Insects smaller than about 1–2 mm (thrips, the tiniest parasitic wasps) fly at Reynolds numbers of only 10–100, where air behaves almost like a thick syrup and ordinary steady-state aerodynamics struggles to generate enough lift. Many of them exploit the clap-and-fling mechanism, first described by Torkel Weis-Fogh in 1973: at the top of every stroke the left and right wings swing together until their leading edges touch or nearly touch (the clap), then immediately rotate apart around that shared edge like a book opening (the fling).
During the fling, air rushes into the widening gap between the wings. This starts each wing's bound circulation from a much higher initial value than a wing accelerating alone from rest could ever achieve through the classical Kutta condition — Weis-Fogh's and later Lighthill's (1973) idealized potential-flow analysis of two hinged plates peeling apart showed the circulation builds essentially as fast as the wings separate, avoiding the slow "wake-shedding" delay that limits an isolated translating wing.
Peel (separation) speed: V ~ (dθ/dt) × c
Fling circulation (simplified): Γ ~ π c V = π c² (dθ/dt)
Reference translating circulation: Γqs = π c U sinα
Reynolds number: Re = U c / ν (νair ≈ 1.5×10⁻⁵ m²/s)
This sim animates that kinematic cycle directly: the wing-pair sweeps up, closes flat as it approaches the dorsal midline (angle-of-attack θ eases to zero at the clap), then peels back open at the start of the downstroke. The rate of that peel — set by the fling peel rate slider — sets dθ/dt and therefore the circulation spike computed live and shown as the fling lift boost (the instantaneous fling circulation divided by the reference translating circulation). The clap gap slider controls how close the leading edges actually get; the model attenuates the circulation spike as the gap widens, since a real gap lets air leak around the edge instead of being trapped and accelerated, weakening the effect — consistent with the finding that partial clap-fling (a "near clap") still helps but less than a true touch.
The scaling used here is a deliberately simplified, illustrative version of the Weis-Fogh/Lighthill idealization — real fling aerodynamics depends on wing flexibility, three-dimensional flow escaping past the wingtips, and viscous effects at these low Reynolds numbers, all of which need full computational fluid dynamics (e.g. Miller & Peskin, 2005) to resolve precisely.