Fig. 3. Mean lift production in dragonfly model wings depends on both the phase-shift between fore- and hindwing stroke cycle and the vertical separation between the two wing hinges. (A) Vortical structures produced by a dragonfly hindwing at 0.35 hindwing stroke cycle. The wing moves from left to right (open arrow). The wing blade element that runs through the center of force at 0.65 wing length is indicated by a white line and the leading edge is indicated by a triangle, as shown in G. Inclination and length of the black vectors indicate direction and relative magnitude of fluid velocity. (B,C) Schematic explanation of how vertical spacing (s) between fore- and hindwing stroke plane changes kinematic phase relationship between both wings (t) due to vortex travel time at 0.35 stroke cycle. When the forewing leads wing motion, destruction of hindwing LEV by the forewing's start vortex depends on the phase-shift between both stroke cycles and the vertical distance between both ipsilateral wing hinges. (D) DPIV image showing the interference between vortical structures produced by fore- and hindwing at 0.15 hindwing stroke cycle. The hindwing leads wing motion by 0.25 stroke cycle. While the forewing (upper blade element) starts wing rotation at the end of the downstroke, its trailing edge vortex interferes with the LEV produced by the hindwing (lower blade element). The hindwing's start vortex still appears to be attached to the wing's trailing edge. (E,F,H,I) Vertical position (E,F) and vertical travel velocity of start vortices (H,I) shed from the fore- (black) and hindwing (blue). The vortex positions were only reconstructed from flow images in which the vortical structures were clearly visible. The data are recorded while the forewing (E,H) and the hindwing (F,I) was leading wing motion by a quarter stroke cycle, respectively. Shaded areas in H and I indicate the period during which the forewing start vortex interferes with the hindwing wake, and the dotted lines indicate maximum downwash velocity measured in the hindwing wake. R=wing length, LEV=leading edge vortex, HW=hindwing, FW=forewing.