Morphing median fin enhances untethered tuna-inspired robotic fish's linear acceleration and turning maneuverability.

Median fins in fish-like swimmers critically govern linear acceleration and maneuvering performance, yet their function remains underexplored in untethered robotic systems. To address this gap, we developed a free-swimming tuna-inspired robotic fish featuring a biomimetic morphing dorsal fin and conducted comprehensive hydrodynamic experiments. Our results demonstrate that dorsal fin erection significantly enhances maneuverability: it reduces head heave by 50%, increases linear acceleration by 27.94%, elevates turning angular velocity by 32.78%, and decreases turning radius by up to 24.89%. CFD simulations also preliminarily validated this mechanism, revealing enhanced vortex effects during median fin erected, along with stronger positive and negative pressure regions around the dorsal fin, which collectively suppress head heave and enhance the wake flow during acceleration. Conversely, the erected fin expands wetted surface area, reducing maximum cruising speed and efficiency during steady swimming. This trade-off mechanism explains why tuna erect median fins transiently during acceleration or turns but retract them post-maneuver to minimize drag. By pre-programming fin folding after acceleration, we confirmed its negligible impact on steady swimming efficiency. This study validates the functional role of morphing median fins in bio-inspired robotics and provides new insights into fin-mediated locomotor control in aquatic organisms.