MIT and EPFL build first bird-scale robot that swims, dives, and flies with flapping wings alone

Researchers at MIT and EPFL have built a 250-gram robot that can swim underwater, dive, launch back into the air, and fly using only flapping wings, no propellers, legs, or folding mechanisms. The robot mimics the roughly 100 bird species that transition fluidly between water and air, a feat no bird-scale machine had previously achieved. Water is about 800 times denser than air, making the transition between mediums exceptionally difficult without swapping hardware. The robot solves this with flexible wings that passively bend up to 90% underwater, reducing motor load and shortening each flap's sweep. In air the wings flap up to 11 Hz; underwater the rate drops to between 0.1 and 6 Hz. The device is neutrally buoyant, avoiding energy waste fighting buoyancy. The critical transition, launching from water into flight, completes in under a second using 8 to 10 wingbeats, but only within narrow conditions: intermediate wing stiffness, a short tail close to the body, and an exit angle near 70 degrees. The work demonstrates a mechanically simple approach to multi-fluid locomotion that could inform future robotic platforms operating across air and water.