Fig. 6. Velocity vectors near the leading edge at three instants at the beginning of the downstroke on a plane y=10 cm (see Fig. 4A for the orientation of the plane relative to wing). L.E., leading edge. The vectors are colored according to the magnitude of absolute velocity (cm s^-1) and are of constant length. The flow separates at the leading edge (A) and the separation point moves down (B,C) as the downstroke is continued.