In a glider review for the Schempp-Hirth Discus, I encountered the following paragraph:
The Discus 2 airfoil is thin (about 14.5 percent), incorporating studies by K.H. Horstmann and Dr. Wuerz (wing) and Luc Boermans (tail).
After researching this a bit, I've been unable to find an explanation for what "14.5 percent" was referring to.
I thought perhaps that it is the ratio (14.5/100) of the thickness of the wing to the width of the wing, but that is really just an educated guess.
The Wikipedia page on airfoils has a section entitled Thin airfoil theory, with two bullets which also might qualify:
(1) on a symmetric airfoil, the center of pressure and aerodynamic center lies exactly one quarter of the chord behind the leading edge (2) on a cambered airfoil, the aerodynamic center lies exactly one quarter of the chord behind the leading edge
Is this "one quarter" the ratio that is mentioned in the article?
I did see this post, but had trouble seeing how the 90% and 99% numbers mentioned in the answer are related.