velocity in relation to barrle twist

[Mikecr]

That was the way I thought as well. RPMs.

I believe that if you explore it further though, you'll find that its revolutions per distance. Meaning -it don't matter how fast its going, or what the RPMs are. If a bullet requires 1 turn every 8", then thats what you have to give it.
Revolutions per distance is set purely in twist(1:8"). No amount of velocity or resulting rpms change that. Only twist rate.
Difficult for me to grasp, but I'm jumpin the fence anyway. Sorry

 

[dwm]

[ 02-08-2004: Message edited by: dwm ]

 

[Mikecr]

This link brings you to a bullet design spreadsheet: http://www.angelfire.com/id/psycho/images/BDESIGN.xls
I don't know the source.
But It looks alot like Bob McCoy's work.

I preloaded a .224 80gr Berger into it and The results show a 7.5 twist as required for SG=1.5, with air density and velocity adjusted to correct all other resultant parameters.

Now if you change velocity only, even dramatically, you will notice that the twist rate required barely changes.
If you think its wrong, or you know of any stability program showing otherwise, Please steer me towards it.

 

[Michael Eichele]

Mikecr,

That is the same download from the Lilja web site. That is correct. The stability factor changes little with velocity, but I will still take the slowest twist I can get!

I tend to lean twards the twist calculator on the jbm page and the reloader's archive program. They both seem to match every bullet twist combo I have tried.

[ 02-08-2004: Message edited by: meichele ]

 

[MAX]

Looks like I was hanging out somewhere else for awhile.

Twist rate is the key because a) substantial change in velocity induced RPM(or S) requires gross change in velocity, which is counterproductive to the task or impossible to achieve; b) remember, GS is required because of that difference in position between CG and CP. You may gain additional GS with increased velocity, but the overturning moments grow as well. It is a fools errand.

Bullets with thin jackets are less tolerant of fast twist rates because of thin jackets(structural), and that the energy of angular momentum squares as a function of velocity like other energy calculations, in this case, RPM(S). These bullets tend to higher uniformity and accuracy, but their operational window is smaller. A smooth bore MAY stabilize one at a slower twist rate, all things being equal, but the difference is very small. So small in fact that changes in atmospheric density may undo that advantage.

I recognize the benefits of slower twists regarding accuracy, but in the main these advantages are discernable only to a small degree in very controlled circumstance, such as benchrest competition. If you wish to shoot a "55 gr bullet" and maybe leave the option open for heavier projectiles, optimize your twist for the heavies as your lighter projectiles will probably shoot about as well. It is a lot easier to deal with too much twist than not enough!

Personally, I would build the gun for the bullet and not play hopscotch.

 

[dwm]

Ok, so how does a gain twist barrel work in this very interesting model?

Also, a bullet will gyroscopically stablize when stood on end on the floor and spun like a top. All those stability equations apply. No forward velocity at all. So much for stability vs forward velocity.

But what about stability vs RPM? No revolutions or RPM, no stability, right? Top falls over when RPM approach zero. But forward velocity and RPM are linked together by the twist rate. Man this just keeps on going around in circles. I'm getting dizzy.

The RPM vs forward velocity thing was very enlightening. As forward velocity increases so does RPM, but strangely enough the bullet still turns 1 revolution in x inches no matter what the forward velocity is. It is fixed by the twist rate. (Ouch,

)

Thanks,

Doug

I am beginning to think all barrels should be gain twist.

By the way, I do have McCoy's book.

[ 02-09-2004: Message edited by: dwm ]

 

[Mikecr]

I thought gain twists reduced potential core from slipping, and increased velocity due to a flatter pressure curve.
But then, I also intially thought that stability was rpm dependent. I still cannot grasp turns/distance, even though I believe it.

 

[MAX]

Gain twist rate is measured by the twist rate at the muzzle, and has no particular significance in regard to GS.

Gyroscopic stability factor increases with range because the rate of linear velocity decays MUCH more quickly than RPM, and aerodynamic forces are likewise diminishing.

The illustration of the static but spinning top is not pertinent. Spitzer bullets, both flat base and boat tail designs, generally have CG aft of CP(center of aerodynamic pressure), and this is the reason that they have to be stabilized by twist in the rifling. LONGER bullets, such as boat tails and copper alloy designs generally have a greater distance betwen the two, thus requiring faster twists. It is noted that even round balls benefit from a small amount of twist, as are all aerodynamically stable projectiles, but there are only three ways I'm aware of to alter the equation for GS. 1)core density, 2) moving the CG, and 3)changing the diameter of the bullet.

Item 1: using lead as a basis for comparison, tungsten is denser, copper less so. Item 2 might be accomplished by having a low density base core such as aluminum, copper, or plastic. In 3, Angular Momentum is changed in this example because at a GIVEN RATE of rotation, there is more angular velocity/energy in a larger caliber bullet and less in a smaller one. The energy change is NOT linear to diameter. GS is all about angular momentum vs the overturning moment of aerodynamic forces. Items 1 and 3 involve change in angular momentum, while number 2 will affect the distance between CG and CP, thus changing the amount of AM required for stability.

I'm not a rocket scientists but can refer you to a couple if you desire(books). It is perhaps easier to understand if you look at the math formulas and give it a bit of thought. I don't find it easy to explain in discussion.
http://www.nennstiel-ruprecht.de/bullfly/index.htm#Formulas

Maybe that'll help.