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Bullet RPM ......

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Steelbenz View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 11:12
I too have heard the " you have to let the bullet go to sleep" statement but like RC, I can't see the math.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote supertool73 Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 11:26
Bryan Litz pretty much disproved the whole going to sleep theory in his book applied ballistics.  At least as far as the bullet having a lot of "yaw" when it first comes out of the tube and them stabilizing more and more as it goes farther and farther to the point that it actually changes accuracy at farther distances.  Here is the section on it.

http://www.appliedballisticsllc.com/epswerve.html
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 13:25

I can describe and possible give you the calculation exactly what do you need from me? Be specific.

I have had many interesting discussions in person and email and phone and have developed things(formulas and tools) that were theories to some of the mentioned people and we put them into practice.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote dsr Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 14:05
Originally posted by Rancid Coolaid Rancid Coolaid wrote:

Could you explain how the bullet becomes more stable in flight?


As the bullet travels down range the aerodynamic force is reduced which can be considered applied at the center of pressure which is not located at the center of gravity. The resulting two forces applied, one not varying (mass) applied at the center of mass, and one decreasing with speed applied at the center of pressure reduces the overturning fores (moment). Making the bullet more stable.

No where in my preceding post did I mention the bullet "going to sleep".

All the above is contained in Litz's book.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 14:24
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steelbenz Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 14:38
LITZ used the term. Still doesn't matter, the longer bullet needs to spin faster for the very reason I've stated. Litz would also agree. As the bullet slows force applied to the bullet makes it want to fall over just like a top, only on a different plane.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote dsr Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 15:53
Steelbenz,

To see what I am describing read the chapter on bullet stability. Specifically the section on dynamic stability vs gyroscopic stability. (ie - the horseshoe graph).

I do agree with your statement on why a longer bullet needs a faster twist rate but I believe the chapter on bullet stability will clarify your understanding of the dynamics better.

Respectfully,
dsr
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 17:22

Vp has much to do with this as well.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote dsr Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 17:37
Originally posted by dsr dsr wrote:

Originally posted by Steelbenz Steelbenz wrote:


As RC noted rotation is degrading as fast as velocity as soon as it leaves the barrel ...
(My emphasis added)


Steelbenz statement that rotation is degrading as fast as velocity is false according to Litz. It remains close to its muzzle value.

Looking at the problem from a different view ( remembering that rotation is basically constant for the bullets flight) and using the example earlier in this thread. A .308 with a muzzle velocity of 3000 ft/s in a 1 in 12 twist rate give 3000 rotations per second.

And let's say for sake of argument ( round numbers) that it has a 1000 yard velocity of 1100 ft/s and since we agree that the rotation is still 3000 rotations per second. Reverse solving for twist rate gives 1 in 4.4 twist rate.

Using JBM Ballistics stability calculator with the following inputs at the muzzle.
.308 caliber, 1.5 inches length, 220 grains, 0.0 plastic tip, 3000 ft/s, twist rate 12, temp 59, pressure 29.92. Gives a stability factor of 1.265 at the muzzle.

And as agreed to for this argument the rotation of the bullet at 1000 yards still has 3000 turns/second with a velocity of 1100 ft/s with an effective twist rate of 1 in 4.4. Re-running the JBM calculations with the new velocity and twist rate gives a stability factor of 6.775.

So we see that the bullet has gained stability.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote dsr Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 17:43
Originally posted by 338LAPUASLAP 338LAPUASLAP wrote:

Vp has much to do with this as well.



I am sorry 338LAPUASLAP I don't know what "Vp" is.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 18:13
Velocity Pressure Wave and Drag or Shock wave from bullets tip and surface area along with the twist rate loss factor and the left and right running bow waves and tail waves resulting in a turbulent tail wave and turbulent wake wave that apply force/pressure as the bullet is pulled downward by gravity and the density factor of the air it travels through resulting in this gradually slowing revs p/m resultant from force rotational drag factor.  (Anyone remember the old free bore boost effect on rpm of bullet?)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 18:13

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 18:16

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Post Options Post Options   Thanks (0) Thanks(0)   Quote dsr Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 18:26
Pressures across the surface of a body can be integrated to produce a resultant vector and a moment on the body.

I can can do the pressure calculations on the surface of a right cone relativly easly in supersonic flow but I don't know any relatively easy closed form solutions to a compression corner, expansion fan, expansion fan, compression corner for a body of rotation.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote dsr Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 18:31
In the top most shadow graph one can see the shock waves as thin black oblique lines at the front and rear of the bullet. The expansion fans are the less distinct oblique lines between the shock waves.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 18:32
Wah Wah did did you you say say?

We all have much to learn.

I understand some of what you posted above in differing degrees in a variety of other fields and scenarios but what is exactly are you referring or making a inference to in regards to the above posted question or the bullet rpm post?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 338LAPUASLAP Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 18:44
I looked back at what I think you were trying to draw conclusion or reference to.  

So that you know you use a factor for the collisions.  Each Lab has their own calculations or Sub factors.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Peddler Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 19:55
I don't care much what it does on the way to the " Boiler Room " just as long as it kills when it gets there!!!

Mine usually do!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Kickboxer Quote  Post ReplyReply Direct Link To This Post Posted: February/18/2016 at 20:01
The more you know about the flight characteristics of the projectile, the better prepared you are for putting it "on target" at whatever range you choose.  If you are a "control freak", you want to know these things...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote budperm Quote  Post ReplyReply Direct Link To This Post Posted: February/19/2016 at 05:52

Bet you've lost sleep over  it......



On second thought, you NEVER seem to sleep so probably NOT... Smile

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