Experimental study of the effect of caudal fin stiffness on taxi locomotion of a robotic flying fish model.

The flying fish is one of just a handful of animals able to locomote through both air and water, excelling at both long-distance gliding flight and agile swimming. In order to leave the surface of the water, the flying fish frequently performs a mode of locomotion unique to the animal known as "taxi". In taxi, the flying fish will be entirely above water except for its caudal fin which remains partially submerged. It flaps its caudal fin vigorously to generate thrust with its unusually large pectoral fins extended until these "wings" generate enough lift to become airborne. Taxiing not only enables a fish to take flight after swimming, but also allows it to extend the duration of its glide after losing height. It is also one of the least studied areas of flying fish locomotion despite being critical to its multimodality. In this presentation, we seek to understand how caudal fin stiffness impacts the resulting flow fields, and therefore hydrodynamics, in flying fish taxi locomotion compared to swimming. We use a biologically relevant robotic model to experimentally study this via water channel experiments with different bioinspired caudal fin designs. High speed video and bulk force and torque measurements are collected to study the resulting kinematics and force generation. We then perform particle image velocimetry on the wake of the partially submerged flying fish robot to visualize the flow field.