Demystifying the Clap & Fling Mechanism: Insights from the Force Partitioning Method (FPM)*

Flying insects employ an intricate flapping motion of their wings to sustain and control flight. Several physical mechanisms have been advanced over the years to account for their large lift coefficients. Using video recordings of hovering Encarsia Formosa (tiny wasps), Weis-Fogh (J. Exp. Biol,1973) was the first to observe the use of the clap & fling (CF) mechanism, where the interaction of the two wings generates additional lift. In CF kinematics, the wings clap together at the end of the upstroke before swiftly flinging apart as the downstroke begins. This synchronized interaction of the wings along with their coupled rotation induces leading edge vortices at the wings first during the clap phase and then again while the wings pronate.  Here we apply the force partitioning method to dissect the lift generation mechanism in wings undergoing CF. The FPM, which is derived using first principles, provides a mathematical partitioning of total pressure force on an immersed body into components due to vortical regions, added-mass effects, and viscous momentum diffusion. FPM is applied to CF kinematics over a range of Reynolds numbers and the results examined to provide new insights into this unique flight mechanism.