If the sensor decides the thermal is strong enough, the UAV shuts off its power. As the plane soars the onboard computer records what the plane senses, so it can estimate the size and strength of the thermal and locate its center, Allen said.
"It's trying to build a map of the thermal and from that fly in a more optimized circle to get the most energy out of it," he said.
"It also estimates the drift of the thermal with the wind so it can stay within it during windy conditions."
Allen and his team launched Cloud Swift on a test flight last fall over Rogers Dry Lake at Edwards Air Force Base.
In the 23 updrafts it detected, the plane gained an average altitude of 565 feet (172 meters). In one strong thermal it climbed 2,770 feet (844 meters).
The aircraft added 60 minutes to its flight time by soaring in the thermals. It could have stayed up longer, were it not for the limitations of the battery that powers the wing motions required for soaring, Allen said.
In future flights Allen and his team hope to teach the plane how to detect and fly in air currents that flow up the sides of hills, called ridge lift.
They would also like to teach the plane a technique called dynamic soaring, which involves shifting between air currents of different speeds.
The techniques would allow the airplane to fly to specific targets, not just loiter around in rising thermals, Allen said.
In addition, Allen's team is collaborating with researchers at the University of Washington in Seattle to extend the concept to multiple UAVs.
Juris Vagners is an aeronautical engineer at the university. He said a team of UAVs could cooperate to find and use thermals like a flock of hawks.
Hawks circle in thermals as they hunt for prey. Periodically one hawk will separate to scout out another thermal. If it finds one, the other birds will wander over to it, Vagners explained. This allows the other hawks to save their energy.
"Biology has figured out how to use cooperation to share the burden," he said, adding that birds are not cooperating in the human sense of the word.
The researchers are programming multiple UAVs to cooperate like the hawks, enabling them to stay airborne much longer than they could on fuel alone.
The researchers have simulated a cooperative flight on computers. They are seeking funding to conduct flight tests with actual vehicles.
Vagners said multiple cooperating UAVs would be useful for tasks such as ongoing surveillance.
Instead of sending out one craft until it runs out of fuel and then launching another, a swarm of UAVs could circle in a thermal, sending one out on a specific mission assignment when required.
"This strategy would allow significant air time and minimize the issues of launch and recovery in order to replenish the craft up there," he said.
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