Fig. 2. Diagram of sectional flow velocities. The wing is travelling through the fluid at forward velocity V_f. (A) Downstroke. The wing is sweeping into the incident flow. The magnitude sectional flow velocity at wing position r is given by r +V_fcos(). (B) Upstroke. The wing is sweeping with the incident flow. The flow velocity at wing position r is given by –r –V_fcos().