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# Drag Function question.

#22
03-09-2004, 12:37 AM
 Silver Member Join Date: Jan 2004 Location: New Mexico Posts: 113
Re: Drag Function question.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>I can not get your website to work. None of the links at the top of the home page are working. I would like to look a the point mass stuff. Let me know what I am doing wrong.<HR></BLOCKQUOTE>

I don't know. It's working for me. It's just HTML. The direct link to the calculations is here.
__________________
JBM Small Arms Ballistics -- http://www.eskimo.com/~jbm
#23
03-09-2004, 05:08 AM
 Bronze Member Join Date: Dec 2002 Location: Asuncion, Paraguay Posts: 39
Re: Drag Function question.

Brad, send me an e-mail if you are interested and I'll try to contact you with HBC.
#24
03-10-2004, 02:07 PM
 Bronze Member Join Date: Sep 2001 Location: San Jose, CA Posts: 66
Re: Drag Function question.

My main complaints about using multiple BCs to deal with drag functions are two: first, it is not obvious what the multiple BCs should be nor the boundaries for each. Second, each change of BC introduces a discontinuity into the output.

A BC is supposed to be a scaling factor so that the same function can be used for a variety of bullets. If we are stuck with G1 based BCs, but the G1 drag model predicts less drag than occurs in reality (as does the G1 function for small arm prjectiles at velocities under 2500 fps), then one solution is to mathematically account for increasing drag experienced by small arms projectiles as they slow and to permit the shooter to adjust the slope of the drag curve. That way, a single G1-based BC can be used and the shape of the curve can be tweaked to match actual performace of the bullet in question.
#25
03-10-2004, 05:08 PM
 Silver Member Join Date: Jan 2004 Location: New Mexico Posts: 113
Re: Drag Function question.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>...then one solution is to mathematically account for increasing drag experienced by small arms projectiles as they slow and to permit the shooter to adjust the slope of the drag curve. That way, a single G1-based BC can be used and the shape of the curve can be tweaked to match actual performace of the bullet in question<HR></BLOCKQUOTE>

Mathematically, there is no difference in using different BCs. It's also the same thing as using a different drag function (e.g. G6, G7) which is different. You can always convert the G1 BC to a G7 BC and then see how much difference there is -- there may not be that much and you can get away with one BC for the velocity range you're interested in.

As for discontinuities, the BC changes the acceleration of the bullet. That's two integrations away from the bullet position/drop. It won't give you a true discontinuity in either the position or velocity.
__________________
JBM Small Arms Ballistics -- http://www.eskimo.com/~jbm
#26
03-10-2004, 06:51 PM
 Bronze Member Join Date: Sep 2001 Location: San Jose, CA Posts: 66
Re: Drag Function question.

Sure, it's a different drag function. But, as already pointed out manufacturers typically publish only the G1 BC. A function that could use a G1 BC but allow the user to change the rate of deceleration to fit the users data would solve the problem.

As to discontinuities, I agree that these aren't "true" discontinuities, but are abrupt changes of slope.
#27
03-11-2004, 07:57 AM
 Silver Member Join Date: Jan 2004 Location: New Mexico Posts: 113
Re: Drag Function question.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>...function that could use a G1 BC but allow the user to change the rate of deceleration to fit the users data would solve the problem.<HR></BLOCKQUOTE>

How are you going to "change the rate of deceleration"? With a constant? It would have to be something that the user could enter and you're already entering BC. I guess I just don't see the point in adding another constant to do the same thing as the BC.

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR>As to discontinuities, I agree that these aren't "true" discontinuities, but are abrupt changes of slope.<HR></BLOCKQUOTE>

I've done some testing with higher order fits to drag curves, it doesn't make any difference. There is no abrubt change of slope in the velocity or position.
__________________
JBM Small Arms Ballistics -- http://www.eskimo.com/~jbm
#28
03-11-2004, 08:01 AM
 Silver Member Join Date: Jan 2004 Location: New Mexico Posts: 113
Re: Drag Function question.

If you want to adjust data, download my free modified point mass program, MPM. I runs from the command line, but is quite powerful. You'll have to enter the dimensions for your bullet and a few other parameters, but you get a lot more information. In the trajectory input file (*.trj), there are two "fudge factors", lift_factor and yaw_drag_factor that can be adjusted. Also you can play with the dimensions of your bullet to get exactly what you're seeing at the range.
__________________
JBM Small Arms Ballistics -- http://www.eskimo.com/~jbm

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