Fig. 4. Section schematic of wings approaching each other to clap (A–C) and flinging apart (D–F). Black lines show flow lines, and dark blue arrows show induced velocity. Light blue arrows show net forces acting on the airfoil. (A–C) Clap. As the wings approach each other dorsally (A), their leading edges touch initially (B) and the wing rotates around the leading edge. As the trailing edges approach each other, vorticity shed from the trailing edge rolls up in the form of stopping vortices (C), which dissipate into the wake. The leading edge vortices also lose strength. The closing gap between the two wings pushes fluid out, giving an additional thrust. (D–F) Fling. The wings fling apart by rotating around the trailing edge (D). The leading edge translates away and fluid rushes in to fill the gap between the two wing sections, giving an initial boost in circulation around the wing system (E). (F) A leading edge vortex forms anew but the trailing edge starting vortices are mutually annihilated as they are of opposite circulation. As originally described by Weis-Fogh (1973), this annihilation may allow circulation to build more rapidly by suppressing the Wagner effect.

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