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Drag functions and drop charts

 
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  #8  
Old 06-21-2006, 07:27 PM
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Location: NC, oceanfront
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Re: Drag functions and drop charts

BC is dependent on drag. From the muzzle or any other point. It varies only because drag varies. The bullet's curve. BC could be used as a constant because of this, but it's used as a variable in software. Unless some other decay factor is used. From what I've seen, it's often used wrong(like using a G1 BC WITH a G7 drag curve, or claiming a ridiculous G1 BC from a bullet which is really following G7 curve at a G7 BC)
Adjustment of a bullet's drag curve to match in ballistic software would be great. Seems like someone was working on this with what they called a 'DK' adjustment(bain). It flattened the curve to match field observed drops at different distances.Not just 2 distances either. Far superior to BC adjustment, which can only actually be correct at 2 points. Sierra's BCs at different velocities offers potential for more, close points. A pretty poor bandaid, but what else could they do given that nearly all ballistic software available has set drag curves which do not match our bullets. The only control Sierra has over this market is in their BC numbers.
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  #9  
Old 06-22-2006, 07:54 AM
JBM JBM is offline
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Join Date: Jan 2004
Location: New Mexico
Posts: 113
Re: Drag functions and drop charts

[ QUOTE ]
BC is dependent on drag. From the muzzle or any other point. It varies only because drag varies. The bullet's curve. BC could be used as a constant because of this, but it's used as a variable in software. Unless some other decay factor is used. From what I've seen, it's often used wrong(like using a G1 BC WITH a G7 drag curve, or claiming a ridiculous G1 BC from a bullet which is really following G7 curve at a G7 BC)


[/ QUOTE ]

Originally, the BC was used to relate a "standard" bullet's drag to that of the bullet in question. The idea was that if you had a good enough drag function, you could use one BC to represent a bullet of the same "type" (or drag curve shape).

I agree that it doesn't make sense to use a G1 BC with a G7 drag function. It's just plain wrong.

I think it's important to keep in mind what is really going on here. You are using two values to find the CD (drag coefficient) of the bullet as a function of mach number (not distance). The CD is proportional to G/BC (there is also a velocity term, density and some constants in there see my website for the formula -- it's at CD and KD). So if you have the right G function, and the right BC, or list of BCs you can get a good value for the CD as a function of mach number depending on how many BCs you want to use. If you use enough, you can use any drag function you want, of course the less the drag function "fits" the bullet, the more BCs you'll have to use.

This gets us to the point of using different drag functions. If you can find a drag function that fits a bullet well, you only have to use one drag function -- in other words, the CD is proportional to G.

As an example, look at a .223 caliber boattail:

1.000 0.268
1.100 0.295
1.200 0.317
1.300 0.335
1.400 0.349
1.500 0.360
1.600 0.367
1.700 0.372
1.800 0.377
2.000 0.383
2.200 0.390
2.500 0.404
3.000 0.439
3.500 0.488
4.000 0.541
4.500 0.595

The first column is the mach number and the second column is the BC (G1). This data is taken from my online Drag/Twist calculation.

For a G7 BC we have:

1.000 0.212
1.100 0.201
1.200 0.192
1.300 0.190
1.400 0.189
1.500 0.188
1.600 0.188
1.700 0.188
1.800 0.189
2.000 0.193
2.200 0.197
2.500 0.202
3.000 0.208
3.500 0.208
4.000 0.209
4.500 0.210

Note that there is much less variation using the G7 drag function. If I had to pick one BC, I'd use the G7 drag function and a BC of about 0.200 (I'm guessing by looking at the data, in practice, it would probably be a weighted average over the velocity range I was interested in, but it wouldn't make that much difference!).

Does that mean that the G1 BC is wrong? If you used software that allowed any number of BCs as a function of mach number (or velocity at some conditions) and put in these 16 BC values for G1 and G7, you wouldn't see any difference. If you picked a single BC and ran it, you would.

So is it wrong to use multiple BCs? If you use a single drag function, then you pretty much have to use multiple BCs for some bullets.

[ QUOTE ]

Adjustment of a bullet's drag curve to match in ballistic software would be great. Seems like someone was working on


[/ QUOTE ]

You're better off using CD versus mach number and just ditching the whole BC idea (that's what my modified point mass program does). Of course there are market considerations that the bullet manufacturers have to deal with that I don't. Can you imagine trying to sell a bullet with a G7 BC of half the value of your competitors G1 BC and trying to explain to all your customers that it has less drag? No thanks.

[ QUOTE ]

this with what they called a 'DK' adjustment(bain). It flattened the curve to match field observed drops at different distances.Not just 2 distances either. Far superior to BC adjustment, which can only actually be correct at 2 points.


[/ QUOTE ]

I'm not sure why you say this. Why two points? I would think it's really only valid at one point. Manufacturers have chosen BC values over a range of velocities to give decent accuracy (I'm not sure how they define that though).

[ QUOTE ]

Sierra's BCs at different velocities offers potential for more, close points. A pretty poor bandaid, but what else could they do given that nearly all ballistic software available has set drag curves which do not match our bullets. The only control Sierra has over this market is in their BC numbers.

[/ QUOTE ]

Yep. I can tell you from personal experience that when you mention CD versus Mach number, the eyes roll back into people's heads and the lights turn out (not for everyone granted, but it's typical).

JBM
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JBM Small Arms Ballistics -- http://www.eskimo.com/~jbm
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  #10  
Old 06-22-2006, 11:24 AM
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Join Date: Jul 2001
Posts: 1,459
Re: Drag functions and drop charts

JBM, thanks for the info. Been using your software for the last few years and am very happy with the results. Nice to see all the changes on your site. Now I can't find anything [img]/ubbthreads/images/graemlins/grin.gif[/img]

I think you hit the nail on the head. BC has become the holy grail that is used/misused to sell product. It really is a meaningless number unless you take into account the assumptions that are made.

Precious few shooters actually know what the BC means in real terms.

I think that many bullet manf are understating their BC so that consumers will not go BS and slag the company.

What would happen if someone said BC 0.625 BUT a G7 curve? Run that through a G1 comparison as on your software and HOLY S...can't be true.

Conversely, there are some who use an 'elevated' BC number by using a more favorable drag curve function in their calculations.

No standards and too much smoke/mirrors.

Only way to know for sure is to launch them in your rifle, your scope and rangefinder. Then you create data that is precisely incorrect except for this one set of circumstances.

Ultimately, if it is reliable and repeatable, that is what matters to LR shooters/hunters. The actual numbers don't really matter.

But it makes for great conversations...

Jerry

PS the drop table I created for that 162gr Amax shooting 7RM agreed with my real world drops out to 940yds to the click. One BC number can work IF the drag curve/scope/rangefinder/shooter/etc. fit. Unfortunately, that is very rare.
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