A team of engineers at MIT has developed a robotic glider capable of both soaring through the air in high winds and riding rapidly along the water’s surface.

In drafting the unique design of their robot, which they call the “wind-powered Unmanned Nautical Air-water vehicle,” or UNAv, the researchers drew inspiration from both biological and man-made designs.

They looked to albatrosses, seabirds capable of flying far distances in windy areas, in designing the robot’s flight capabilities. They fused that influence with the design of sailboats, allowing the robot to glide atop the water just as efficiently as it flies.

The combination proves to be more than the sum of its parts. The UNAv is able to move 10 times faster than sailboats, and requires just a third of the wind power required by albatrosses to fly.

The UNAv is also quite lightweight, weighing in at a modest 6 pounds.

Why Albatrosses?

“Albatrosses are very special creatures,” said Gabriel Bousquet, a former postdoc in MIT’s Department of Aeronautics and Astronautics, who led the development of the UNAv as part of his graduate thesis.

Albatrosses, he noted, fly thousands of miles through some of the harshest weather conditions in the world.

“For albatrosses, it seems that the bigger the storm, the better — and most striking, they travel those huge distances without even flapping their wings,” he said.

Albatrosses are able to use the wind to power their flight, allowing them to travel far distances through strong winds and inclement weather, all the while exerting minimal energy.

A year ago, Bousquet, along with two other MIT researchers, Jean-Jacques Slotine, professor of mechanical engineering and information sciences and of brain sciences, and Michael Triantafyllou, the Henry L. and Grace Doherty Professor in Ocean Science and Engineering, published a study on the mechanics of the albatross’s unique flight pattern.

Albatrosses use a unique technique called dynamic soaring, in which they move in and out of high- and low-speed layers of air. In the aforementioned study, the researchers identified a mechanism that they termed “transfer of momentum,” in which the bird gathers momentum from high speed layers of wind and dives down into slower layers, thereby using that momentum to propel itself without constantly flapping its wings.

Bousquet also noticed that sailboats use the same principle of transferring momentum to move through the water. That is, sailboats gather momentum from the air using the sail and transfer it to the water using the keel, propelling the boat forward along the water.

Noting the similarity between the mechanisms of albatross flight and sailboat travel, the researchers sought to create a vehicle that combines the distinct advantages that each design bears.

Albatrosses, with wings to provide natural lift, are adept at staying aloft mid-air, while sailboats transfer momentum between two layers, air and water, of very different speeds.

“We then realized that if we merged the ideas of a sailboat and that of an albatross, by adding a sail and keel sail to a glider airplane, we could bring the best of both world together, and make a system that would both be up to 10 times faster than sailboats, and require one third as much wind as albatrosses to fly using wind power,” said Bousquet.

Designing the UNAv

In their design, the researchers merged the concepts. They used an autonomous glider frame designed by Mark Drela, professor of aeronautics and astronautics at MIT, and equipped it with a tall, triangular sail and a specially designed keel.

“We went ahead and designed an albatross-sized glider, equipped with a keel, to fly like an albatross, and inject momentum into the water like a sailboat,” said Bousquet. “In practice, the keel looks like a small vertical wing extending from the belly of the glider, and the tip of that small wing is designed to become immersed when the glider skims about 50 cm above the surface.”

They also attached a group of instruments to the glider that would help the robot gauge its movement. This includes a GPS, inertial measurement sensors, auto-pilot instrumentation, and ultrasound, to track the height of the glider above the water.

In order for the glider to operate properly, it must be able to gauge and dictate its movement with incredible precision, particularly at the moment when it transitions between flying in the air to dipping its keel in the water.

“The UNAv needs to have an on-board, automatic control that is able to both fly at extreme low height, and dip its keel in the water with centimetric accuracy, and at the same time it needs to be able to control the force generated by the keel extremely well,” said Bousquet.

In experiments conducted in the fall of 2016, the researchers took a prototype of the UNAv that lacked a sail for a test ride on Boston’s Charles River. Without a sail, the UNAv lacked a propulsion mechanism, so the team attached it to a boat and towed it until it gathered enough speed to fly on its own.

As they hoped, their glider was able to fly above the water at 20 miles an hour, dip down to submerge its keel in the water, steer away from the boat with the adjustment of its keel, and fly back up out of the water.

The experiments proved that their device can, indeed, fly efficiently using air and water power.

What’s Next?

Moving forward, the researchers intend to complete a full prototype of the UNAv that includes a sail and demonstrate wind-powered propulsion.

If they are successful, the UNAv could be used to monitor swaths of the oceans that are difficult to reach cost-effectively. UNAvs could theoretically be used to gather valuable data about our oceans. In particular, Bousquet notes, they could be used to gather information about oceanic CO2 absorption.

“How CO2 is absorbed by the oceans is driven by very complex processes, related to physics, biology and chemistry, and understanding them requires a lot of in-situ monitoring,” he said. “In particular, the southern ocean is a major CO2 absorber, but because it is both windy and remote, it is one of the least monitored. I imagine hundreds of UNAvs roaming the southern oceans and collect data for scientists to use. UNAvs could also tremendously help search-and-rescue at sea, and monitoring of the world’s fisheries and protected areas.

The novel design of the UNAv also demonstrates an important contribution to the emerging field of bio-inspired engineering. The team’s infusion of biological mechanisms into an engineered system portends an exciting future of similar designs.

“We are living a very exciting time, where engineering capabilities are becoming advanced enough to really translate bioinspiration to engineered systems—such as the UNAv,” said Bouquet. “Think of how maneuverable swifts or bats are, how energy efficient albatrosses and frigate birds. There is still a wide gap between the flight performance of these animals and engineered systems, and I expect that in the next decade, a lot of ideas glanced from biological systems will find their way into engineered systems.”