"When [the hand-launched Tactical Long Endurance Unmanned Aerial System] TALEUAS encounters a thermal it senses the lift and spirals around to take advantage of it. Vultures and eagles use the same technique, and Kevin Jones, who is in charge of the project, says he has often found TALEUAS sharing the air with these raptors. On some occasions, the birds found that the thermals they were attempting to join were too weak for their weight, as the drone is more efficient at gliding than they are.
TALEUAS’s endurance is limited only by the power requirements of its electronics and payload, which at the moment are battery powered. Dr. Jones and his team are, however, covering the craft’s wings with solar cells that will generate power during the day, and are replacing its lithium-polymer battery with a lithium-ion one capable of storing enough energy to last the night. That done, TALEUAS will be able to stay aloft indefinitely.
TALEUAS does, however, depend on chance to locate useful thermals in the first place. Roke Manor Research, a British firm, hopes to eliminate that element of chance by allowing drones actively to seek out rising air in places where the hunt is most likely to be propitious. As well as thermals, Mike Hook, the project’s leader, and his team are looking at orographic lift, produced by wind blowing over a ridge, and lee waves caused by wind striking mountains. Their software combines several approaches to the search for rising air. It analyses the local landscape for large flat areas that are likely to produce thermals, and for ridges that might generate orographic lift. It also employs cameras to spot cumulus clouds formed by rapidly rising hot air.
Perhaps the most ambitious scheme for a robot glider, however, is the artificial albatross proposed by Philip Richardson of the Woods Hole Oceanographic Institution, in Massachusetts. Like its natural counterpart, this artificial bird harnesses wind shear—the difference in wind speed at different heights—in a technique called dynamic soaring.
The air is quite still near the surface of the sea even when it is blowing strongly just a few metres above, so an albatross can rise to gain height like a kite in a breeze, then glide down in any direction. By repeating this trick it can fly thousands of kilometres without flapping its wings, and by tacking it can travel anywhere, regardless of the wind direction. Dr Richardson hopes to replicate this with his robot bird. If he can, he will surely break all records for the time a heavier-than-air artifact has stayed aloft."