# Homework Help: Aerodynamics- Coefficient of Drag

1. Feb 24, 2015

### tsukuba

1. The problem statement, all variables and given/known data
Hello, Lets say I was given a problem where I have to calculate CD

I find everything except CD
Now, CD = CD0 + K CL2
I know how to calculate K but I don't know what CD0 is. Would that be a given? or do I have to find it myself? If so, how?

Then theres another formula for CD which is
= CDmin + K (CL - CL0 )2

In this case what is CL0

Thanks for your help

2. Relevant equations
D=T= 1/2 ρ V2 S CD

3. The attempt at a solution
No solution required. Just need an explanation as to how I get those numbers

2. Feb 25, 2015

### paisiello2

3. Feb 25, 2015

### tpv

Whenever dealing with equations, it is necessary to let the textbook define the variables - as not all textbooks use the same definitions. All that said, in aerodynamics, it is normal to define the drag coefficient as the sum of the profile drag and the induced drag.
CD = Cd + k CL2
The left side of this equation applies to wings of fixed length. The first term on the RHS is the profile drag and it is usually defined as the drag coefficient associated with an "infinite wing" length - that is to say, what you obtain when measuring drag on a wing that spans the width of a wind-tunnel. The second term on the RHS is the induced drag, and results from the fact that a wing of finite length has more drag than a wing of infinite length. Therefore, if this is what your equation is referring to, then the CD0 term in your question is the profile drag, and you would obtain it by finding wind-tunnel tests for the wing you are considering. NACA was started to provide such things.

However, as profile drag itself follows the same basic curve as the total drag, it is also possible that your equation is being applied to a wing of infinite length. In that case, CD0 would simply be the drag coefficient when CL = 0. (ie the intercept of the curve with the CD axis).

I hope this helps.

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