Researchers at MIT and the Swiss Federal Institute of Technology Lausanne (EPFL) have developed a 250-gram robotic bird capable of swimming underwater, launching into the air, and flying. The breakthrough vehicle, which relies on flexible flapping wings rather than propellers, could provide a cost-effective tool for environmental monitoring.
Watching a bird dive beneath the water’s surface is a natural maneuver that has long confounded engineers. While roughly 100 bird species—including puffins, loons, and gulls—seamlessly transition between flight and swimming, recreating this ability in a small-scale robot has proven difficult. Most amphibious robots traditionally rely on separate propulsion systems, such as propellers or legs, to manage the vastly different physical properties of air and water.
Engineering the Flapping-Wing Aerial-Aquatic Vehicle
The team at MIT and EPFL has unveiled a flapping-wing aerial-aquatic vehicle,
or FAAV, that accomplishes the transition using a single set of wings.
The core challenge for the researchers was the density of water, which is about 800 to 1,000 times denser than air. To compensate for this, the robot uses passive flexibility in its wings. The wings bend by up to 90 percent underwater, which limits the load on the motor and shortens the effective sweep of each flap. This design allows the robot to vary its flapping frequency from 0.1 to 6 hertz underwater, increasing to as much as 11 hertz in the air.
Optimizing the Transition from Water to Air
The most precarious moment for the robot is the “split second” when it attempts to breach the surface. Without the benefit of feet for paddling, the FAAV relies entirely on its wings and a precisely angled tail to generate the necessary lift. Experimental testing in water tanks and a lake revealed that a 70-degree exit angle provides the most reliable results.
“If you look at birds, most birds need to paddle their feet at the surface to take off. And the question was, do we need the same for robots? And it turns out we don’t. No one’s been able to fly out of the water with wings.”
Raphael Zufferey, assistant professor of mechanical engineering at MIT
The research, published in the journal Science, highlights a trade-off in wing stiffness. While softer wings improve underwater speed and energy efficiency, they often lack the rigidity required to generate sufficient lift once airborne. The team determined that moderately flexible wings offered the best balance for both mediums.
Future Applications in Marine Research
The project serves as both a technological test bed and a scientific tool. By manipulating variables like wing size and flapping frequency, researchers can test theories about why real diving birds evolve specific physical traits. For instance, the robot’s performance data suggests that diving birds may reduce their wingspan underwater to increase speed rather than simply to conserve energy, a hypothesis that is difficult to test on living animals.
Beyond biomechanics, the researchers envision a practical future for these amphibious drones. Because the robot can be built for approximately US$300 in materials, it offers a low-cost alternative for monitoring sensitive ecosystems.
“Our dream vision is for oceanographers, marine biologists, and members of coastal communities to launch this robot from a boat, or from shore, and it would fly close to the area of interest, such as an iceberg or a port facility, or over a pod of whales. It would dive into the water to take a measurement or collect a sample, and fly back to deliver the data at a fraction of the cost of traditional methods. Then it could go back out to dive for more.”
Raphael Zufferey, assistant professor of mechanical engineering at MIT
Next Steps for Autonomous Deployment
As the team moves forward, they plan to refine the wing design to allow for directional turning, potentially enabling these robots to operate as sophisticated, mobile sensors in some of the most inaccessible parts of the ocean.
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