## 6Dof bullet parameters

Started by mman, December 05, 2013, 11:24:44 AM

#### mman

#30
Quote from: admin on December 14, 2013, 07:32:20 PM
In intlift2 the machnumber is an input value. Not computed from anything
In bfx the speed of sound is taken to be constant, I have to look up its value?
If I'm going to use intlift and bfx 3dof solver together there has to be a conversion formula from m/s to mach number. Here is one:
http://www.engineeringtoolbox.com/speed-sound-d_82.html

But there are others. I'm just wondering which to use.

#### mman

#31

thx, have a look at the code in ad0915628 - i mentionned in a previous post that i salvaged much of it. however 6dof equations of motions are not so difficult to implement. the hard thing is to get the aerodynamic coefs right,

#### mman

#33
Quote from: admin on December 14, 2013, 11:46:46 PM
thx, have a look at the code in ad0915628 - i mentionned in a previous post that i salvaged much of it. however 6dof equations of motions are not so difficult to implement. the hard thing is to get the aerodynamic coefs right,
Okay, I didn't know it was this SPINNER code you meant.. It's a shame because it could have been pretty accurate at least according to pp presentation of it. And intlift doesn't give all the coefficients but it's a good start.

First version of gyroscopic stability (SG) calculator attached.
I used intlift2 results as an external table as Robert suggested. Sheet is example calculation for Berger 70 gr. VLD bullet. Bullet mass is actually additional input value and one can use it to "SOLVE JACKET THICKNESS" by pressing a button as jacked thickness is hard to measure. Then code iterates jacket thickness until bullet mass given is equivalent with bullet mass calculated from dimensions.

I spent a while wondering what's wrong with ogive ratio. Then I realized that these ogive ratios are not equivalent with other ogive dimensios in Litz's book.

Cool! a pitty that I cannot embark on the matter myself, I have other science to do.

I know that you can shoot the distances to test your own calculations. Can you measure the distance that your bullets start to toggle (shoot keyholes) because their velocity goes below that of sound (and the nose pressure gets a factor of 3 to 5 higher?).

I would reload as low as safe, to have low muzzle velocities and put paper targets every 10 meters in the neighbour hood where the bullets goes below the velocity of sound.

Probably statistics is involved, hence you should have statistics of the number of keyholes per meter....

?

#### mman

That is certainly possible. However I don't have the calculations to compare at the moment. This keyholing at transsonic velocity is usually due to dynamic instability which I cannot calculate. I would need couple more aerodynamic coefficients: spin damping moment coefficient and magnus moment coefficient.

If you look at lapua's radar drag data you can see possible effect of instability for various bullets at subsonic velocity regime. Drag usually seems to increase quite rapidly. Possibly there you could see keyholing as well.

but the missing values are small?

#### mman

Well, no. When it comes to dynamic stability, without them two guys and pitch damping moment coefficients I have nothing.

Look at McCoy's pages 233-234 and 37-39. It should explain it.

#### mman

Robert anything new on bfx implementation?