A flapping-wing robot that can both swim underwater and fly through the air is helping scientists rethink how diving birds manage life in two radically different worlds. "In addition to shedding light on the morphological and behavioral adaptations of aerial-aquatic animals, the design principles described here lay the foundation for a class of robots that can be used for limnology, oceanography, marine ecosystem monitoring, and coastal management," the authors write. Roughly 100 bird species are capable of both flying through the air and propelling themselves underwater using only their wings. While these birds use a similar flapping motion in both environments, they adjust by slowing their wingbeats and reducing their wing area underwater. Because water is nearly 1,000 times denser than air, moving efficiently in each medium requires very different forces and wing movements. However, studying these behaviors and movements in live animals is challenging, and computer simulations struggle to model the complex interactions between flapping wings, fluid forces, and the transition from water to air accurately. As a result, the physiological adaptations and compromises that allow birds to move through such disparate environments efficiently remain relatively unknown. According to Raphael Zufferey and colleagues, robotic models offer a valuable alternative because they adhere the same physical principles as living animals while enabling researchers to control their design and movement precisely.
In this study, Zufferey et al. present flapping-wing robots capable of flying through the air, swimming underwater, and transitioning seamlessly between the two environments – aerial-aquatic vehicles designed to explore the physical challenges faced by diving birds and the design strategies that make dual-environment locomotion possible. The modular, 250-gram robot features a streamlined fuselage, two flexible membrane wings, and a movable tail. It is also fully waterproof, untethered, and equipped with onboard electronics, allowing the authors to adjust wing-flapping frequency and tail position wirelessly, to examine systematically how wing size, flexibility, and flapping influence movement in air, underwater, and during transitions between the two environments. By comparing data from diving birds with their experimental observations, Zufferey et al. found that complex wing-folding mechanisms are not essential for aerial-aquatic locomotion. Instead, an effective balance of wing flexibility, size, and flapping frequency is sufficient to achieve similar performance. What's more, experiments revealed that smaller wings increase underwater speed but do not improve swimming efficiency, suggesting that reduced wing size in diving birds may primarily enhance maneuverability and prey pursuit rather than conserve energy. Wings with intermediate flexibility provided the best overall performance, improving underwater propulsion while still generating sufficient lift for flight. Because flying requires less energy than swimming, the authors discovered that it was more efficient for the robot to leave the water and fly than to remain submerged over longer distances. The study also showed that wing-powered takeoff from the water is possible without leg assistance, although it requires substantial power.
