Objectives This study aims to develop a concept design for an unmanned aerial-aquatic cross-medium vehicle capable of flying in the air and navigating underwater, featuring repeatable medium transitions and superior hydrodynamic performance.

Methods After analyzing the shape of batoid fish evolved through natural selection for good fluid dynamics performance, the research employed 3D scanning and mathematical fitting methods to conduct configuration studies of the unmanned vehicle. To achieve covert propulsion underwater, numerical analysis methods were used to determine the propulsion amplitude and frequency by fitting the swimming gaits of batoid fish. In order to protect the rotor blades and enhance airborne efficiency, an innovative soft hybrid cross-medium approach was developed using ducted rotor devices as flight propulsion units. Numerical simulation methods were employed to study the unmanned vehicle's rapid underwater movement, aerial performance, and cross-medium capabilities.

Results The results show that during underwater navigation, with a biomimetic fish fin swinging cycle of 2 seconds and a maximum swing angle of 15°, the unmanned vehicle achieves a speed of over 3 knots. During aerial flight, with an attack angle of 7° for the hybrid vehicle and rotor speed set at 3000 rpm, the aerial speed exceeds 100km/h. During the transition from air to water, the average load is 0.17 MPa, with the maximum load occurring at abrupt structural edges, reaching up to0.46 MPa stress. Structural strength calculations were performed for applying 0.17 MPa load at the center bottom and 0.46 MPa load at the bottom edge of the vehicle. The maximum stress occurs at the center bottom, measuring 47.94 MPa, with maximum deformation of 0.17 mm.

Conclusions The designed cross-medium unmanned aerial-aquatic vehicle satisfies the proposed requirements for both aerial and underwater operation. Furthermore, the fluid loads and structural safety during the air-to-water transition scheme have been assessed, ensuring the vehicle's capability to safely and repeatedly shift between aerial and aquatic environments.