This 250-Gram Robot Can Swim Underwater Then Fly into the Sky Using Its Flapping Wings Like a Diving Bird

The bird-fish hybrid could one day be used to study aquatic animals underwater and then fly back to the base with data.

Loons, gulls, puffins, and petrels are strange little acrobats. These birds plunge into water chasing fish, then burst back into the air and cruise at highway speeds. About 100 species of birds pull off this trick, flying through both air and water with the same pair of wings. How they manage two fluids that differ a thousand-fold in density — and especially how they smash back through the surface — has long puzzled biologists. Now two universities have gotten together to make an interesting robot duplicate.

MIT and EPFL in Switzerland have now built a machine that does something similar. Meet FAAV — the “flapping-wing aerial-aquatic vehicle” — a 250-gram, untethered robot with a slender carbon-fiber body, a small motorized tail, and two flexible membrane wings that beat like a bird’s.

It cruises through the air at approximately 6.3 meters (20.7 feet) per second, swims underwater at nearly one m/s (3.2 feet), and — most impressively — is designed to launch itself out of the water and back into flight without propellers, without folding its wings, and without paddling feet. The results were published in Science.

“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,” says Raphael Zufferey, assistant professor of mechanical engineering at MIT.

“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.”

Two mediums, one wing

The problem is that air and water behave nothing alike. Water is around 800 times thicker than air, so a wingbeat that works smoothly in the sky suddenly has to push against something much heavier underwater. In theory, a bird or robot would need to flap its wings far faster in air than in water to stay efficient. But real diving birds do not follow that perfect formula. They make a practical compromise, using a flapping rhythm that works well enough in both places. FAAV does the same.

The trick, the team discovered, is flexibility. Rather than mimicking the intricate way birds fold their wings underwater, they let FAAV’s wings bend passively. In dense water they deform by up to 90%, shrinking the effective stroke and easing the motor’s load. In the air, they stiffen enough to generate lift.

FAAV uses one small electric motor to power all of its wing movement. The motor is controlled in a way that lets the robot push hard when it is moving slowly underwater, but ease off when its wings are moving quickly in the air.

×

Get smarter every day...

Stay ahead with ZME Science and subscribe.

Daily Newsletter
The science you need to know, every weekday.

Weekly Newsletter
A week in science, all in one place. Sends every Sunday.
No spam, ever. Unsubscribe anytime. Review our Privacy Policy.

Thank you! One more thing...

Please check your inbox and confirm your subscription.

In the air, FAAV flaps quickly, about five to 11 times per second. Underwater, it can slow down dramatically, sometimes to just one wingbeat every 10 seconds. Instead of putting all the electronics inside a heavy waterproof shell, the engineers coated each part in flexible silicone. That kept the robot light. The waterproofing added only 13 grams, or about 5% of the robot’s total weight.

The hardest thing FAAV does — and the reason a bird-scale machine had never done it before — is the water-to-air transition.

“Clearing the wings from the water surface just after transition was the most difficult part,” Zufferey told ZME Science in an email.

Earlier aerial-aquatic robots relied on chemical combustion, a power tether, or dedicated propellers to break the surface. FAAV does it with wings alone.

The window is narrow. Exit takes less than a second and about 8 to 10 wing strokes. The tail has to sit close to the body; a long one drags and pitches the robot back down. And the launch angle must be roughly 70 degrees — too shallow and the submerged tail pulls it back, too steep and it tumbles backwards.

Even then, Zufferey is candid that the full mission hasn’t been strung together yet. He said that, thus far, diving and taking off have been done separately.

Swimming underwater and transitioning into flight has been shown as one continuous maneuver, but the complete cycle — cruise, plunge, swim, launch, cruise again — is still to come.

What the robot taught them about birds

Because a robot obeys the same physics as an animal but can be pushed into conditions none would tolerate — flapping absurdly slowly, or with no legs at all — FAAV has already produced a few surprises.

RelatedPosts

Why birds crash into planes and cars like a deathwish
The world’s most evolutionary distinct birds
Your smartphone could be a digital ornithologist: software recognizes birds from photos
Urban seagulls know exactly when to strike to steal your food — we have the GPS data to prove it

Smaller wings, the team found, swim faster but not more efficiently. That suggests when birds fold their wings underwater, they’re chasing speed and shorter dives, not saving calories — reversing a common assumption. The wing-only launch is also so power-hungry that it likely explains why most diving birds still use their feet: only light-bodied fliers like kingfishers and dippers can afford to skip the paddle. Heavier divers need the extra shove.

FAAV also found its way into the same efficient flapping rhythm used by many animals that move through air or water. Fish, birds and insects often settle into this range because it helps them move efficiently without wasting energy. The engineers did not program the robot specifically to hit that target. It arrived there on its own, suggesting that FAAV is using the same kind of natural rhythm that evolution has found again and again.

For now, FAAV isn’t autonomous.

“All operations are timed sequences with no user input. Ultimately we will have the robots reconnect with the user in flight,” Zufferey said.

The goal is for it to eventually decide when to dive and surface, and to fly back to reconnect with a human operator. Zuffery said on one battery charge, the team estimates it could fly approximately six kilometers (3.7 miles) or swim about two (1.2 miles). Beyond roughly 15 meters, it’s more energy-efficient to leap out and fly than to swim.

Zufferey imagines it launched from a boat or shoreline, dipping in to sample a coral reef, an iceberg’s edge, or a pod of whales, then flying home to deliver the data before diving again.

“You could send this out not just every week, but every hour,” he said.