This study focuses on the flapping mechanisms found in recently developed biometric flapping-wing air vehicles (FWAVs). FWAVs mimic the flight characteristics of flying animals, providing advantages such as maneuverability, inconspicuousness, and excellent flight efficiency in the low Reynolds number region. The flapping mechanism is a critical part of determining the aerodynamic performance of an FWAV since it is directly related to the wing motion. In this study, the flight characteristics of birds and bats are introduced, the incorporation of these flight characteristics into the development of FWAVs is elucidated, and the utilization of these flight characteristics in the development of FWAVs is explained. Next, the classification and analysis of flapping mechanisms are conducted based on wing motion and the strategy for improving aerodynamic performance. Lastly, the current research gap is elucidated, and potential future directions for further research are proposed. This review can serve as a guide during the early development stage of FWAVs.

In flapping-wing air vehicles, the flapping mechanism is directly related to the movement of the wing making it one of the major factors in determining aerodynamic performance. In this study, a method to increase aerodynamic performance using the flapping mechanism is discussed. This paper presents a twist-coupled mechanism that can increase thrust by combining twisting motion with flapping motion. The proposed mechanism generates twisting motion by the 4-bar planar link mechanism and flapping motion by the 4-bar spatial link mechanism. The mechanism can be driven by only one actuator by connecting two crankshafts with a pair of gears and rotating them at once. Here, we define the design parameters and constraints and search for the optimal design parameters to maximize aerodynamic force. Optimization is carried out by a genetic algorithm, a global optimization algorithm, combining kinematic and aerodynamic analyses. We then search for the design parameters that maximize thrust. Based on our optimization results, the proposed mechanism has the figure-of-eight wingtip trajectory motion like the flying animals. The aerodynamic efficiency of the proposed mechanism was validated by an aerodynamic measurement test comparing a reference mechanism that can only generate flapping motion without twisting motion. For comparative validation, prototypes of the proposed mechanism and the reference mechanism were designed and fabricated. Thrust and lift were measured by the wind tunnel test. From the wind tunnel test, it is confirmed that the proposed mechanism can generate aerodynamic loads more efficiently than the reference mechanism.