Fig. 1. Cross section of an airfoil illustrating geometry of the principal model parameters. (A) The arrows indicate that the wing is translating to the left due to root-flapping and rotating about the point r due to active supination or pronation. Point i is the location of the incident flow, which is illustrated in (B). The subscript (r) has been removed from _r, _i and c for clarity. (B) The relevant flow velocity components due to root-flapping and wing rotation and their resultant, _(r,t). Also illustrated are the wing pitch, , the geometric attack angle, _g, and the angle of incidence, ', acting at the point i in (A).