Drag Function question.

[baldeagle713]

Ok so let me see if I got this strait.
Say I load up a 142 grain 6.5MM SMK. I'd use the G1 Function. For a 140G Burger VLD I'd use the G7 function and a 140g AMAX I'd use the G5? OR Did it just get Boild down to using the G1 Function for every thing?

I get confused EZ Fellows so KISS (KEEP iT SIMPLE & Sencer)for me please.

 

[JBM]

<BLOCKQUOTE><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><HR> Ok so let me see if I got this strait. Say I load up a 142 grain 6.5MM SMK. I'd use the G1 Function. For a 140G Burger VLD I'd use the G7 function and a 140g AMAX I'd use the G5?<HR></BLOCKQUOTE>

The short answer is that you use the G function for which you have ballistic coefficients. If you have a G1 BC for the VLD you have to use the G1 drag function otherwise any trajectory you calculate is wrong.

I've not seen anything except G1 BCs from Sierra so you'd always use a G1 drag function from them. The BC on Bergers site for the 140 grain VLD looks like a G1 BC so you'd use a G1 drag function there too -- same for the AMAX.

Most, if not all manufacturers provide G1 BCs so use the G1 curve.

 

[John Burns]

Brad,

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.

Thanks

 

[TiroFijo]

I of course (and humbly) agree with JBM above posts.

On the question of which drag curve fits best a certain bullet (IF you have the correct BC data to use other than the G1 function) I've asked about this to a gentleman (HBC) that frequents www.benchrest.com and has the appropriate equipment to test the bullets:

"I have two Oehler Model 43's with two acoustic targets. I set one target at 101.6 yards and the second at 287.6 yards and the sky screens are spaced 24 feet apart for good resolution. Thus I simultaneously measure BC twice for each round fired.
I have never set the acoustic targets more than 400 yards distance but simulate shooting longer distance by shooting a full charge load and another at about 80%.

The G7 drag table will fit most long range match bullets very well, such as the 6mm Sierra 107 grain MK, The Berger 105 VLD, the more streamlined of the 1000 yard bullets, and the 300 grain .338 MK is another example. Some of the older design MK's with short ogives fit the G1 table better. I have just one test with the Sierra 30 cal. 155 grain MK and it fit the G5 data best, based on that test.

I have experimented with very sharp pointed 6mm boat tail bullets with meplats down to 0.010" diameter and they always fit the G6 data best even though the G6 projectile is a flat base but it has a very sharp meplat. The 100 grain 6mm Lost River bullet fits the G6 table also.

If in doubt when selecting a drag table for a 1000 yard bullet, choose the G7. The G7 also fits some of the short range BC bullets best even though they are flat base (i.e. 68 grain hpfb match) but actually the ogive length of those short range bullets is a higher percentage of the bullet overall length than most 1000 yard bullets.

The results are very good with the G7 data. I think you can expect most long range bullets (VLD), when checked against the G7 data, on holding a BC of well under 2% variation."

 

[JBM]

I'd be curious to know how my CGI implementation of McDrag agress with your measurements. You can find it here
McCoy's paper on McDrag said it can be as accurate as 2% at high mach numbers.

Thanks!

[ 03-08-2004: Message edited by: JBM ]

[ 03-08-2004: Message edited by: JBM ]

 

[TiroFijo]

Brad, I was just quoting these results from a friend. I will send him a mail to your site so you two can compare notes.

 

[baldeagle713]

Thank you again.

 

[JBM]

<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.

 

[TiroFijo]

Brad, send me an e-mail if you are interested and I'll try to contact you with HBC.

 

[Blaine Fields]

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.

 

[JBM]

<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.

 

[Blaine Fields]

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.

 

[JBM]

<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]

I should think longer before I hit "Add Reply".

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.