Trajectory optimization of a flapping-wing robot in a wind field

Owing to wave obstruction, the speed of the horizontal wind field decreases as it approaches the sea surface. Consequently, the wind field that increases with height is referred to as a gradient wind field. On the sea surface, albatrosses exploit this gradient wind field to glide efficiently, enabling them to fly thousands of kilometers using wind energy. As an emerging class of bionic flying robots, flapping-wing robots mimic birds’ flying methods. This wind energy utilization behavior holds significant potential for flapping-wing robots, offering a promising solution to address their current endurance limitations. To utilize wind energy effectively, albatrosses rely on their exceptional gliding characteristics. This study replicates the aerodynamic features of albatrosses, including their high aspect ratio and superior lift-to-drag ratio, to enhance the performance of the independently developed flapping-wing robot, USTB-Hawk. Given that the improved flapping-wing robot must transition between flapping flight mode and gliding flight mode, this paper also introduces a mode-switching mechanism. This mechanism, based on a ratchet stop system and a flapping phase detector, ensures stability in the gliding posture during flight experiments. In addition to aerodynamic characteristics, albatrosses primarily exploit wind energy by continuously ascending and descending within the gradient wind field, achieved through efficient planning of their gliding trajectory. To simulate the gliding trajectory of the improved flapping-wing robot, it is essential to determine its aerodynamic characteristics in gliding posture, including lift and drag data across different angles of attack. This study conducts fluid mechanics simulations on the improved flapping-wing robot. In the simulation, the robot’s design is simplified, with some complex structures removed, to reduce computational costs and model complexity. The results indicate that the gliding posture of the flapping-wing robot avoids a stall state within an angle of attack range of −10°– 20°. In addition, the lift generated by the robot is sufficient to counteract gravity at angles of attack between 2.86° and 20°. With the enhanced aerodynamic characteristics identified, this study further investigates the gliding trajectory of the flapping-wing robot by integrating the gradient wind field model with the kinematic model of the gliding posture for various trajectory angles. Considering that the trajectories of different entry angles vary under a constant wind field, this study conducts a detailed analysis of the gliding trajectories corresponding to different initial heading angles for the same track angle. Trajectories with track angles of −30°, 0°, 30°, and 60° are selected for flight experiments in a real wind field. The experimental results reveal that the energy consumption of a gliding flight is significantly lower than that of a flapping flight over the same distance. These findings demonstrate that the flapping-wing robot can effectively utilize wind energy and enhance its endurance by strategically planning its gliding trajectory within the gradient wind field.