# What is the meaning of the percentage used to describe the thinness of this glider's airfoil?

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.

• 14.5% is a relative thickness (thickness divided by chord) Commented Jul 5, 2017 at 22:22
• You could call it the 'cross-sectional aspect ratio'.
– amI
Commented Jul 5, 2017 at 22:51

14.5% given in the article is the (maximum) thickness to chord ratio, expressed in percentage. It is usually used to explain how thin (or thick) the airfoil is. Note that this is a characteristic of the airfoil.

From the NASA page Wing geometry definitions:

Airfoil thickness to chord ratio, image from NASA page Wing geometry definitions

... The straight line drawn from the leading to trailing edges of the airfoil is called the chord line. ... The maximum distance between the two lines is called the camber, which is a measure of the curvature of the airfoil (high camber means high curvature).

The maximum distance between the upper and lower surfaces is called the thickness. Often you will see these values divided by the chord length to produce a non-dimensional or "percent" type of number.

• Well, at least the answer was on the page "intended for college, high school, or middle school students" :-). Seriously, the linked page does a great job of putting this stuff into layman's language. Thanks for the answer! Commented Jul 6, 2017 at 20:23

• @bclarkreston, yes, the black line is the chord. It is also the $x$ axis, because the profile is normalized to have the leading edge at origin and the trailing edge at $(1, 0)$. The gray line is the camber line. Commented Jul 6, 2017 at 18:58