Boat tail design ????

[BT Sniper]

Here is a ? for you guys. Why is the design of boat tails a straight taper? Wouldn't it be more aerodynamic to have a curved design. Say a 3 or 4 diameter ojive finished of with flat base of course.

[Bullshop Junior]

It would seem if it was better that is what would be made. And the strait angle is way easyer to make.

[windrider919]

If you search for VLD / Very Low Drag bullets on the net you will find some made of solid copper that have a base shape like that. However, research done on torpedo shape by the US Navy and aerodynamic research by a German scientist named Kamm ( and before that Krupp testing of projectile shape) showed that the best shape for the trailing edge was a cone or truncated cone of max 12 degrees a side / 22 degrees included. With the truncated cone the airflow behind the boat tail kept the shape of the [missing] cone without significant drag vortex. With a rounded base the airflow separates once the angle goes over 14 degrees and goes turbulent, increasing drag over what even a flat base has. Supposedly, too, when the bullet speed drops through the sonic barrier the round based bullet is unstable and will yaw and tumble off course even though spinning for stability because the turbulence flops from side to side behind the bullet chaotically instead of staying centered . Oddly enough, this does NOT apply to round ball bullets or golf balls, baseballs, etc. Only long, projectiles.

http://i290.photobucket.com/albums/ll249/windrider919/LM-102.jpg
LM-102 Kupfer VLD bullet with very low drag tail

http://i290.photobucket.com/albums/ll249/windrider919/LM-101-KupferKohleVLDwmicrogroovesf.jpg
LM-101 Kupfer VLD conventional BT

[BT Sniper]

Exelent reply, thanks guys. I had read too that 10-12 degrees on the boat tail was the magic number.

Windrider, Very well put. Thanks.

Straight taper it is!

Good shooting guys,

Brian

[MIBULLETS]

Many even go as far as 7 degrees. I have heard 7 - 12 degrees with the lower number being the higher ballistic coefficient.

[303Guy]

... this does NOT apply to round ball bullets or golf balls,Actually, it does. The airflow breaks away just past the 'equator' of a sphere. A golf ball is dimpled to improve it's aerodynamics. The dimpling causes the airflow to break away further back from it's 'equator' giving it a boat tail effect. I wonder how that principle could be applied to boolits?

[windrider919]

Actually, I read an article that NASA had tried the dimples on the skin of a plane and it did NOT work to keep smooth airflow. In fact the plane barely could fly. It works on small round objects but not on anything else. The Navy has tried a synthetic porpoise skin that did increase speed of the ship. And the Air Force tested tested patches of plastic that had a surface duplicating sharks skin on a F-16 in critical aerodynamic areas and found they could increase turning rate and lower speed while maintaining stable flight but that at high speed turbulence was so bad they killed the project. So surfaces can make a great difference but its hard to predict what works and what does not. I read an article about 20 years ago where a 1 foot spheres were dimpled like a golf ball and shot out of an air cannon. The balls were both in scale (large dimples, same number as on a real golf ball) and in size ( small dimples , actual size as a golf ball but numbering thousands to cover the surface of a 1FT sphere) and neither flew as well as a smooth sphere. The point is, the dimples work in a particular situation but not anything else. My thought is "Since dimples do NOT work on much, who thought to put them on a golf ball in the first place?"

But my actual comment was about the rapid "whipping" of the turbulent vortex airflow trailing a round based long projectile as being different than the "consistant" / 'steady' / 'semetrical' airflow behind a round ball. It is very hard to describe in words but the very high speed film of the shockwaves showed it.

[BT Sniper]

http://i636.photobucket.com/albums/uu87/BTSniper/croppedbullet.png

OK here is what I had in mind. Did a little photo shop work on one of my bullets. Basicly sliced the bottom off and pasted a mirrored image of the tip of the bullet in it's place. Chopped off the tip (now on the base of the bullet) at a point that looked good.

It would seem more streamlined and not sure the angle would exceed 12 degrees. If it did it certainly could be changed.

I know it all has to do with airflow and seperation with the surface of the base of the bullet, supersonic shock waves and all that. Actually spent alot of time studying aerodinamics from an airline stand point but this idea was just a though that occupied the limited space in my head for a few minutes the other day.

Boat tails don't even come in to play till 300 yrds or maybe even further then I'm not sure I could make a bullet to match quality to begine with. It has been a fun topic none the less.

Good shooting,

BT

[BT Sniper]

I bet a smooth glof ball would have a hard time staying put on the green. Imagine a pitching wedge shot from between say 75-100 yrds and you are expecting the back spin of the ball to stop it on the green. Without any dimples it would have very little bite. I know the dimples some how make it fly straighter too but oh well golf balls make great targets :Fire: back to bullets.

[bohica2xo]

Golf balls are bullets to me. I launch them 400+ yards from a weapon...

But anyway, back to rifle projectiles.

While planes, golf balls, bullets, and birds all fly through the air, bullets went down a different path af few centuries ago. Seems some guy began cutting helical grooves in the bore...

Bullets are gyrostabilized, and generally spinning with a surface speed that is 5 to 15% of the foreward velocity at the muzzle. That changes everything. As the bullet slows down on the way to the target, it does not lose rotational speed nearly as fast. All sorts of factors in the equation.

There is still some debate regarding the transition from cylinder to boat tail. Sharp, radiused, rebated? I believe what you are proposing has been tried & abandoned by several people before you. The Coanda effect is thought to actually increase drag with a design like that.

If you really wat to drive yourself crazy, start hanging out with the 6PPC shooters...

B.

[Lead pot]

Actually, it does. The airflow breaks away just past the 'equator' of a sphere. A golf ball is dimpled to improve it's aerodynamics. The dimpling causes the airflow to break away further back from it's 'equator' giving it a boat tail effect. I wonder how that principle could be applied to boolits?

You mean like this bullet??
http://i22.photobucket.com/albums/b302/940Leadpot/bulletdimp250j4.jpg

[StarMetal]

Actually, it does. The airflow breaks away just past the 'equator' of a sphere. A golf ball is dimpled to improve it's aerodynamics. The dimpling causes the airflow to break away further back from it's 'equator' giving it a boat tail effect. I wonder how that principle could be applied to boolits?


Probably gas leaks if the dimpling is too deep.

Joe

[45 2.1]

Actually, it does. The airflow breaks away just past the 'equator' of a sphere. A golf ball is dimpled to improve it's aerodynamics. The dimpling causes the airflow to break away further back from it's 'equator' giving it a boat tail effect. I wonder how that principle could be applied to boolits?

Already has been on round balls. A roll with a rasp improves flight characteristics.

[MightyThor]

Another consideration is that, at ignition the gas pushes on the bullet base and, on a boattail must also push on the taper in a certain way. I would guess that the gas pushing on an ogive instead of a flat taper would induce all kinds of variable pressures on the boat tail that would be less predictable rather than more so.

[ANeat]

You could always load it in a sabot

[9.3X62AL]

Already has been on round balls. A roll with a rasp improves flight characteristics.

I see yet another experiment with the RB T/C Hawken on the horizon. Thanks a bunch, 45-2.1!