Airfoil - Airfoil Terminology

Airfoil Terminology

The various terms related to airfoils are defined below:

• The suction surface (a.k.a. upper surface) is generally associated with higher velocity and thus lower static pressure.
• The pressure surface (a.k.a. lower surface) has a comparatively higher static pressure than the suction surface. The pressure gradient between these two surfaces contributes to the lift force generated for a given airfoil.

The geometry of the airfoil is described with a variety of terms.

A key characteristic of an airfoil is its chord. We thus define the following concepts:

• The leading edge is the point at the front of the airfoil that has maximum curvature.
• The trailing edge is defined similarly as the point of maximum curvature at the rear of the airfoil.
• The chord line is a straight line connecting the leading and trailing edges of the airfoil.
• The chord length, or simply chord, is the length of the chord line and is the characteristic dimension of the airfoil section.

The shape of the airfoil is defined using the following concepts:

• The mean camber line is the locus of points midway between the upper and lower surfaces. Its exact shape depends on how the thickness is defined;
• The thickness of an airfoil varies along the chord. It may be measured in either of two ways:
  • Thickness measured perpendicular to the camber line. This is sometimes described as the "American convention";
  • Thickness measured perpendicular to the chord line. This is sometimes described as the "British convention".

Two key parameters to describe an airfoil’s shape are its maximum thickness (expressed as a percentage of the chord), and the location of the maximum thickness point (also expressed as a percentage of the chord).

Finally, important concepts used to describe the airfoil’s behavior when moving through a fluid are:

• The aerodynamic center, which is the chord-wise length about which the pitching moment is independent of the lift coefficient and the angle of attack.
• The center of pressure, which is the chord-wise location about which the pitching moment is zero.