Albatross robot takes flight
May 18, 2018
MIT engineers have designed a robotic glider that can skim along the
water’s surface, riding the wind like an albatross while also surfing
the waves like a sailboat.
In regions of high wind, the robot is designed to stay aloft, much like
its avian counterpart. Where there are calmer winds, the robot can dip a
keel into the water to ride like a highly efficient sailboat instead.
The robotic system, which borrows from both nautical and biological
designs, can cover a given distance using one-third as much wind as an
albatross and traveling 10 times faster than a typical sailboat. The
glider is also relatively lightweight, weighing about 6 pounds. The
researchers hope that in the near future, such compact, speedy robotic
water-skimmers may be deployed in teams to survey large swaths of the
“The oceans remain vastly undermonitored,” says Gabriel Bousquet, a
former postdoc in MIT’s Department of Aeronautics and Astronautics, who
led 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.”
Bousquet will present details of the robotic system 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 study on the
dynamics of albatross flight, in which they identified the mechanics
that enable the tireless traveler to cover vast distances while
expending minimal energy. The key to the bird’s marathon voyages is its
ability to ride in and out of high- and low-speed layers of air.
Specifically, the researchers found the bird is able to perform a
mechanical process called a “transfer of momentum,” in which it takes
momentum from higher, faster layers of air, and by diving down transfers
that momentum to lower, slower layers, propelling itself without having
to continuously flap its wings.
Interestingly, Bousquet observed that the physics of albatross flight is
very similar to that of sailboat travel. Both the albatross and the
sailboat transfer momentum in order to keep moving. But in the case of
the sailboat, that transfer occurs not between layers of air, but
between the air and water.
“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.”
An albatross glider, designed by MIT engineers, skims the Charles River.
Bousquet also realized that the speed at which both an albatross and a
sailboat can travel depends upon the same general equation, related to
the transfer of momentum. Essentially, both the bird and the boat can
travel faster if they can either stay aloft easily or interact with two
layers, or mediums, of very different speeds.
The albatross does well with the former, as its wings provide natural
lift, though it flies between air layers with a relatively small
difference in windspeeds. Meanwhile, the sailboat excels at the latter,
traveling between two mediums of very different speeds — air versus
water — though its hull creates a lot of friction and prevents it from
getting much speed. Bousquet wondered: What if a vehicle could be
designed to perform well in both metrics, marrying the high-speed
qualities of both the albatross and the sailboat?
“We thought, how could we take the best from both worlds?” Bousquet
Out on the water
The team drafted a design for such a hybrid vehicle, which ultimately
resembled an autonomous glider with a 3-meter wingspan, similar to that
of a typical albatross. They added a tall, triangular sail, as well as a
slender, wing-like keel. They then performed some mathematical modeling
to predict how such a design would travel.
According to their calculations, the wind-powered vehicle would only
need relatively calm winds of about 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,” Bousquet says. “It’s very
efficient, and you can travel very fast, even if there is not too much
The team built a prototype of their design, using a glider airframe
designed by Mark Drela, professor of aeronautics and astronautics at
MIT. To the bottom of the glider they added a keel, along with various
instruments, such as GPS, inertial measurement sensors, auto-pilot
instrumentation, and ultrasound, to track the height of the glider above
“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,” Bousquet says.
The researchers decided to test this “critical maneuver” — the act of
transitioning between flying in the air and dipping the keel down to
sail in the water. Accomplishing this move doesn’t necessarily require a
sail, so Bousquet and his colleagues decided not to include one in order
to simplify preliminary experiments.
In the fall of 2016, the team put its design to the test, launching the
robot from the MIT Sailing Pavilion out onto the Charles River. As the
robot lacked a sail and any mechanism to get it started, the team hung
it from a fishing rod attached to a whaler boat. With this setup, the
boat towed the robot along the river until it reached about 20 miles per
hour, at which point the robot autonomously “took off,” riding the wind
on its own.
Once it was flying autonomously, Bousquet used a remote control to give
the robot a “down” command, prompting it to dip low enough to submerge
its keel in the river. Next, he adjusted the direction of the keel, and
observed that the robot was able to steer away from the boat as
expected. He then gave a command for the robot to fly back up, lifting
the keel out of the water.
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, prove that the team’s conceptual device can
travel successfully, powered by the wind and the water. Eventually, he
envisions fleets of such vehicles autonomously and efficiently
monitoring large expanses 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 research was supported, in part, by the Link Ocean Instrumentation