## progress with custom drag tables

Started by admin, August 18, 2011, 02:38:05 PM

Good news, at least for me.

after some hefty programming I managed to extend BfX with the ability work with custom drag tables.

In the figure you see how it will work. To the right you see the custom drag table, in this case one for a Lapua bullet and supplied via Lapua's QuickTime software.

To the left you see a comparision of the results.

After I have removed some bugs, an update of the getting started documentation and rigorous testing I will release the new addin within a week or two.

#### mman

Impressive! This is good news!

This works now   ( that is not clear from the first post here )

#### mman

I can't get this to work. Could you explain what values drag table range must include... What is lapus's quick time sofware?

#4
One of the last worksheets in that workbook.

once you have a dragtable put together, give it an Excel name ...

alternatively, send me the spreadsheet and i'll will have a look.

#### mman

It works now... Thanks. It was m (3,2) that I was missing. Does the program linearly interpolate drag coefficient between given values? What is the conversion coefficient used when mach number is converted to m/s (ICAO conditions)?

#7
The Siacci methods needs a parametrization in the form of d= 1461.5128 * a*v^b (or something like that, i have to look it up) where d is the drag function and v is  the (metric) velocity. I calculate with the aid of some logarithms the constants a and b from the tabulated d's and their metric velocities. This is done for the ICAO velocity of sound. One could say that there is a powerlaw  interpolation deployed, although the powerlaw behaves almost linearly in the interval

Presently the only way to correct for temperatures other than ICAO is through adaption of the BC (with bfx_c), there is now no way to communicate the temperature from the excel "space"  to the procedure that uses the mach value to compute drag. At the moment it would require to incorporate the temperature in every BfX function - making them  awkward. I am expecting to introduce a "table" (a range in excel)  for bc. This table contains then the bc, temperature, humidity and pressure, making BfX_C superflous yet keeping the formulas simple. This would be a rather straightforward thing to do now the ground breaking work with the drag tables has been done. However, I find the 6dof implementation much more attractiveto work on - this will happen before X mas I hope. Before that, however, I want to implement neural network software in Excel. Not only do I need it for something else, I think it can be handy for 6dof speedup and - in a  far future - the implementation of software that does the aerodynamics on a more elementary level than what we do with bc's and tables.

the m (3, 2) I added later, and was not included in the screenshot of the prototype you saw on the forum. (However I have to include it in BfX_help() )

For what are you using the drag functions?

#### mman

#8
Quote from: admin on September 17, 2011, 09:40:33 PM
For what are you using the drag functions?

Lapua's functions for lapua bullets. Customised drag functions for subsonic airgun and rimfire projectiles..

m/s -> mach number conversion is useful when creating customised drag function from velocity / range information (chronograph data).

do you have such a dragfunction for R50?

#### mman

#10
Do you mean RWS R50 22 lr. ammo ? I have nothing yet... First I have to work with CED to make sure that measured velocities are accurate enough.

And secondly I think drag coefficient curve is gun depended at least for some degree. This is the hard part when using subsonic lead projectiles. This means that everyone should calculate their own gun/ammo specific bcs for rimfire and airgun bullets.

McCoy made a complete ballistic study of the R50 and the Eley Tenex. Interesting literature ...