Trajectory Planning with a Bioinspired Flexible Propulsor for Sustained Thrust Production across the Air-Water Interface

When jumping from the water to catch prey, archer fish generate sustained propulsion while exiting the water with minimal surface disturbance. Inspired by this behavior, we developed a bioinspired flexible propulsor controlled by a custom apparatus that independently varies flapping frequency, amplitude, and vertical translation velocity. We quantified the performance of propulsors of varying stiffnesses across a wide parameter space of trajectories, measuring propulsive forces with a 6-axis load cell and capturing wake structures using 2D PIV. Thrust and efficiency metrics were analyzed from partially- and fully-submerged fixed-depth cases, as well as cases with constant vertical velocity. These fixed and variable depth results were used to design trajectories ideal for generating sustained thrust as the propulsor exits the water, and the resulting performance of these trajectories was analyzed. These findings provide a framework to design propulsion strategies for effective transitions from aquatic to aerial environments.