If you crossed an albatross with a sailboat, what would you get? According to a team of MIT engineers, you'd end up with a hybrid robotic glider that enjoys the best of both the aerial and on-water worlds. The team's Albatross glider soars like a bird and skims the water like a sail craft, yet requires less wind than an albatross needs to stay aloft, and can travel 10 times faster than a sailboat.
The albatross is a miracle of bioengineering. These legendary birds of good omen have a physiology that makes them living long-range gliders. Their long, tapering wings boast spans of up to 12 ft (3.7 m) and feature strong leading edges and tendons that allow the wing joints to lock in place.
These birds are such masters of the air they can stay aloft for years at a time, only dipping down periodically to snatch a fish from the sea, and nonstop migrations of 10,000 km (6,200 mi) are routine. But although they have a remarkable ability to make incredible flights spanning whole oceans with very little muscular effort, they're not very good at powered flight, as anyone who has seen one try to takeoff can vouch for. They also aren't noted for their speed.
Gabriel Bousquet, Jean-Jacques Slotine, and Michael Triantafyllou of MIT looked into the aerodynamics of the albatross to find out how it manages its epic voyages. What they found was that the birds regularly change altitude, riding in and out of high- and low-speed layers of air. The difference in air velocity isn't very much, but by shifting between these layers, the albatross exploits what is called "transfer of momentum." That is, by diving down from a higher, faster layer into a lower, slower one dumps energy from the upper layer to the lower one, causing the bird to speed up without flapping its wings.
According to the MIT team, the principle and the mathematics to describe it is the same as the mechanism that makes a sailboat work. A sailboat moves because it sits on the interface between two very different media – air and water. The sail sticks up into the wind and the keel is thrust down into the water. As the wind blows across the sails, the wind's energy or momentum is transferred into the water and the boat pushes forward, though not very fast because of the fiction.
What made this discovery of interest to the researchers wasn't how similar to two were, but their dissimilarities. The albatross can exploit very small differences in wind speed, while the sailboat could exploit very large ones. Was there a way to combine these two properties in a single device?
The result is a robotic glider airframe designed by Mark Drela, professor of aeronautics and astronautics at MIT that weighs about 6 lb (2.7 kg) with albatross-like wings spanning 3 m (10 ft) and a knife-like keel that can be dipped into the water. It also has GPS, inertial measurement sensors, an autopilot, and an ultrasound altimeter.
"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," says Bousquet.
The idea is that when the wind is strong, the glider can soar and take advantage of the air layers, but when the winds die down, it can dip its keel and speed up again. The result is a vehicle that can cover a third more distance than an albatross for the same wind, and attain speeds 10 times greater than an average sailboat. Along with the wings and keel, the robotic glider also has a triangular sail to help it along on the water. The team says that calculations indicated that the vehicle could reach speeds of 20 knots (23 mph, 37 km/h when the winds were blowing at only 5 knots (5.8 mph, 9.3 km/h).
Late last year, the team tested the glider's ability to transition between air and water gliding. For this, the sail was removed to simplify the results and the robotic craft was taken to the MIT Sailing Pavilion and launched onto the Charles River in Boston, Massachusetts. Since the sail was needed to bring the craft up to speed, the glider was attached to a fishing pole and suspended behind a whaler boat that towed it under power.
At 20 mph (32 km/h), the glider lifted off from the water and went into autonomous flight. Then, under remote control, it was able to dip its keel back in the water, steer, and lift the keel again for pure flight.
The hope is that the success of the conceptual robot will lead to more advanced vehicles that could one day be deployed in flocks (fleets?) to survey large areas of the sea.
"The oceans remain vastly under-monitored," says Bousque. "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."
The research will be presented at the IEEE International Conference on Robotics and Automation, which will run from May 21 to 25 in Brisbane, Australia
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