Wandering albatrosses exploit the vertical gradient of wind velocity (wind shear) above the ocean to gain energy for long distance dynamic soaring with a typical airspeed of 36 mph. In principle, albatrosses could soar much faster than this in sufficient wind, but the limited strength of their wings prevents a much faster airspeed. Recently, pilots of radio-controlled (RC) gliders have exploited the wind shear associated with winds blowing over mountain ridges to achieve very fast glider speeds, reaching a record of 498 mph in March 2012. A relatively simple two-layer model of dynamic soaring predicts maximum glider airspeed to be around 10 times the wind speed of the upper layer (assuming zero wind speed in the lower layer). This indicates that a glider could soar with an airspeed of around 200 mph in a wind speed of 20 mph, much faster than an albatross. It is proposed that recent high performance RC gliders and their pilots’ expertise could be used to develop a high-speed robotic albatross UAV (Unmanned Aerial Vehicle), which could soar over the ocean like an albatross, but much faster than the bird. This UAV could be used for various purposes such as surveillance, search and rescue, and environmental monitoring. A first step is for pilots of RC gliders to demonstrate high-speed dynamic soaring over the ocean in realistic winds and waves.
The hand-launched Tactical Long Endurance Unmanned Aerial System (TALEUAS) is being developed at the Unites States’ Naval Postgraduate School in Monterey, California. It needs an electric propeller to get airborne, but give it a few minutes to reach a reasonable altitude and TALEUAS can fly all day just by riding rising currents of warm air called thermals.
When 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, indeed, the birds found that the thermals they were attempting to join it in were too weak for their weight, as the drone is more efficient than they are at gliding.
TALEUAS’s endurance is limited only by the power requirements of its electronics and payload, for at the moment these 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. Such software replicates the behaviour of a skilled sailplane pilot—or a vulture—in knowing where to find rising air and where to avoid downdraughts.
SOURCES – Economist, RC Soaring Digest