Why is an airfoil shaped like a teardrop?

Question

I understand the reason airfoils are cambered on the top: to create lift. But one would assume this would result in aircraft wings having a semicircle-shaped design. Why is the cross-section of an airfoil instead more reminiscent of a teardrop on its side and cut horizontally? Why is the front thicker than the back?

Answer 1

The airfoil shape is optimized for two features

• a maximal lift coefficient $c_L$, to get a big lift force
• a minimal drag coefficient $c_D$, to get a small drag force

Maximizing the lift coefficient $c_L$ alone would, as you said, demand for a semi-circle-like shape, like this:

Minimizing the drag coefficient $c_D$ alone would demand for a tear-drop-like shape (with a round front end and a thin back end. See the streamlined body in the image below.

_{(image from Wikipedia - Drag coefficient)}

Putting these two requirements together you arrive at the typical airfoil shape which is kind of a compromise between the semi-circle and the tear-drop shapes from above.

_{(image from Wikipedia - Airfoil)}