Lin Y.-JChang S.-KLai Y.-HYang J.-T.JING-TANG YANG2022-03-222022-03-22202120545703https://www.scopus.com/inward/record.uri?eid=2-s2.0-85114632847&doi=10.1098%2frsos.202172&partnerID=40&md5=e6442224f8122bdfb5614d3cc2a9d5d6https://scholars.lib.ntu.edu.tw/handle/123456789/598057Unlike other insects, a butterfly uses a small amplitude of the wing-pitch motion for flight. From an analysis of the dynamics of real flying butterflies, we show that the restrained amplitude of the wing-pitch motion enhances the wake-capture effect so as to enhance forward propulsion. A numerical simulation refined with experimental data shows that, for a small amplitude of the wing-pitch motion, the shed vortex generated in the downstroke induces air in the wake region to flow towards the wings. This condition enables a butterfly to capture an induced flow and to acquire an additional forward propulsion, which accounts for more than 47% of the thrust generation. When the amplitude of the wing-pitch motion exceeds 45°, the flow induced by the shed vortex drifts away from the wings; it attenuates the wake-capture effect and causes the butterfly to lose a part of its forward propulsion. Our results provide one essential aerodynamic feature for a butterfly to adopt a small amplitude of the wing-pitch motion to enhance the wake-capture effect and forward propulsion. This work clarifies the variation of the flow field correlated with the wing-pitch motion, which is useful in the design of wing kinematics of a micro-aerial vehicle. ? 2021 The Authors.Butterfly flightForward propulsionInduced flowWake-capture effectWing-pitch motionjournal article10.1098/rsos.2021722-s2.0-85114632847