SRYGLEY, R.B.*; THOMAS, A.L.R.: Flow Visualizations with Free-flying Butterflies Reveal a Variety of Unconventional Lift-generating Mechanisms

Flying insects generate very high lift forces- typically twice as much lift per unit wing area as conventional wings achieve- and sometimes an order of magnitude more. Three hypotheses compete to explain the mechanism insects use to generate these very high forces: delayed stall (leading edge vortices), rotational mechanisms, and wake capture. Each of these mechanisms has received some support from experiments with large-scale mechanical models and tethered insects. Here we present flow visualizations of free-flying Red Admiral butterflies (Vanessa atalanta), and use critical point theory to describe the observed flow. In free flight, butterflies use an unexpected and striking diversity of aerodynamic mechanisms to generate force. We find evidence for wake capture, three different types of leading edge vortex, and for the ability to switch between active and inactive upstrokes. Leading edge vortices were most prevalent when accelerations were large. Minimizing leading edge vortices during steady forward flight reduces the drag imposed by capturing streamwise fluid and accelerating it into a spanwise flow. Wake capture was not evident when the insect accelerated forward ahead of its shed vortex, indicating that wake capture is in part contingent on passive interactions between the shed vortex and the flapping wings, and in part under active control of the insect. We also find evidence for rotational mechanisms- a clap and fling- used to generate thrust and circulation. The ability to generate aerodynamic forces in many qualitatively different ways on each of a series of successive wing beats may explain how butterflies flutter.