HAWKS and albatrosses soar for hours or even days
without having to land. Soon robotic gliders could go one better,
soaring on winds and thermals indefinitely. Cheap remote sensing for
search and rescue would be possible with this technology, or it could be
used to draw up detailed maps of a battlefield.
Glider pilots are old hands at using rising columns of heated air to gain altitude. In 2005 researchers at NASA's Dryden Flight Research Center
in Edwards, California, flew a glider fitted with a custom autopilot
unit 60 minutes longer than normal, just by catching and riding
thermals. And in 2009 Dan Edwards, who now works at the US Naval Research Laboratory in Washington DC, kept a glider soaring autonomously for 5.3 hours this way.
Both projects relied on the glider to
sense when it was in a thermal and then react to stay in the updraft.
But thermals can be capricious, and tend to die out at night, making
flights that last several days impossible, says Salah Sukkarieh
of the Australian Centre for Field Robotics in Sydney. He is designing
an autopilot system that maps and plans a glider's route so it can use a
technique known as dynamic soaring when thermals are scarce. The glider
first flies in a high-speed air current to gain momentum, then it turns
into a region of slower winds, where the newly gained energy can be
converted to lift. By cycling back and forth this way, the glider can
gain either speed or altitude.
"Theoretically you can stay aloft
indefinitely, just by hopping around and catching the winds," says
Sukkarieh, who presented his research at a robotics conference in
Shanghai, China, last month.
Inspired by albatrosses and frigate birds, the operators of radio-controlled gliders
have used dynamic soaring to reach speeds of more than 600 kilometres
per hour by flying between two regions of differing wind speeds.
To plan a path for dynamic soaring you
need a detailed map of the different winds around the glider. So
Sukkarieh is working on ways to accurately measure and predict these
winds. He recently tested his autopilot on a real glider, which made
detailed wind-speed estimates as it flew.
The system has on-board sensors,
including an accelerometer and altimeter, which measure changes in the
aircraft's velocity and altitude to work out how the winds will affect
the glider. From its built-in knowledge of how wind currents move, the
system was able to work out the location, speed, and direction of nearby
winds to create a local wind map.
By mapping wind and thermal energy
sources this way and using a path-planning program, the glider autopilot
should be able to calculate the most energy-efficient routes between
any two points. The system would be able to plot a path up to a few
kilometres away when the wind is calm but only over a few metres when
turbulent, as the winds change so quickly, says Sukkarieh.
He says that the amount of energy
available to a glider is usually enough to keep it aloft for as long as
it can survive the structural wear and tear. He plans to test the
mapping and route-planning systems more extensively in simulations, to
be followed by actual soaring experiments.
"I think we have some examples from
nature that mean this should be possible," says Edwards, who is not
involved in Sukkarieh's research. "We're just taking our first baby
steps into doing it autonomously."
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