



Question
Airfoil Percent Thickness Computation
Hey guys,
A simple question that I think I know the answer to, but would like to verify: How is an airfoils' percent thickness computed? In the attached JPG, does the "simplified airfoil" have a 5% (max Yvalue) or a 2.5% (average Yvalue) or some other value percent thickness? Thanks, Todd 




Thanks nuevo! Right now I don't have an airfoil program that's easy to use, but as time permits I'm working on learning XFLR5....
So for an airfoil with a curved top surface AND a curved bottom surface (as most airfoils are), do airfoil programs still use the max value on the Yaxis or does it also subtract off the bottom surface giving a relative distance between the two or do they perform some other (presumably calculus based) algorithm? 



It's the max value between the top and bottom surface. So, if you have a 10" chord and 0.85" maximum distance between the top and bottom surface, you have an 8.5% airfoil. It can be symmetrical, flat bottom or under cambered, all are 8.5%.




Todd,
Here's a great online book that's free to read: http://www.desktop.aero/appliedaero/...lgeometry.html Also here is a great book too on polars: http://www.b2streamlines.com/books/booktitles.html Curtis Montana 



Thanks for the links Curtis!
In the past I counted on Profili2 to do all the work for me of designing airfoils. I no longer have a PC, but have found the light and switched over to a Mac There is XFLR5 for a Mac (it's a Unix based OS). I just haven't learned to use XFLR5 yet, but I will.... 



I didn't have time before. Below is an image showing a sample airfoil I generated. The chart at the bottom displays the thickness of the airfoil (10%), and the point where that max thickness occurs (30% chord). I drew a yellow line showing approximately where the max thickness occurs. Hoping this picture will clear up any questions you had.




Yep, nuevo, thanks!
My next step is to determine what percent thicknesses will be needed to maintain a constant "rise" value on the Yaxis as the Xaxis grows. This "airfoil" (well actually an obtuse shaped triangle, not the shown right triangle) will be used as a flap on a wing that has a constant TE thickness. A bit of math and I should have it ready to send off to Laszlo at Compufoam...... 


Joined Feb 2010
201 Posts

Which version of XFLR5 are you using.
My version do not provide such mistake you found. The last one is 6.02 and the 6.03 is n the way... I strongly recommand everybody to look at XFLR5 website regularly to get updating. Marc 
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