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Estimating drag coefficient

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Estimating drag coefficient

Unread postAuthor: SpudBlaster15 » Fri Jul 27, 2007 2:41 pm

Does anyone know of a method of accurately estimating the drag coefficient of a projectile?

Thanks.
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Unread postAuthor: noname » Fri Jul 27, 2007 2:45 pm

If you have a chronograph, you could probably get a rough idea by shooting an aerodynamic projectile from 50 feet, then shooting one that isn't aerodynamic.

1st thing that came up on Google. It'll work if you're shooting arrows!
http://homepage.ntlworld.com/joetapley/amf.htm
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Unread postAuthor: Pilgrimman » Fri Jul 27, 2007 2:47 pm

That depends on more variables than I'd like to think about. Some are: air density/composition, wind resistance (you'll need calculus to figure it out for irregular shapes), temperature, presence of thermals, and whether the projectile spins, so it is stable, and doesn't go end over end, thus screwing your calculations. I don't even want to think about the complexity if sabots or wadding were involved... :lol:

Anyone who fully understands this, please provide a link, or at least supplement my post. I hope I've helped somewhat... :roll:
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Unread postAuthor: SpudBlaster15 » Fri Jul 27, 2007 2:56 pm

Pilgrimman, I am referring to the drag coefficient of a projectile, not an entire trajectory calculation.

Noname, I don't have a chronograph, otherwise I would use that method.
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Unread postAuthor: Pilgrimman » Fri Jul 27, 2007 2:59 pm

OHHHHH gotcha! Well then, just assume it is stable in flight, and go from there! The density and such still affects it, but I don't know what kind of degree of accuracy you want. Ignore the thermals thing.

Edit: Here's a link, but I don't know how reputable it is...

http://en.wikipedia.org/wiki/Drag_coefficient
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Unread postAuthor: boilingleadbath » Fri Jul 27, 2007 3:47 pm

The density does NOT affect the Cd of an object.

Anyway, if you know the hang time of your projectile, the mass of it, and your muzzle velocity, it is possible to calculate the Cd...
But not using the current public release of Johanna's range calculator, because that doesn't handle vertical movement in a realistic fashion.

Which means that you either have to wright your own calculator (not really difficult) or use the GGDT's exterior ballistics tool (which is probably accurate enough...)
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Unread postAuthor: Pilgrimman » Fri Jul 27, 2007 3:49 pm

It doesn't???? I would think that at a higher pressure, or in the presence of heavier particles that the resistance would be greater... Oh well, thanks for correcting me... 8)
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Unread postAuthor: paaiyan » Fri Jul 27, 2007 3:52 pm

WEll the drag coefficient is how much drag it has. Density isn't a property that would affect that. The types of things that affect drag are aurface area, shape, the smoothness of the surface etc.
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Unread postAuthor: jimmy101 » Sat Jul 28, 2007 12:31 pm

I don't think there is any way to "accurately" calculate the Cd from first principles.

You might want to look here, scroll down to the Other Projectiles section and there is a table where I've collected Cd's for various things.
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Unread postAuthor: boilingleadbath » Sat Jul 28, 2007 3:43 pm

Oops, you meant the density of the air.

Terribly sorry for the misunderstanding there: the density of the projectile will NOT effect the path of the object, but that of the air WILL.

The drag force formula is 1/2pv<sup>2</sup>C<sub>d</sub>A... and "p" is, after all, density.

As a note, I've starter work on another utility, "Hang Time Ballistics Live".
This will allow you to find the Cd, Apogee, Hang time, impact velocity, and Muzzle velocity of your projectile by inputing all the following:
Cd, Appogee, Hang time, impact velocity and Muzzle velocity.


...just kidding. (heh, lol, that'd be an easy program to make...)
Actually, you input the following:
Altitude above sea level, Temperature, projectile mass, projectile diameter, and weather or not it landed in a tree...
Plus TWO of these:
Cd, Apogee, Hang time, Muzzle velocity

I'm not expecting great accuracy when you try to find the missing data with the Apogee-Hang time input pair, but there's nothing I can do about that.
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Unread postAuthor: cdheller » Sat Jul 28, 2007 5:20 pm

search for
"ballistic coefficient "
?might be a better term for what your after?

there are a few free programs around, some let you input shape and sizes

http://www.uslink.com/~tom1/calcbc/calcbc.htm

http://www.eskimo.com/~jbm/calculations ... tions.html
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