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Old Feb 01, 2011, 05:10 PM
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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 Y-value) or a 2.5% (average Y-value) or some other value percent thickness?

Thanks, Todd
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Old Feb 01, 2011, 05:22 PM
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I don't have any facts to back my estimate but I'd say 5%.
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Old Feb 01, 2011, 06:32 PM
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Airfoil programs display maximum % thickness. 5% in your example.
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Old Feb 01, 2011, 07:33 PM
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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 Y-axis 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?
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Old Feb 01, 2011, 08:58 PM
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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%.
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Old Feb 01, 2011, 09:25 PM
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ok great, thanks for the explanation!
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Old Feb 02, 2011, 08:03 AM
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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
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Old Feb 02, 2011, 11:29 AM
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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....
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Old Feb 02, 2011, 12:07 PM
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Todd, XFLR5 is a 1 stop shop for analysis. It's a little unfriendly at first, but well worth the effort to learn. Go for it!
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Old Feb 02, 2011, 12:40 PM
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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.
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Old Feb 03, 2011, 10:27 AM
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Yep, nuevo, thanks!

My next step is to determine what percent thicknesses will be needed to maintain a constant "rise" value on the Y-axis as the X-axis 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......
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Old Feb 09, 2011, 07:16 AM
Marc PUJOL
Joined Feb 2010
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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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