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Darts, aerodynamic jump, and freeflight....

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Darts, aerodynamic jump, and freeflight....

Unread postAuthor: jeepkahn » Sat Jan 02, 2010 3:40 pm

http://www.arl.army.mil/arlreports/1999/ARL-TR-1872.pdf

Stumbled across this article for KE round flight characteristics, and thought it might come in handy for those of us that are into advanced projectile designing...

The short of it is how it relates to drag/fin stabilized rounds in the transition from launch to freeflight(aka aerodynamic jump) and how the launch disturbances affect flight before aerodynamics take over...

Even shorter version: If you use stabilized(non-spin) projectiles, barrel porting is your friend...
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Re: Darts, aerodynamic jump, and freeflight....

Unread postAuthor: USGF » Sat Jan 02, 2010 3:57 pm

jeepkahn wrote:http://www.arl.army.mil/arlreports/1999/ARL-TR-1872.pdf

Stumbled across this article for KE round flight characteristics, and thought it might come in handy for those of us that are into advanced projectile designing...

The short of it is how it relates to drag/fin stabilized rounds in the transition from launch to freeflight(aka aerodynamic jump) and how the launch disturbances affect flight before aerodynamics take over...

Even shorter version: If you use stabilized(non-spin) projectiles, barrel porting is your friend...


Jeep, I for one will look it over. Thanks and Happy New Year.

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Unread postAuthor: jeepkahn » Sat Jan 02, 2010 4:17 pm

It doesn't say that you need barrell porting, but with what it does describe it makes it quite easy to deduce that if launch disturbances(aka high residual muzzle pressure) can be reduced before the projectile exits the muzzle(via muzzle porting) there will be significantly less yaw created by muzzle blast, allowing for a faster transition to stabilized freeflight and truer trajectories and better accuracy and better retention of KE....
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Unread postAuthor: ramses » Sat Jan 02, 2010 5:38 pm

jeepkahn wrote:It doesn't say that you need barrell porting, but with what it does describe it makes it quite easy to deduce that if launch disturbances(aka high residual muzzle pressure) can be reduced before the projectile exits the muzzle(via muzzle porting) there will be significantly less yaw created by muzzle blast, allowing for a faster transition to stabilized freeflight and truer trajectories and better accuracy and better retention of KE....


so porting is your friend...
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Unread postAuthor: Moonbogg » Sat Jan 02, 2010 11:59 pm

My only concern with porting is that if you have a tight fitting projectile and the porting isn't perfectly calculated, then there will be a breaking effect on the round prior to exit.
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Unread postAuthor: ramses » Sun Jan 03, 2010 10:42 am

Moonbogg wrote:My only concern with porting is that if you have a tight fitting projectile and the porting isn't perfectly calculated, then there will be a breaking effect on the round prior to exit.


braking, not breaking. :D

The braking will be minimal. You won't pull a vacuum since there are holes to relieve the vacuum. The effect of a few inches of barrel should be negligible if you are at a velocity where you are dealing with penetrators.

Plus the lower velocity would be offset by less drag from less yaw.

Porting is your friend...
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Unread postAuthor: jeepkahn » Tue Jan 05, 2010 10:01 am

ramses wrote:
Moonbogg wrote:My only concern with porting is that if you have a tight fitting projectile and the porting isn't perfectly calculated, then there will be a breaking effect on the round prior to exit.


braking, not breaking. :D

The braking will be minimal.
Plus the lower velocity would be offset by less drag from less yaw.

Porting is your friend...


When dealing with several hundred pounds of M/E, an ounce of barrel friction induced drag will be more than made up for in gains in aerodynamic efficiency, I would venture to say that having the projectile stop accelerating prior to exiting the barrel, the gains in AJ stability would INCREASE downrange velocity.... But only in non-spin stabilized rounds, and the effect on balls and non-drag stabilized projectiles will be virtually nil...
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