Fig. 7. Streamlines of the fluid flow around two wings (A) and one wing (B) at a Reynolds number of 8 and around two wings (C) at a Reynolds number (Re) of 128 for a fling half-stroke. The arrow on the left wing shows the direction of the normalized force acting on the wing at each time (i–vi). The wings begin at an angle of attack of 90° and rotate about the trailing edge to an angle of attack of 45°. (A) During rotation, attached leading edge vortices are formed on each wing and no trailing edge vortices are formed (i–iii). When translation begins, small attached trailing edge vortices begin to form (iii–v). As the trailing edge vortices grow in size relative to the leading edge vortices, lift is reduced. The leading edge vortices, however, remain larger than the trailing edge vortices for most of the half-stroke (v–vi). (B) In the one wing case, attached leading edge and trailing edge vortices are formed during rotation (i–iii). When translation begins, equally sized leading and trailing edge vortices are attached to the wing, creating substantially lower lift forces in comparison to the two-winged case (iii–vi). (C) At a Reynolds number (Re) of 128, attached leading edge vortices are formed on each wing and no trailing edge vortices are formed initially (i–iii). When translation begins, however, the leading edge vortices are shed, and trailing edge vortices are formed (v–vi). The trailing edge vortex grows in size and is subsequently shed from the wing as a new leading edge vortex begins to form. (D) Flow visualization of fling at Re=30 by Maxworthy (1979). Similar to case A, a pair of large leading edge vortices is formed and remains attached to the wing during rotation. A smaller pair of trailing edge vortices is formed and grows during translation. (E) Flow visualization of fling at Re=1.3 x10⁴. Similar to case C, a pair of large leading edge vortices (1) forms during rotation and is shed during translation. A new pair of leading edge vortices forms during translation (2).