Incidence

When an aircraft travels in the plane of symmetry (that is, the direction of flight is parallel to the plane of symmetry), the angle between the direction of motion and the direction of the chord of a profile, as shown in Figure 1.12, is called the geometrical incidence of the profile, denoted by the Greek letter a. The angle a is also called angle of attack.

For an airplane as a whole the geometrical incidence will be defined as the angle between the direction of motion and the chord of the aerofoil. When the chords of various profiles of a wing are parallel the incidence is the same at each section. When the chords are not parallel the incidence varies from section to section and the wing has twist. The value of the geometrical incidence would be altered if a different line were chosen as chord.

In this situation, it will be beneficial to understand the difference between the wing with the chords of its profiles at different locations along the span parallel to each other and the wing with the chords of its

Ch°irl &

profiles at different locations along the span not parallel. We know that the profiles are the cross-sections of the wing geometry, at different locations of the span, in planes parallel to the mid-plane (xz-plane in Figure 1.3) passing through the nose and tail tips of the airplane. Therefore, only for a wing which has its left and right wings parallel to the y-axis in Figure 1.3 the chords of its profile will be parallel, and the wing will be termed cylindrical wing. For a wing with its left and right parts not parallel to the y-axis, the chords will not be parallel, and the wing will be termed a twisted wing.