22-10-2012, 05:10 PM
Autonomous Soaring for Improved Endurance of a Small Uninhabited Air Vehicle
ABSTRACT
A relatively unexplored method to improve the endurance of an autonomous aircraft is to use buoyant plumes of air found
in the lower atmosphere called thermals or updrafts. Glider pilots and birds commonly use updrafts to improve range,
endurance, or cross-country speed. This report presents a quantitative analysis of a small electric-powered uninhabited air
vehicle using updrafts to extend its endurance over a target location. A three-degree-of-freedom simulation of the uninhabited
air vehicle was used to determine the yearly effect of updrafts on performance. Surface radiation and rawinsonde balloon
measurements taken at Desert Rock, Nevada, were used to determine updraft size, strength, spacing, shape, and maximum
height for the simulation. A fixed-width spiral path was used to search for updrafts at the same time as maintaining
line-of-sight to the surface target position. Power was used only when the aircraft was flying at the lower-altitude limit in
search of updrafts. Results show that an uninhabited air vehicle with a nominal endurance of 2 hours can fly a maximum of
14 hours using updrafts during the summer and a maximum of 8 hours during the winter. The performance benefit and the
chance of finding updrafts both depend on what time of day the uninhabited air vehicle is launched. Good endurance and
probability of finding updrafts during the year was obtained when the uninhabited air vehicle was launched 30 percent into
the daylight hours after sunrise each day. Yearly average endurance was found to be 8.6 hours with these launch times.