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cd value

Unread postPosted: Sat Apr 05, 2008 11:36 am
Author: POLAND_SPUD
what is the exact cd of marbles or other smooth spheres ? or where can I find such information?(drag coefficient, you know the thing that you have to put into GGDT, I am not reffering to balistic coef.)

EDIT sorry for that spelling error I meant drag coef not flow coef. :oops:

Unread postPosted: Sat Apr 05, 2008 11:43 am
Author: pat123
I have been wondering the same thing. I think the flow coefficient would refer to the valve not the ammo.

edit: ok drag coefficient I also think a perfect sphere is .4 but the average marble is not a perfect sphere I don't know how much it would change it by.

Unread postPosted: Sat Apr 05, 2008 11:48 am
Author: Hubb
I'm not too sure myself, but have you seen this link? It may help you.

Unread postPosted: Sat Apr 05, 2008 11:55 am
Author: Jared Haehnel
I believe a perfect sphere is .4...

Unread postPosted: Sat Apr 05, 2008 11:56 am
Author: POLAND_SPUD
thx hub but I've already checked that page before writing this thread... sorry for that spelling mistake in my first post I hope you all understand that I am asking about the value that you have to put in External Balistic Calculator

Unread postPosted: Sat Apr 05, 2008 1:51 pm
Author: Hubb
PM D_Hall. It's his program so he should be able to help.

Unread postPosted: Sat Apr 05, 2008 2:15 pm
Author: boilingleadbath
The Cd of a sphere is <a href="http://aerodyn.org/Drag/speed-drag.html#sphere">somewhat dependent</a> upon mach number.

.42 is a good guess.

Unread postPosted: Sat Apr 05, 2008 2:23 pm
Author: jrrdw
boilingleadbath wrote:The Cd of a sphere is <a href="http://aerodyn.org/Drag/speed-drag.html#sphere">somewhat dependent</a> upon mach number.

.42 is a good guess.


Is this number ".42" refering to low and high spots in the marbles surface?

Unread postPosted: Sat Apr 05, 2008 3:55 pm
Author: POLAND_SPUD
look here http://www.thehalls-in-bfe.com/GGDT/usa ... istic.html it means that it is for subsonic projectiles

I expect that the value is lower than 0.4 or 0.42 because they fly quite far and they still have enogh energy to cause some dammage...

I'll try to PMing D_hall about it and then I post his response

Unread postPosted: Sat Apr 05, 2008 4:14 pm
Author: jrrdw
The ratio of the drag on a body moving through air to the product of the velocity and surface area of the body. (dictionary.com)

How far and how close to a given area are you trying to place the marbles? The curve of the flight will have real time effect on every single shot. On marbles inparticular because of inperfections in glass surfaces.

High tech for marbles, ehh?

Unread postPosted: Sat Apr 05, 2008 4:25 pm
Author: POLAND_SPUD
well I am not expecting pinpoint accuracy... I understand that marbles aren't ideal projectiles, becasue of imperfections of their surface and small differences in their size but anyway sphere is quite common shape for ammo so it would be good to know the exact value (esspecialy becasue it has such a great impact)

I don't understand that part about the curve of the flight, could you elaborate on that ??
EDIT oh now I get it, you were talking generaly about accuracy but I thought you meant that the curve of flight has effect on the value of drag coeff or something like that.... :D

Unread postPosted: Sat Apr 05, 2008 4:51 pm
Author: jrrdw
Round balls curve in flight because of the imperfections in the surface catch air and spin the ball creating drag on one side and curving flight of the ball. Thats the best discription I can put to words.

The super balls I shoot have a seem around the middle and ever single one curves in flight, the more pressure I shoot them with the further they fly straight, but all ways curve at some point in flight.

It's quit funny to shoot them at low enough pressures, 15/20psi you can watch them vear off to one side or another. 15 feet or better from center line. Cracks me up every time!

"oh now I get it, you were talking generaly about accuracy but I thought you meant that the curve of flight has effect on the value of drag coeff or something like that"

It most deffenitly will have more of a effect because anything the turns without maintaining steady momentum slows.

Unread postPosted: Sat Apr 05, 2008 5:24 pm
Author: POLAND_SPUD
@jrrwd yeah I know what you mean. You should try shooting marbles on a sunny day - they shine so much that you can easily see them flying - that's something really impressive

what about inducing spin on marbles (and other round ammo as well)? I did tried doing it with a homemade hop-up the results were not that bad - I could clearly see that mables were flying stright (well at least in the plane perpendicular to the direction of spin) but I experienced some problems with elevation - I mean that they were reasonably accurate in the horizontal plane

I suspect that it was only casued by the ghettoness (yeah I know there isn't such word but you get the idea) of my homemade hop-up or small differences in size of ammo. AFAIK airsoft hop-ups work quite well right ?

I've just recived a message form D_Hall, he didn't give a direct answer but he provided a link ---> http://en.wikipedia.org/wiki/Drag_coefficient# now I feel like an idiot... well not exactly like a complete idiot but anyway an idiot...
I've seen that page but I was not sure whether it is correct... well check it on your own but the way I understand it it means that cd value (drag coef) of smooth sphere is 0.1 (0.4 for rough sphere.... ) now the question is -> how smooth does the sphere have to be to be regarded as a smooth sphere? :shock:

Unread postPosted: Sat Apr 05, 2008 5:34 pm
Author: jrrdw
"AFAIK airsoft hop-ups work quite well right ?" I've never seen anybody use hop-ups on airsoft. Airsoft was not around when I was growing up. I see youngen's useing them now, but I'm not to familure them.

The only advantage I can think of useing spin on round ammo is curving it the same direction every time. Lave them in there kinda like morters.

Do a experiment, shoot at different psi's and see how far they fly straight at different pressures, if you can still see them that far out, lol.

Unread postPosted: Sat Apr 05, 2008 5:46 pm
Author: Antonio
I actually found out the Cd (drag coefficient) for a bb airsoft ball myself. I found out it was about 0.25 It is fairly easy if you have a chrono or you can make one yourself, like my mic chrono. If you measure the speed at different ranges. You can graph it in excel and see how much in speed the ball slow down.

F=m*a> from the graph u can take the deceleration'' of the bb. If you know the mass of the bb (I used 0.12g) then you can calculate the Drag force on the bb. Cd=Drag force/(0.5*1.225*cross-sectional area ball*velocity^2). As the velocity decreases during its flight the Cd also changes. So it all becomes pretty complicated. To get a, you make a graph of speed against distance. speed^2/distance should give you the acceleration. As the line (speed/distance) is prob not straight, you should choose a point where your speed is close to opperation speed. Yeah.. thats how I got mine. I did it with a spring airsoft gun for consistancy.
If anybody wants the excel sheet just ask me, as though I still gotta retrieve it lol