HELP drag/lift coefficient using renolds number

[Nemrav]

what is stubbing me is that I can't seem to find a formula that allows me to calculate drag or lift effectively and this is why :

to get lift, you need the lift coefficient

to get the lift coefficient, you need to know the lift.....

so as far as the coefficient for either is concerned, its out the window for both, found out that Reynolds number can do it but....

what's the formula that uses the Reynolds number ?

if you find the formula, please list what each variable means.....

and as a starting place I found these sites : http://www.civil.uwaterloo.ca/enve214/files/reynolds_number_and_drag.pdf

http://www.physicsforums.com/showthread.php?t=139377

and no, I tried NASA, they only provide the paradox formulas....

please note that I have only so far tried to get the drag coeffient formula, I'l bump the thread when I search for lift coefficient....

Edited March 27, 2014 by Nemrav

[PUNiSH3R]

You work from empirical data for a known airfoil known as a "polar diagram". Dr. Hepperle has a decent explanation on his site, http://www.mh-aerotools.de/airfoils/hdipolar.htm. There is also a decent book for the non-mathematically minded called "Understanding Polars Without Math".

[K^2]

You do not compute lift/drag from Reynolds Number. A table of values that is either experimentally obtained or numerically evaluated for a given wing foil/geometry is only valid for a given Reynolds Number. So you need to take your specific wing, compute Reynolds Number for your aircraft, and that will give you a specific table or polar that you should be using to obtain the data you need.

So if you go to the link that PUNiSH3R posted, you'll see at the top left of the diagram "NACA 4412 12%" and "Re = 10⁵". This is for a wing of "infinite" span with geometry NACA 4412 and is valid at Reynolds Number 10⁵.

To be honest, I'm not sure what the 12% refers to. It might be the maximum thickness as fraction of the chord, but that seems redundant, as "4412" already specifies that.

[architeuthis]

That first link touches on it. Drag coefficients are complicated functions of geometry, Reynolds numbers, Mach numbers, Froude numbers (for partially submerged ship hulls) and relative surface roughness effects, therefore determining them analytically is often only done as an exercise for simple shapes. More often they are determined numerically, or empirically. Typically for aircraft and rockets Cd is not highly dependent on Re because all else being equal Cd approaches a constant value for high Re.