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new twist on b.c.'s
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<blockquote data-quote="JBM" data-source="post: 11994" data-attributes="member: 1969"><p><strong>Re: new twist on b.c.\'s</strong></p><p></p><p><BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR></p><p>One thing I didn't catch though was the "CD" you mentioned. What is that, and how do you calculate it? </p><p><HR></BLOCKQUOTE></p><p></p><p>CD is the drag coefficient. It's what everybody is trying to avoid by using BCs. The drag of the bullet is proportional to the cross sectional area of the bullet, the density of the air, CD and the velocity squared [there are some constants and the mass in there too]. A drag function, associated with a BC, is used to find the CD of the bullet as a function of mach number. So to find a trajectory, you can use a BC and a drag function or just the CD. The idea is that the curve of CD versus mach number is the same for like bullets, e.g. flat base, boattails, etc. The drag function models the shape of the curve and the BC adjusts it so that the magnitude is correct. It's not perfect -- just take a look at Sierra's lists of BCs for different bullets, they change with bullet velocity. If they used a G7 drag function which has a different curve, they could probably get away with fewer BCs. Take a look at my MPM program (it's free) -- is uses the CD and a few other coefficients to find the trajectory.</p><p></p><p> <BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>I was wondering if you could maybe tell me the formula for figuring bc.<HR></BLOCKQUOTE></p><p></p><p>I just do it iteratively. Find a trajectory and adjust it until it works. Not real elegant though. Unfortunately, there isn't a simple formula because the equations of motion are a differential equation. There is no simple solution to the equations. You must integrate them to get the trajectory. </p><p></p><p> <BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>I called Art Pejsa and asked him for the formula, but he was such an arrogant bugger I couldn't stand being on the phone long enough with him to get a straight answer.<HR></BLOCKQUOTE></p><p></p><p>Yeah, I talked to him ONCE too.</p><p></p><p>[ 09-23-2004: Message edited by: JBM ]</p></blockquote><p></p>
[QUOTE="JBM, post: 11994, member: 1969"] [b]Re: new twist on b.c.\'s[/b] <BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> One thing I didn't catch though was the "CD" you mentioned. What is that, and how do you calculate it? <HR></BLOCKQUOTE> CD is the drag coefficient. It's what everybody is trying to avoid by using BCs. The drag of the bullet is proportional to the cross sectional area of the bullet, the density of the air, CD and the velocity squared [there are some constants and the mass in there too]. A drag function, associated with a BC, is used to find the CD of the bullet as a function of mach number. So to find a trajectory, you can use a BC and a drag function or just the CD. The idea is that the curve of CD versus mach number is the same for like bullets, e.g. flat base, boattails, etc. The drag function models the shape of the curve and the BC adjusts it so that the magnitude is correct. It's not perfect -- just take a look at Sierra's lists of BCs for different bullets, they change with bullet velocity. If they used a G7 drag function which has a different curve, they could probably get away with fewer BCs. Take a look at my MPM program (it's free) -- is uses the CD and a few other coefficients to find the trajectory. <BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>I was wondering if you could maybe tell me the formula for figuring bc.<HR></BLOCKQUOTE> I just do it iteratively. Find a trajectory and adjust it until it works. Not real elegant though. Unfortunately, there isn't a simple formula because the equations of motion are a differential equation. There is no simple solution to the equations. You must integrate them to get the trajectory. <BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>I called Art Pejsa and asked him for the formula, but he was such an arrogant bugger I couldn't stand being on the phone long enough with him to get a straight answer.<HR></BLOCKQUOTE> Yeah, I talked to him ONCE too. [ 09-23-2004: Message edited by: JBM ] [/QUOTE]
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