de cockerpunk

casting such a ball would not work. .

Clearly you know little about molding. Hint: not all molds are two halves.



You're welcome.

[Bill Douglas]

Originally Posted by tabs

The dimples would allow gas to escape around them as the ball travels down the barrel. Also the dimples would not allow the rifling in the barrel to fully grab the ball.

Yes.

And the ball needs to be about two thou bigger than the bore to get a perfectly tight seal - no gasses escaping. Dimples pimples won't help. They can be very accurate; who was the fella who said "Don't worry about the English, they are 1,000 yards away."

[cockerpunk]

Originally Posted by island911

Ha!

Physics, eh? So what's your excuse for being so weak on the subject? Theoretical physics?

Clearly you know little about molding. Hint: not all molds are two halves.

You're welcome.

adding more seams ..... yeah, thats a bad plan for any projectile, esp a round ball.

you will loose even more accuracy for every new seam you add.

[jyl]

Why do black powder rifle fire round balls? Is it just an accident of history, or is that shape actually optimal?

[island911]

Originally Posted by cockerpunk

adding more seams ..... yeah, thats a bad plan for any projectile, esp a round ball.

you will loose even more accuracy for every new seam you add.

doubling down to pull more from yer ass, I see.

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