Objectives This study aims to develop a concept design for an unmanned aerial-aquatic cross-medium vehicle that can fly in the air and navigate underwater, and features repeatable medium transitions and superior hydrodynamic performance.

Methods After analyzing the shape of batoid fish that evolved good fluid dynamics performance through natural selection, 3D scanning and mathematical fitting methods are employed to conduct a configuration study of the unmanned vehicle. To achieve covert propulsion underwater, numerical analysis methods are used to determine the propulsion amplitude and frequency by fitting the swimming gaits of batoid fish. To protect the rotor blades and enhance airborne efficiency, an innovative soft hybrid cross-medium approach is developed that uses ducted rotor devices as flight propulsion units. The numerical simulation method is then 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 3 000 r/min, the aerial speed exceeds 100 km/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 to 0.46 MPa stress. Structural strength calculations are 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 0.02 mm.

Conclusions The designed cross-medium unmanned aerial-aquatic vehiclesatisfies the proposed requirements for both aerial and underwater operation. Furthermore, the fluid loads andstructural safety during the air-to-water transition scheme are assessed, ensuring the vehicle's ability to safelyand repeatedly shift between aerial and aquatic environments.