Two Stage Boat Tail

D

Damon

Guest
I had a thought the other day that's been rattling around my head.

We know that the idea behind the boat tail is to reduce the base area, but we can't get too crazy with them because if you push the angle too far, the flow separates from the boat tail and negates its effect. So we're stuck with what we recognize as an optimum angle. What is it- 10 degrees or something like that?

Has anyone ever tried a two stage design where the boat tail angle increases half way into the boat tail (therefore reducing base area even more)? I can't believe I'd be the only one to think of such a thing, and I don't know much about bullet making or manufacturing concerns.

Anyone ever go down that rat hole?
 
Damon said:
..............................

We know that the idea behind the boat tail is to reduce the base area, but we can't get too crazy with them because if you push the angle too far, the flow separates from the boat tail and negates its effect.........
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we do?
 
Don't we?

Seriously, though. My limited understanding is that base drag being reduced is what we're after, and reducing the base area is what accomplishes that. Am I oversimplifying? Probably. Just wondering if anyone ever played with non-conical or multi-angle boat tails in an effort to make them more efficient.
 
what would you call rebated boattails? Henry Childs did some testing, and found many designs showed no improvement, but one did. Can't remember just what was what, though...
 
I guess that's my question. Is the rebated boat tail the result of much tinkering that has failed to produce results, or simply a stab at the problem that happens to be relatively easy to manufacture?
 
Reducing the drag too much can cause stability issues. The drag created by the base of the bullet increases the stability. A typical boat tail bullet requires a faster twist than a flat base already. The increase in B.C. is minimal after a boat tail is lengthened at a typical 10 degree angle for more than .15". You may still gain a little, but not a lot.

Michael
 
I recall that there were quite a few European projectiles with bases other than flat in times past, but as I recall, they all were hunting projectiles with designs specific to performance on specific sized game. Brenneke was one manufacturer that comes to mind, but a quick Google didn't turn up the projectile I was looking for which started with a steep boat tail form then took a second step in to virtually a point. Maybe I'm thinking of RWS.

Of course, Lapua produces/produced a rebated boat tail projectile.
 
mturner said:
You may still gain a little, but not a lot.
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To keep flow attached to a surface that is diverging away from the streamlines is difficult. Tricks like turbulence generators, suction and blowing can work, but how can this be accomplished on a bullet that costs less than a dollar? Solve this problem and you could be a billionaire. It's important not just for bullets, but also planes, ships and anywhere drag limits performance.

See the flow behavior in a diverging channel in this video at about 13:40 and after: http://www.youtube.com/watch?v=wMxK2GtFFq0&list=PL0EC6527BE871ABA3

Cheers,
Keith
 
mturner said:
Reducing the drag too much can cause stability issues. The drag created by the base of the bullet increases the stability. A typical boat tail bullet requires a faster twist than a flat base already. The increase in B.C. is minimal after a boat tail is lengthened at a typical 10 degree angle for more than .15". You may still gain a little, but not a lot.

Michael
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Yup. Pretty much everything with bullet design is a compromise, one factor weighted against another. You could design a bullet for a specific task, but you change one thing, even muzzle velocity, and the design parameters change.

R.G. Robinett made me up some 210-grain 30s on an 1.400-inch jacket, with a .175″ boattail, that stabilize in a 12-twist barrel (on "not frigid" days) at less than 3,000 fps MV. A longer boattail, like the VLD design, wouldn't be stable.

So what? Well, shot dispersion due to CG offset increases with rate of twist...

Another whole approach would be to increase pressure on the base of the bullet. That way, we'd need less twist for gyroscopic stability. Little "arrow fins" that pop out -- & VLDS in a 15-twist...but why a VLD, as you've already compromised on lower drag...

Actually, after the drag on the base, the next place to get lower drag is at the nose. But there are compromises there, too.
 
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supersonic, SUPERsonic, SUPERSONIC!!!

What's with "fluid dynamics" and "laminar flow" etc?

IMO the boattail is there to break the edge......and to add wt without adding bearing surface.
 
alinwa said:
supersonic, SUPERsonic, SUPERSONIC!!!

What's with "fluid dynamics" and "laminar flow" etc?

IMO the boattail is there to break the edge......and to add wt without adding bearing surface.
Click to expand...

Yes, it's supersonic, but there is still a boundary layer that detaches just like laminar and subsonic turbulent flow. Separation involves the same principles, it's just that supersonic flow has even more inertia, so it is even harder to get it to turn a corner. The shock wave adds something new, which may be your point. If the National Committee on Fluid Mechanics Films (the organization that made the video I linked) had done a video on supersonic flow, I would have recommended it instead.
 
alinwa said:
supersonic, SUPERsonic, SUPERSONIC!!!

What's with "fluid dynamics" and "laminar flow" etc?

IMO the boattail is there to break the edge......and to add wt without adding bearing surface.
Click to expand...


Look at Bryan Litz's book starting on page 253. He does a good job explaining how a boat tail reduces drag and the effect on air flow if you have too much angle.
 
This seems to be mainly a discussion of a bullet in flight, but don't forget about the fact that you have to get it up to speed in a barrel with a bunch of pressure behind it....

Hovis
 
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