Characteristics of Airfoils

To understand how lift is generated, let’s look at the important properties of an airfoil.

• Angle of attack $\alpha$ is the angle between the chord line of the airfoil and the oncoming airflow. Increasing the angle of attack increases lift (up to a certain point).

• Chord length $c$ is the straight-line distance from the leading edge to the trailing edge of the airfoil.

The planform area is calculated as $A=c\ell$, where $c$ is the chord length and $\ell$ is the span of the wing.

The aspect ratio of a wing is defined as the ratio of its span $\ell$ to its average chord length $c$:

$$r_A=\frac{\ell }{c}$$

Stall

As the angle of attack increases, the lift coefficient $C_L$ also increases—up to a certain critical angle. Beyond this angle, the boundary layer separates form the surface of the wing, leading to stall. This results in a dramatic loss of lift, a sharp increase in drag, and unsteady/turbulent flow around the airfoil.