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cd value

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Unread postAuthor: POLAND_SPUD » Sat Apr 05, 2008 5:55 pm

ok I'll post it in a new topic so that no one would miss it ->>>

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:

@jrrwd yeah of course I meant curving them in one direction otherwise it wouldn't make sense

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.

lol I can see whether they fly straigh or not... but i don't usually see where they hit.. well to be more precise I see them when they hit the concrete wall surounding my property (it is located about 100 - 120m away from the place where I shoot and I am generaly trying to avoid hiting it becasue there is a road behind it 8) ).. some of the marbles explode on hitting it...

anyway really try shooting marbles on a sunny day it really looks cool when you see them flying in random directions

EDIT
@ant thx! but now I am confused why it isn't 0.1 or something close to 0.1 ?

D_hall also wrote that
these things do not scale perfectly. A sphere of size X doesn't necessarily have the same Cd as a sphere of size Y. Will it be close? Yes. Identical? Not necessarily.
(I hope he doesn't mind quoting his message but paraphrasing isn't something I do with pleasure)

I suspect that the problem with scaling cd value, small imperfections on the surface of BBs as well as calculation errors might have affected your result (0.25)...(but still 0.25 sounds reasonable for me)

now what does it mean for spudding community ?
that we cannot tell for sure what's the exact cd value for a given spherical ammo without testing it first but we may assume that it is close to 0.25
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Unread postAuthor: jrrdw » Sat Apr 05, 2008 6:23 pm

ant wrote: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


Unless I missed something your not calculating in the distance change as the projectile curves.

Example: Point A to point B in a straight line is 100yards/meters what ever you prefer add the curve and now you have point C, being how the curve slows the ball down point C is at a shorter distance, changing the equation.

Keep in mind my math sucks, I get this from deductive reasoning, it only makes sense for it to happen this way. Simple phyisics correct?
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Unread postAuthor: D_Hall » Sat Apr 05, 2008 6:29 pm

The biggest issue is that there is no ONE answer to this question.

Different people running different experiments will get different results. Ant mentions running his own experiments. Great! Now, how accurate was his mass measurement? How accurate was his chronograph? How perfect where his spheres? What were the weather conditions? All these things matter. Really, they do!

Further, even if your data is perfect, these things are not truly linear. A perfect sphere 1 m in diameter will NOT have the same Cd as a perfect sphere 1 cm in diameter. Will it be close? You bet! Will it be identical? Nope. In theory, you can scale with Reynold's number but in practice it never really works out if for no other reason than the fact that there is no such thing as a perfect sphere (ball barings are pretty damned close though).

Let me put it this way... In a former life I had access to aerodynamic data produced by the best facilities NASA and the military possess. That data generally came with a caveat... +/- 5%. Now, if NASA can't nail their Cd down to closer than 5%, just what makes you think you can?

All that said, Wikipedia has a pretty nice little chart on Cds for various shapes (although their understanding of how this stuff is applied to cars, planes, etc. is lacking).
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Unread postAuthor: Antonio » Sat Apr 05, 2008 8:21 pm

jrrdw wrote:Unless I missed something your not calculating in the distance change as the projectile curves.

Example: Point A to point B in a straight line is 100yards/meters what ever you prefer add the curve and now you have point C, being how the curve slows the ball down point C is at a shorter distance, changing the equation.

Keep in mind my math sucks, I get this from deductive reasoning, it only makes sense for it to happen this way. Simple phyisics correct?


Oh yeah I missed out one key point> I meant average speed over the length instead of speed. So when I used my mic chrono graph I didnt really have to bother about that factor as it calculates the average speed anyways:)

Cd is linearly dependent on the cross sectional area. Thus a 3 time bigger area gives a 3 time bigger Cd, but yeah then you still dont know where the flow seperates etc.
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