Aerofoil geometry

If a horizontal wing is cut by a vertical plane parallel to the centre-line, such as X-X in Fig. 1.4, the shape of the resulting section is usually of a type shown in Fig. 1.6c.

This is an aerofoil section. For subsonic use, the aerofoil section has a rounded leading edge. The depth increases smoothly to a maximum that usually occurs between £ and way along the profile, and thereafter tapers off towards the rear of the section.

If the leading edge is rounded it has a definite radius of curvature. It is therefore possible to draw a circle of this radius that coincides with a very short arc of the section where the curvature is greatest. The trailing edge may be sharp or it, too, may have a radius of curvature, although this is normally much smaller than for the leading edge. Thus a small circle may be drawn to coincide with the arc of maximum curvature of the trailing edge, and a line may be drawn passing through the centres of maximum curvature of the leading and trailing edges. This line, when produced to intersect the section at each end, is called the chord line. The length of the chord line is the aerofoil chord, denoted by c.

The point where the chord line intersects the front (or nose) of the section is used as the origin of a pair of axes, the x-axis being the chord line and the j-axis being perpendicular to the chord line, positive in the upward direction. The shape of the section is then usually given as a table of values of x and the corresponding values of y. These section ordinates are usually expressed as percentages of the chord, (100x/c)% and (100yjc)%.