Maybe you can present your function/measurements to the forum and then we can supply you with either the parametrisations, or a bc, or some comparisions (e.g. the special purpose drag function versus g7...).

The VBA code for the drag function fit is in the workbook, I just checked it. You have to switch on the developer tab in Excel to get access to it.

Quote from: 375CT on November 27, 2013, 04:48:32 AM

Sorry for asking but I still need to relate what's in the workbook and Pejsa's book, because I'll like to calculate custom curves myself and any help on how to compute those "a" and "b" values is greatly appreciated.

here is the excel spreadsheet with which i determined the splines.

Quote from: 375CT on November 26, 2013, 03:57:10 AM

Still trying to understand how to load Lapua's Doppler data on BfX.

have a look at MMANs ballistic calculator to see examples:

http://www.2shared.com/file/YEE1WaWl/Ballistic_calculator.html

I want to remember that Pejsa parametrized the drag function for four velocity regions (splines). He did that with four functions of the form cd=a*velocity^b, a and b being the parameters. This allows to derive formulas for velocity, drop etc as function of time and distance etc. The formulas depend on a and b.

What I did for the drag functions (e.g. G7)  that are included in BfX is that I parametrized them myself. However, the included drag functions require more velocity intervals for a good parametrisation. Typically 10 velocity regions were necessary and as much sets of a,b values.

The custom drag function, listing drag for a set of typically 40 velocity regimes, are parametrized automatically by BfX. BfX calculates the parameters a and b for each velocity interval of the supplied custom drag table. Instead of having only four parameter regions that Pejsa's drag function has, this use of custom drag tables yields typically  40 of such intervals.

But for our computers this is not a problem.

from an earlier post the following comparision

You wrote

Is it by taking the dataset and calculating a series of "banded BCs", corresponding to each bracketed Cd zone?

No, the drag function allows to directly calculate the deacceleration.

Hello on the forum!

The custom dragfunction is used instead of the standard ones, e.g. g7. For a customized drag function for a specific bullet, the ballistic coefficient (BC) is then 1 lb/in^2. Other dragfunctions do require a BC.

On page 20 of the GettingStarted workbook the custom drag functions are explained

One can (could?) obtain custom drag functions for Lapua bullets.
These drag functions where obtained bij analyzing radar data, probably via differentiating deacceleration as function of speed.

MMAN on this forum has the most experience in using customized dragfunctions. He has probably an idea how good they are. For my purposes using g7/g1 was good enough. Have a look at MMANs sophisticated ballistic calculators made with BfX on this forum.

You can make the comparison yourself quite easily. If one has a Lapua drag function one can with BfX compare its output (velocity as function of time/range) to the G7 one with ballistic coefficients from Bryans Book. Take care, at large ranges, say above 1000m, the elevation is such that BfX looses accuracy.

Congratulations to the team,

and I am sorry for you. Nice spreadsheet.

Cool. How is/was the world championship?

I have upgraded the software and it supports Excel 2013 now!

I have tested it for the following combinations
windows 7 64bit with 64 bit Excel 2013
windows 7 64bit with the 32 bit Excel 2010,
I cannot test the other versions currently.

Also the site was updated a little.

Quote from: gvp on June 17, 2013, 10:44:47 PM


- us(P, d_air)  BFX does not calculate the speed of sound, it always assumes that it has the value at Standard temp and pressure STP. This gives minor deviations with other computer codes. On the otherhand, the calculation of speed of sound introduces inaccuracies too, and I have never encountered a proper treatment of the subject. On the otherhand I did not look for it. I, personnally, can only guess why a drag function should be related to the mach number of a projectile.

- d_air(P,T,H)     =>     us(P,T,H) - I  never looked into the matter. Wikipedia has an article about it.

- BC(M, D, shape, u, P, T, H) or BC(M, D, shape, u/us) the ballistic coefficient does not depend on air properties. It depends on all the physical properties of the projectile alone and is normalized on the properties of the projectile the bc's drag function belongs to. A first guess would be to normalize on the sectional density of the drag functions standard bullet, hence assuming the projectile has the same geometrical shape.

- Cd(shape, u, P, T, H) or Cd(shape, u/us) There is truth in both formula's. BFX uses standard drag functions at STP and gives with BfX_C a method to correct for air density effects.

The other questions I cannot answer now, I have to look in the books, it has been more than two years ago that I studied the subject

- Lets say that we have a projectile with unknown BC ... the right way to build the BfX model for it is:
the right way to determine the bc is to measure velocity as a function of distance and reproduce with bfx_vx the values. Use a fit procudure to find the optimum bc. you can download the excel files from my site.

Alternatively you can determine the Cd of a projectile also from these velocity measurements your selves. I have to look in the theory for the proper definitions.


- Last question: why the BfX custom drag functions need as an input the length of the projectile ?
I don't remember. I think I do not use the input, but require it for the sake of future developments. I have to look in the code.

the other thing you can do is to fix a spare telescopic sight to your barn, aiming it at a fixed point at 1000m. Then as a function of atmospheric conditions, measure the displacement of the crosshairs.

cos theta = x/sqrt(x^2+d^2) [rad], where x is the horizontal distance and d the drop.

Can you explain your formula?