A team of MIT engineers have built a robotic glider that can skim along the surface of the water, riding the wind like an albatross and simultaneously surfing the waves like a sailboat.
In regions where there is high wind, the robot is engineered to remain aloft, a lot like its avian counterpart. However, in regions where there are calmer winds, the robot can dip a keel into the water to ride like an extremely efficient sailboat.
The robotic system, which takes ideas from both biological and nautical designs, can cover a particular distance using one-third as much wind as an albatross and travels 10 times faster than a regular sailboat. The glider is also comparatively lightweight, weighing around six pounds. The researchers anticipate that in the near future, such compact, fast robotic water-skimmers may be used in teams to survey large areas of the ocean.
“The oceans remain vastly under-monitored,” says Gabriel Bousquet, a former postdoc in MIT’s Department of Aeronautics and Astronautics, who directed the design of the robot as part of his graduate thesis. “In particular, it’s very important to understand the Southern Ocean and how it is interacting with climate change. But it’s very hard to get there. We can now use the energy from the environment in an efficient way to do this long-distance travel, with a system that remains small-scale.”
Details of the robotic system will be presented by Bousquet this week at IEEE’s International Conference on Robotics and Automation, in Brisbane, Australia. His collaborators on the project are 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.
The Physics of Speed
Last year, Bousquet, Slotine, and Triantafyllou published a paper on the dynamics of albatross flight, in which they detected the mechanics that enable the untiring traveler to cover great distances while using up nominal energy. The key to the bird’s lengthy voyages is its ability to fly in and out of high- and low-speed layers of air.
Specifically, the team learned the bird is able to perform a mechanical process known as a “transfer of momentum,” in which it takes momentum from higher, faster layers of air, and by diving down conveys that momentum to lower, slower layers, thrusting itself without having to constantly flap its wings.
Fascinatingly, Bousquet noticed that the physics of albatross flight is quite similar to that of sailboat travel. Both the albatross and the sailboat transfer momentum so as to keep moving. But with the sailboat, that transfer happens not between layers of air, but between the water and air.
“Sailboats take momentum from the wind with their sail, and inject it into the water by pushing back with their keel,” Bousquet explains. “That’s how energy is extracted for sailboats.”
Bousquet also grasped that the speed at which both an albatross and a sailboat can move relies upon the same basic equation, associated to the transfer of momentum. Fundamentally, both the boat and the bird can travel faster if they can either stay aloft easily or interact with two layers, or mediums, of extremely different speeds.
The albatross manages well with the former, as its wings offer natural lift, though it flies between air layers with a comparatively small difference in wind speeds. On the other hand, the sailboat is better at the latter, traveling between two mediums of extremely different speeds—air versus water—though its hull produces a lot of friction and prevents it from gaining much speed. Bousquet speculated: What if a vehicle could be built to perform well in both metrics, combining the high-speed potentials of the albatross and the sailboat?
“We thought, how could we take the best from both worlds?” Bousquet says.
Out on the Water
The researchers drafted a design for such a hybrid vehicle, which in due course resembled an autonomous glider with a 3-meter wingspan, similar to that of a common albatross. They incorporated a tall, triangular sail, in addition to a slender, wing-like keel. They then performed a few mathematical modeling to calculate how such a design would travel.
According to their calculations, the wind-powered vehicle would only need moderately calm winds of around 5 knots to zip across waters at a velocity of about 20 knots, or 23 miles per hour.
We found that in light winds you can travel about three to 10 times faster than a traditional sailboat, and you need about half as much wind as an albatross, to reach 20 knots. It’s very efficient, and you can travel very fast, even if there is not too much wind.
The team put together a prototype of their design, using a glider airframe designed by Mark Drela, professor of aeronautics and astronautics at MIT. On the glider’s bottom, they incorporated a keel, together with various instruments, such as GPS, auto-pilot instrumentation, inertial measurement sensors, and ultrasound, to monitor the height of the glider above the water.
The goal here was to show we can control very precisely how high we are above the water, and that we can have the robot fly above the water, then down to where the keel can go under the water to generate a force, and the plane can still fly.
The researchers prepared to test this “critical maneuver” —the action of transitioning between flying in the air and dipping the keel down to sail in the water. Realizing this move doesn’t essentially require a sail, so Bousquet and his colleagues decided not to include one so as to shorten preliminary experiments.
The team tested the robot design from the MIT Sailing Pavilion out onto the Charles River in the fall of 2016. As the robot did not have a sail and any mechanism to help it to start, the team hung it from a fishing rod attached to a whaler boat. With this arrangement, the boat towed the robot along the river until it touched about 20 miles per hour, at which point the robot autonomously “took off,” riding the wind by itself.
Once it was flying independently, Bousquet used a remote control to trigger a “down” command on the robot, which caused it to dip sufficiently low to submerge its keel in the river. Next, he adjusted the direction of the keel and saw that the robot was capable of steering away from the boat as estimated. Then with another command, he made the robot to fly back up, thereby removing the keel out of the water.
“We were flying very close to the surface, and there was very little margin for error—everything had to be in place,” Bousquet says. “So it was very high stress, but very exciting.”
The experiments, he says, indicate that the team’s conceptual device can travel effectively, driven by the wind and the water. In the long run, he visualizes fleets of such vehicles independently and efficiently tracking large areas of the ocean.
“Imagine you could fly like an albatross when it’s really windy, and then when there’s not enough wind, the keel allows you to sail like a sailboat,” Bousquet says. “This dramatically expands the kinds of regions where you can go.”
This study was supported partly by the Link Ocean Instrumentation fellowship.
The UNAv, a wind-powered UAV for ocean monitoring: performance, control and validation