A new robot swims underwater with powerful flaps, bursts out of the water, and takes to the air — all using the same pair of flapping wings.
In a study published in Science, researchers report a 250-gram flapping-wing robot, demonstrating how flexible wings, adjustable flapping frequencies, and careful design choices allow seamless transitions between water and air. The study, they say, offers new insights into how diving birds achieve the same feat and opens possibilities for aquatic-aerial robots.
“We developed flapping-wing aerial-aquatic vehicles that can fly in the air, swim under water, and transition between the two media,” the authors write.
The researchers have long been perplexed by wing-propelled diving birds that flap through both air and water. The three-order-of-magnitude difference in density between air and water creates steep challenges for any vehicle trying to operate in both. Diving birds solve this by changing flapping frequency and sometimes reducing wing area, but studying exactly how they do it in the wild is difficult.
To overcome those limitations, Raphael Zufferey — a mechanical engineer at Massachusetts Institute of Technology — and his colleagues built a 250-gram, untethered robot with a slender fuselage, symmetric membrane wings, and an adjustable tail. The design allowed systematic testing of wing size, stiffness, and flapping rates in controlled experiments.
The robot adjusted its flapping from roughly 1 Hz underwater to as high as 11 Hz in air. Highly flexible wings — far more compliant than bird bones — proved especially effective. They passively deformed underwater to lower loads on the motor, enabling higher frequencies and smoother transitions than rigid wings would allow.
Medium-sized wings with moderate stiffness offered the best overall performance. They generated enough lift and thrust for flight while still permitting efficient swimming. The robot achieved average flight speeds of 6.3 meters per second and could cover about 6 kilometers on a single battery charge in the air, or 2 kilometers while swimming. Superhydrophobic coatings helped shed water rapidly during takeoff.
Tail length and the angle at which the robot emerged from the water were critical for success. A short tail reduced unwanted pitching moments as the body left the surface. An emergence angle near 70 degrees produced the most reliable exits.
Massachusetts Institute of Technology
“Medium wing stiffness played an equally crucial role in successful transitions,” the researchers explain. They observed that wings that were too flexible struggled with takeoff and flight, while overly stiff wings caused destabilizing oscillations at the surface.
The robot also demonstrated plunge-diving, entering the water from a 5 m/s aerial approach and immediately transitioning to underwater propulsion. Although water egress demanded high power, the overall energy costs compared favorably with those measured in living animals.
Journal Reference: Science. DOI: 10.1126/science.aeb6744
