Engineers seek lessons from birds with huge wingspan
The giant-winged, wandering albatrosses that live on the Kerguelen Islands, a speck in the southern Indian Ocean, are built for distance. Their flights can range for thousands of miles as the birds glide low on winds over the open ocean for weeks at a time.
Such long flights require energy efficiency. Aerospace engineers and researchers in Germany and France are gaining a better understanding of how these birds soar with seemingly little effort. Insights from their research could help optimize flightpaths and control surfaces of gliders and unmanned air vehicles, craft novel uses of ever-smaller global positioning systems and point the way to a more precise understanding of flight habits and patterns.
Study results show wandering albatrosses rely on a flight mode called dynamic soaring, which enables them to draw energy from moving air in the layers of horizontal wind shear just above the sea surface. The foraging birds, tracked using ultra-light global positioning devices, never fly in straight lines. Instead, they make use of a repetitive curved trajectory, climbing windward from sea level to about 30 ft., turning leeward and descending back to sea level, where they skim along at low altitude, curving back to a windward position.
This cycle takes about 15 sec. The birds' speed varies between 20-70 mph, and researchers calculate the albatrosses gain energy during the windward climb on the order of 360% in relation to their starting point. Even when a bird reaches the top of its maneuver, the energy level continues to increase, reaching maximum during the descent and then slowly dissipating through the skim below.
The research hinges on fieldwork done a few years ago on the islands of Kerguelen—aptly known also as the Desolation Islands—located in the middle of a triangle between Australia, Africa and Antarctica. “It's a little less isolated because you can get phone calls now,” says biologist Anna Nesterova.
Gottfried Sachs, a flight-system dynamics expert at the Technical University of Munich, assembled the research team, which included then-doctoral candidate and integrated navigation engineer Johannes Traugott, who did the sensor optimization and data analysis, and Nesterova, who studies king penguins and internal navigation. She is noted for her expertise in bird-handling.
They set out from a tiny French military and science station on Kerguelen and set up on Cape Ratmanoff, the easternmost point of Courbet Peninsula, an 8-hr. hike away. “It's primitive. Very windy. No trees or bushes. Quite bare,” Nesterova says. A mile-long penguin colony stretches over the beach, and up against it is the colony of albatrosses. They nest on the ground, using the flats as runways to take off into the wind. Bird pairs take turns foraging over the sea and incubating a single egg. Traugott and Nesterova tracked 20 birds; the longest log was for the first six days and 3,000 mi. of a 30-day forage.
To track the birds, Traugott and Nesterova used global-positioning sensors from Italian and German team partners Technosmart Europe and E-Obs GmbH, both specialists in tagging birds. Traugott then generated data with post-processing software.
By sifting through the source data with custom software, the team was able to measure albatross maneuvers with a resolution of 4 in., down from an original resolution of 3 ft. Current geodesy-grade satellite navigation systems are not rugged enough to work strapped to a bird's back under the conditions off Kerguelen. Post-processing the GPS data became vital in building the team's total energy plots for the birds, and it may be the source of the most interesting aspects and lessons regarding aeronautics.
The team's studies, published in peer-reviewed, open-source journal PLOS One, conclude that the albatross gains its cyclic energy boost in the upper curve of its dynamic-soaring maneuver. The difference in wind speed between high and low altitudes is key for the albatrosses' dynamic soaring, Traugott says. The birds awed the team in other ways, too. “It takes amazing offshore navigation skills for them to find their nests after four weeks of foraging over open water,” he says.
Sachs continues to work with birds, conducting precision studies of intermittent flight patterns and studying why birds do not need a vertical fin. “Flap-glide-flap has advantages over continued flapping,” he says. Applying engineering math to that flying technique might shed even more light on birds' energy-efficient motion.
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Read the article by Sachs, et al. at bit.ly/17RD89j