Recent Posts

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21
Ballistics / Pejsa and 3DOF PM
« Last post by 375CT on 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.
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Updates / BfX updated to run under Excel 2015 32 and 64 bits
« Last post by admin on October 18, 2015, 10:26:11 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...
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Member projects / Best spreadsheets july 2015
« Last post by admin on July 21, 2015, 10:41:07 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
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General discussion / LabRadar
« Last post by admin on April 24, 2015, 07:56:47 PM »
26
Ballistics / Re: How to calculate Cd
« Last post by mman on 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.
27
Ballistics / Re: How to calculate Cd
« Last post by 375CT on January 09, 2015, 03:11:15 AM »
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.

Mman, thanks again for your kind reply.

Are you aware of any other method, besides McDrag to compute Cd?
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Ballistics / Re: How to calculate Cd
« Last post by 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.
29
Ballistics / Re: How to calculate Cd
« Last post by 375CT on January 08, 2015, 05:09:27 AM »
Mman,

Sorry for the late reply!

While the example you posted is clear enough, my point was/is if you are aware of an analytical approach to solve for Cd given

1) You don't know/have any initial approximate G value.
2) You only have downrange measured velocities.

Or...you always have to start with a BC calc?

Thanks!
30
Ballistics / Re: How to calculate Cd
« Last post by mman on November 28, 2014, 06:30:55 PM »
Example:

Step 1
You have a bullet with known G7 BC of 0,261. It's 0.264" and 123 grains. Now you can calculate form factor (i7) for it. i7 = weight/7000/(caliber^2*BC) --> i7 = 123/7000 / (0.264^2 * 0,261) = 0,966. This means that bullet's drag is 96,6 % of G7 projectile drag.

Step 2
You can use G7 drag data table to find out Cd for velocity in question (for G7 projectile). See picture below how to do it using G7 graph.
Or using robert's BFx function in excel =BfX_Cd(900;"G7") this gives you 0,262 as a Cd.

Step 3
Since you know that for your bullet drag is 96,6 % of G7 projectile you can calculate for the bullet in question that CD(900m/s) = 0,966* 0,262 = 0,253

Clear enough?



Now if you are measuring some odd shape bullet that doesn't fit very well to any know drag data (G1, G5, G6, G7 etc..) you can start to work with the closest one and using your measured velocities find out where it goes wrong. Then just do your own custom drag table to modify it as necessary. This is possible since Robert kindly added this option as well. You could use some "best fit" tool to do this but good old trial and error works as well.
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