#31

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!

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!

#32

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.

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.

#33

Quote from: mman on November 12, 2014, 05:59:51 AM

375CT,

I've been away for long time but if you haven't found a solution yet I have a suggestion. First just use Robert's spreadsheets to calculate BC from velocity measurements. Then use BC drag tables to find out Cd for velocity in question. As you know Cd for bullet is highly velocity depended and it doesn't make sense to calculate single average value for whole supersonic velocity range.

Mman,

Thanks for the tip, but please could you elaborate a little more over the "use BC drag tables" ? Perhaps I'm missing something here and a small example will help me out. Thanks as usual for your help!

#34

375CT,

I've been away for long time but if you haven't found a solution yet I have a suggestion. First just use Robert's spreadsheets to calculate BC from velocity measurements. Then use BC drag tables to find out Cd for velocity in question. As you know Cd for bullet is highly velocity depended and it doesn't make sense to calculate single average value for whole supersonic velocity range.

I've been away for long time but if you haven't found a solution yet I have a suggestion. First just use Robert's spreadsheets to calculate BC from velocity measurements. Then use BC drag tables to find out Cd for velocity in question. As you know Cd for bullet is highly velocity depended and it doesn't make sense to calculate single average value for whole supersonic velocity range.

#35

Quote from: meccastreisand on October 15, 2014, 04:40:54 PM

unhide Ballistics Table 0 and find cell A23.

Did that, but that cell is for BC adjustments for non-std conditions. Still no luck!

#36

Version 6.9 has been posted and is ready for download. It has a new hit probability calculator included. It's still kinda primitive but improvements will be made.

#37

unhide Ballistics Table 0 and find cell A23.

#38

Quote from: meccastreisand on October 02, 2014, 05:10:03 PM

There are 30 or so hidden tabs that have all the calculations. Find Ballistics Table 0

Thanks a lot for your help. I did what you say but still I cannot find where the Cd are derived from velocities readings. Somewhere else to look at?

#39

There are 30 or so hidden tabs that have all the calculations. Find Ballistics Table 0

#40

Quote from: meccastreisand on September 24, 2014, 11:49:10 PM

When you click on the excel file at the download share it'll show you an image of the spreadsheet. At the top will be an open link and an arrow pointing downward that will download it for you.

Thanks, I finally got it downloaded, but I've revised it and haven't found where Cd is calculated. Please could you tell what tab is or where I should look in?