Parachuting at low Reynolds number with bristled wings

Miniature flying insects such as thrips have been observed to intermittently cease flapping and float passively downwards by spreading their bristled wings. This type of drag-based parachuting can lower their falling speed and aid in long-distance dispersal. Though bristled wings have been shown to reduce drag in the flapping flight of tiny insects, it is unknown whether bristled wings also reduce drag during parachuting. Forewing inter-wing angle (θ) of 97.4° was measured from a published video of parachuting thrips, along with a falling speed of 0.6 m/s and span-based Reynolds number (Re_s) of 40. Numerical simulations of steady flow past a non-bristled wing pair and two bristled wing pairs varying in the number of bristles were used to examine drag coefficients (C_D) and inter-bristle flows, for Re_s ranging from 20-400 and θ ranging from 20°-180°. Force measurements were conducted on equivalent physical models that were towed in an 8-foot long fluid-filled tank. Irrespective of wing design, fluid barriers were observed within inter-bristle gaps at Re_s=20. These fluid barriers maintained constant aerodynamic loading for θ ≥ 100° by maximizing drag force and minimizing leakiness. Irrespective of Re_s, densely bristled wings provided maximum aerodynamic loading for θ ≥ 100°.