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How to generate equations/formula for any airfoil

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Old   April 18, 2010, 11:17
Default How to generate equations/formula for any airfoil
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Hi,

I understand that for NACA00xx airfoils, there's a formula which can output the y coordinate given the x coordinate.

However, if I need a similar formula for a general airfoil whereby I know x/y coordinates, how can it be done?

Any recommendation?

Thanks alot!
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Old   April 18, 2010, 16:21
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If your points are well behaved (no errors in the data) you can always use an interpolation function to obtain formulas for the upper side and the lower side of your airfoil. After all you can easily fit a polynomial function on a given set of points.

If using interpolation won't give you a nice surface (this will be the case for non accurate data) you can always use a least square approximation with orthogonal polynomials (e.g. Chebyshev) to obtain an analytical formula for your airfoil.

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Old   April 19, 2010, 00:15
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Thanks DoHander.

In fact, the airfoil I'm interested in is the s1020 airfoil, as shown over here:



Currently I have a total of 60 points for this airfoil. Based on your experience, which mtd do you think it's better to get a good fit? Is it possible to use the 30pts for the top and bottom airfoil with the least square approximation?

Thanks again!
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Old   April 19, 2010, 10:52
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Yes, I think you can use the least square methods with 30 points. Probably you can use Matlab to solve the problem, if I remember correctly it has a least square fit, not sure however if Matlab use simple polynomials or orthogonal polynomials.

In the past I've used a C# code to solve the problem, but any programming language you know will work.

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Old   April 19, 2010, 12:14
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If you download some software such as designfoil you can make most airfoils i think. You can also export these to CAD software as well.
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Old   April 19, 2010, 19:01
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EDIT: Nevermind ...
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Old   April 19, 2010, 23:56
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Just use a cubic spline interpolation. Matlab has builtin functions for doing this. Fit separate cubic splines for top and bottom curves.
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