Fish Out of Water: Enabling a Flying Fish Robot to Taxi to Study the Effect of an Asymmetric Caudal Fin on Multi-Medium Locomotion

Abstract

Achieving multi-medium locomotion, especially between water and air, has been difficult for engineers. However, it presents no challenge to nature’s flying fish, who can easily traverse both the sea and the air utilizing the same anatomical structure. Better understanding flying fish anatomy could help unlock hydro-aero locomotion; unfortunately, studying flying fish has proven to be very difficult. They are too fast in water, they cannot be held captive, and their material properties and structure significantly change post-mortem. Princeton’s Bioinspired Adaptive Morphology (BAM) Lab aims to solve the mystery of how and why flying fish fly by studying their structure through an engineering approach: a bioinspired robotic model organism (RMO). A component of biological interest in the flying fish is its asymmetric caudal fin, which is common across all species of flying fish and is not present in many other types. The BAM Lab has studied the effect of the asymmetric caudal fin of the RMO for swimming, which has revealed that an asymmetric caudal fin produces higher thrust and lift than a symmetric one does. One goal of this thesis is to examine the effect of the caudal fin shape in taxiing. To make this possible, this thesis focuses on advancing the RMO to give it the ability to taxi. The design changes to the RMO focused on reducing the overall weight to lower the thrust requirements to exit the water. Due to the lack of stability in free swimming, a new experimental setup was also designed that prescribed the trajectory to a 30 degree exit angle. Using this rig, the design changes proved to be very successful as the new RMO was able to reach taxiing height at a wide range of flapping frequencies, whereas in relative comparison, the original RMO was only barely able to reach taxiing height at very high frequencies. Additionally, further force and moment characterization tests were performed with the new RMO at a 30 degree angle at a submerged and taxiing height with the different fins. The results support previous findings that the asymmetric tail provides higher thrust and lift and likely contributes to the flying fish’s incredible ability to navigate both mediums.