Butterflies fly using efficient propulsive clap mechanism owing to flexible wings

<p class="Teaser">Butterflies look like no other flying animal, with unusually short, broad and large wings relative to their body size. <span>Previous studies have suggested butterflies use several unsteady aerodynamic mechanisms</span> <span>to </span>boost force <span>produc</span>tion with <span>upstroke wing</span> <span>clap </span>being a prominent feature<span>.</span> When the wings clap together at the end of upstroke the air between the wings is pressed out, creating a jet, pushing the animal in the opposite direction. <span>Although </span>viewed, for the last 50 years, <span>as a </span>crucial <span>mechanism</span> in insect flight<span>, quantitative </span>aerodynamic <span>measurements of the clap in freely flying animals are lacking</span>. Using quantitative flow measurements behind freely flying butterflies during take-off and a mechanical clapper, we provide aerodynamic performance estimates for the wing clap. We show that flexible butterfly wings, forming a cupped shape during the upstroke and clap, thrust the butterfly forwards, while the downstroke is used for weight support. We further show that flexible wings dramatically increase the useful impulse (+22%) and efficiency (+28%) of the clap compared to rigid wings. Combined, our results suggest butterflies evolved a highly effective clap, which provides a mechanistic hypothesis for their unique wing morphology. Furthermore, our findings could aid the design of manmade flapping drones, boosting propulsive performance.</p>