Interaction of the wakes of two flapping wings on control forces and moment

Control forces and moments of a flapping-wing system are highly affected by wakes and their interaction, but most studies have relied on estimations with an aerodynamic model, which cannot cover the effects of the wakes and the interaction. In this study, the effect of the wakes on the longitudinal control force and moment was investigated. A mean sweeping angle in various sets of wingbeat motion profiles was considered as a control input, and the forces and moments obtained from a rigid wing with an aspect ratio of four, which were controlled by a scaled-up robotic model, were converted to that on a virtual body. The results showed that adjusting a mean sweeping angle, which is originally expected to produce a nose-down moment and a consequent thrust to move forward by tilting a lift force, rather produced a considerable backward force due to the interaction of the wakes. This backward force was larger than the estimations, and the difference gradually increased with an increase in the mean sweeping angle in most of the motion profiles. The force was equivalent to 12.7% of the lift force at maximum, sufficient to resist an initial transition of a flapping-wing system during rapid maneuvers, which needs a strong control force. This suggests that at least for a mean sweeping angle, the effect of wakes must be taken into account. (C) 2021 Elsevier Masson SAS. All rights reserved.