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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Drag Function question.
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<blockquote data-quote="JBM" data-source="post: 32240" data-attributes="member: 1969"><p>For the most part the military doesn't really use BCs and Drag functions anymore (in talking with Bob McCoy before he died). They use CD versus Mach Number. Of course the ratio of the drag function and BC gives you CD (with some constants and a velocity term thrown in) so mathematically it doesn't really matter which G function you use as long as you have the right BCs (that's the real trick). Of course if you're using 10-25 BCs you might as well use CD and Mach number. As for not being accurate down range, it sounds like the low velocity BCs aren't correct.</p><p></p><p>My online programs use some old second order curve fits to published G functions. My MPM program doesn't use drag functions at all except in output. My personal code, PM, is a point mass program and it uses linear interpolation into the ARL (used to be BRL) tables of G functions as a function of mach number.</p><p></p><p>The real utility in drag functions and the reason that you want to find the "right" one that you can limit the number of BCs you have to use. As an extreme example, if you bother to create a drag function for the 300 gn MK, you could use it over the whole velocity range with a BC of 1.0. Of course your drag function is really just the CD as a function of mach number (with the same constants and the velocity term).</p><p></p><p>Berger sent me some data for their 210 grain VLD. It showed BCs as a function of mach number, velocity and drag function. The G7 drag function varied from 0.316 to 0.338 over the range of 1500-3500 f/s. The G1 BC varied from 0.590 to 0.715.</p></blockquote><p></p>
[QUOTE="JBM, post: 32240, member: 1969"] For the most part the military doesn't really use BCs and Drag functions anymore (in talking with Bob McCoy before he died). They use CD versus Mach Number. Of course the ratio of the drag function and BC gives you CD (with some constants and a velocity term thrown in) so mathematically it doesn't really matter which G function you use as long as you have the right BCs (that's the real trick). Of course if you're using 10-25 BCs you might as well use CD and Mach number. As for not being accurate down range, it sounds like the low velocity BCs aren't correct. My online programs use some old second order curve fits to published G functions. My MPM program doesn't use drag functions at all except in output. My personal code, PM, is a point mass program and it uses linear interpolation into the ARL (used to be BRL) tables of G functions as a function of mach number. The real utility in drag functions and the reason that you want to find the "right" one that you can limit the number of BCs you have to use. As an extreme example, if you bother to create a drag function for the 300 gn MK, you could use it over the whole velocity range with a BC of 1.0. Of course your drag function is really just the CD as a function of mach number (with the same constants and the velocity term). Berger sent me some data for their 210 grain VLD. It showed BCs as a function of mach number, velocity and drag function. The G7 drag function varied from 0.316 to 0.338 over the range of 1500-3500 f/s. The G1 BC varied from 0.590 to 0.715. [/QUOTE]
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