## Recent posts

#21
##### Ballistics / Re: Pejsa and 3DOF PM
Last post by admin - December 07, 2015, 03:44:22 PM
Pejsa makes two assumptions to derive his formulas:
1) at a given mach interval the drag function can be approached by a power of the mach number g(m)=a*m^b a and b are real numbers. With the custom drag funtions BfX determines for each interval a and b. Then applies Pejsas method. Numerical integration assumes a constant drag value for a given interval.

2) a flat fire approximation

ad 1) if the  mach interval is small enough Pejsa method, and the software has to sum the effects over many intervals, Pejsa method becomes is equivalent to a numerical integration, probably equalling the accuracy of a  runge kutta integration. BfX has the ability to use a measured drag function, e.g. the ones of lapua. BfX results matches here  the ones of a 3dof model, see the workbooks mman posted.

ad 2) a 3dof is able to go beyond the flat fire approximation

BfX is all about generating tables. However, Pejsa publishes many simple formulas which one can use with an electronic calculator are sufficiently accurate for most purposes. In that case it is much faster than 3dof.

#22
##### Ballistics / Pejsa and 3DOF PM
Last post by 375CT - December 05, 2015, 06:02:54 AM
Folks, I have a friend asking for the main differences between Pejsa and Point Mass (3DOF), which are in your opinion those?

Besides the traditional answer, of a closed form vs a numerical approach, the slope (N) I'd like to have a summary of the differences and strengths of both if possible.

thanks in advance for the help.
#23
##### Updates / BfX updated to run under Excel...
Last post by admin - October 18, 2015, 11:26:11 PM
I have updated BfX to support Excel 2015.
Fixed also a bug that might prevented it to support Excel 2013. I cannot test this...
#24
##### Member projects / Best spreadsheets july 2015
Last post by admin - July 21, 2015, 11:41:07 AM
Dear all,
It is getting a mess. Let us collect what we have produced during the years.

can you post your best spreadsheets and a small description here, status 2015?

Some of them I will put them on the download page of www.bfxyz.nl
#25
##### General discussion / Re: LabRadar
Last post by mman - April 28, 2015, 05:51:28 PM
#26
Last post by admin - April 24, 2015, 08:56:47 PM
#27
##### Ballistics / Re: How to calculate Cd
Last post by mman - January 09, 2015, 06:55:29 AM
I can't give you direct references I'm afraid but I have seen some of them along the way. For me those are not very interesting since most are not that accurate except perhaps limited projectile shapes.

Let's see if robert can do a better job with his new project.
#28
##### Ballistics / Re: How to calculate Cd
Last post by 375CT - January 09, 2015, 03:11:15 AM
Quote from: mman on January 08, 2015, 04:21:25 PM
375CT,

There are lot's of analytical and numerical ways to estimate Cd directly. However the method I just told you is one of the most accurate and still very simple.
If you don't know the actual BC of your bullet use measurements and drawings from Litz's book to estimate form factor and calculate BC from it (I have done this numerous times and usually get the BC right with the accuracy of 3-4 % which is better than I can manage with CFD). All you need for this method is good reference bullet. When you have BC then just use G-drag curve as shown before to calculate Cd.

Are you aware of any other method, besides McDrag to compute Cd?
#29
##### Ballistics / Re: How to calculate Cd
Last post by mman - January 08, 2015, 04:21:25 PM
375CT,

There are lot's of analytical and numerical ways to estimate Cd directly. However the method I just told you is one of the most accurate and still very simple.
If you don't know the actual BC of your bullet use measurements and drawings from Litz's book to estimate form factor and calculate BC from it (I have done this numerous times and usually get the BC right with the accuracy of 3-4 % which is better than I can manage with CFD). All you need for this method is good reference bullet. When you have BC then just use G-drag curve as shown before to calculate Cd.
#30
##### Ballistics / Re: How to calculate Cd
Last post by 375CT - January 08, 2015, 05:09:27 AM
Mman,