When One Wing Does the Work of Two

The Engineering Challenge of Moving Through Air and Water

Every flying robot is designed for the air, while every underwater robot is built for water. The reason is simple: the two environments place very different demands on propulsion. Water is nearly 800 times denser than air, which changes how lift, thrust, and drag are generated. A wing that performs efficiently in flight usually creates excessive resistance underwater, while a propeller designed for swimming cannot generate the lift needed for flight. As a result, most aerial-aquatic robots rely on separate propulsion systems, adding weight, complexity, and energy demands.

Researchers at the Massachusetts Institute of Technology (MIT) and the Swiss Federal Institute of Technology Lausanne (EPFL) explored a different approach. Instead of combining two propulsion systems, they investigated whether a single pair of flapping wings could generate movement underwater, transition through the water's surface, and continue flying. The result is the Flapping-wing Aerial-Aquatic Vehicle (FAAV), an experimental robot inspired by diving birds that demonstrates this concept using one continuous flapping-wing mechanism.

Learning From Birds Instead of Machines

Many seabirds regularly move between air and water while searching for food. Puffins, guillemots, and petrels use the same wings to fly through the air and propel themselves underwater. Rather than changing propulsion systems, they adapt the motion of their wings to suit each environment. Reproducing this ability in a robotic platform has remained a difficult engineering problem because artificial wings must satisfy two very different aerodynamic and hydrodynamic conditions.

To better understand this natural solution, the research team studied bird locomotion before designing the robot. They analyzed wing dimensions, flapping motion, and body posture to identify characteristics that could be transferred into an engineering system. The resulting prototype weighs less than 300 grams and consists of a lightweight body, two flexible membrane wings, and a movable tail that controls the robot's orientation during swimming, transition, and flight.

One Pair of Wings With Multiple Functions

The most important contribution of this research is not simply that the robot can both swim and fly. It is that the same flapping-wing mechanism performs every stage of movement without switching to another propulsion system.

An electric motor drives both wings through a repeated flapping motion. Underwater, the flexible membrane wings naturally bend under hydrodynamic forces, reducing resistance while maintaining forward propulsion. As the robot leaves the water, the wings recover their aerodynamic shape and begin generating lift for flight. This passive structural adaptation allows the robot to move between two very different environments without changing its mechanical configuration.

The researchers also coated the membrane wings with hydrophobic nanoparticles that help water drain quickly during takeoff. At the same time, the movable tail continuously adjusts the robot's body angle to maintain stability. Through repeated experiments, the team found that a tail angle of approximately 70 degrees, combined with medium-sized wings and a flapping frequency of about five wing beats per second, produced the most reliable transitions between swimming and flying.

What the Research Demonstrated

The project was designed as an engineering investigation rather than a product demonstration. Instead of testing a single configuration, the researchers evaluated multiple wing sizes, flapping frequencies, and tail positions in laboratory water tanks before conducting outdoor experiments in Lake Geneva. Each experiment helped identify how different design choices influenced swimming performance, flight stability, and the transition between the two environments.

One of the study's most significant findings involved the transition from water to air. Many diving birds use their feet to paddle across the surface before becoming airborne. The researchers demonstrated that the robot could leave the water and continue flying using only its flapping wings. This result suggests that additional propulsion systems may not always be necessary for aerial-aquatic robots when wing flexibility, body orientation, and flapping motion are carefully coordinated.

Although the FAAV is an experimental research platform, the work contributes to a broader understanding of multi-environment robotics. The researchers suggest that future systems based on similar principles could support environmental monitoring, marine research, infrastructure inspection, and wildlife observation by allowing a single vehicle to operate in both air and water without changing propulsion systems.