Aerodynamic effects on an emulated hovering passerine with different wing-folding amplitudes

Bird flight involves complicated wing kinematics, especially during hovering flight. The detailed aerodynamic effects of wings with higher degrees of freedom (DOFs) remain to be further investigated. Therefore, we designed a novel multiarticulate flapping-wing robot with five DOFs on each wing. Using this robot we aimed to investigate the more complicated wing kinematics of birds, which are usually difficult to test and analyze. In this study the robot was programmed to mimic the previously observed hovering motion of passerines, and force measurements and particle image velocimetry experiments. We experimented with two different wing-folding amplitudes: one with a larger folding amplitude, similar to that of real passerines, and one with only half the amplitude. The robot kinematics were verified utilizing direct linear transformation, which confirmed that the wing trajectories had an acceptable correlation with the desired motion. According to the lift force measurements, four phases of the wingbeat cycle were characterized and elaborated through camera images and flow visualization. We found that the reduction in folding amplitude caused a higher negative force during upstrokes and also induced a greater positive force at the initial downstroke through 'wake capture'. This could increase the vertical oscillation while hovering despite a minor increase in average force production. This phenomenon was not observed during forward flight in previous studies. Our results provide a critical understanding of the effect of wing folding which is required for designing the wing kinematics of future advanced flapping-wing micro aerial vehicles.