Calculation of mean camber line

[Tim S.]

Hello,

I would like to calculate the mean camber line of an arbitrary airfoil. Do you know in which script you are calculating this? I created a simplified script for calculating the halfway point between every x-location, but that doesn't capture the LE and TE well. I will explore further to refine this, but in parallel wanted to check how you calculate it in OpenVSP.

Regards,

Tim

[Brandon Litherland]

The camber degen surface is written out from Degen Geom.  So the resulting average of the upper and lower Bezier curves is already available.  You only need to specify what resolution you care about in the model.  There are some example MATLAB functions in the OpenVSP/matlab/ folder that will plot Degen objects like sticks, plates, and surfaces.  

[Rob McDonald]

I do not do a 'real' mean camber line calculation.  I have thought about this, but have not had the time to implement and test an algorithm.

OpenVSP splines the airfoils in an equal-arc-length parameterization.

Our mean camber lines are taken as the mid-point of equal parameter points on the top and bottom surface.

Rob

[Tim S.]

If I understood it correctly you have the same parametrization of the top and bottom airfoil spline in the x-direction, meaning the current algorithm will move through the x-positions and for the same  coordinate the y-position for the top and bottom spline is evaluated and then divided by two in order to have the mean camber line. This is how I have implemented it as well for OpenAeroStruct. The deviation I am getting to OpenVSP lies in the <1% range which is fine. 

Thanks for clearing this up, I saw (after posting) that you already talked about this in an earlier thread, just wanted to make sure I understood the calculation method correctly.

[Rob McDonald]

I think you have the right idea.

The top and bottom surface need 'matching' parameterizations -- whatever that means.

Airfoil files are usually specified with Y values at certain X points.  The obvious parameterization for that is an X- parameterization.  When you take an airfoil file and average matching top / bottom points, you get a certain mean camber line.

I parameterize the upper and lower curves with an arc-length parameterization (scaled to [0,1] for each).  Then, equal parameter points are paired and averaged.  This mean camber line will be slightly different than the equal x-coordinate.

Neither one is the true mean camber line (as defined by thin airfoil theory) but that takes more work to calculate -- and I haven't gone to the trouble yet.  Someday...

Rob

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