http://ozreport.com/pub/images/Stanford-OSTIV08.pdf

Birds frequently use the energy present in atmospheric currents to conserve their energy while flying. Although energy in the form of thermal updrafts is routinely used by pilots of full-scale and model sailplanes, the energy in atmospheric turbulence has not been utilized to its full potential. The emergence of ultra-light sailplanes has opened up the possibility of utilizing this form of ‘gust-soaring’. This paper deals with the design of simple control laws to extract energy from atmospheric turbulence and their application to small manned as well as unmanned aircraft. A simulation-based optimization procedure to design control laws for energy extraction from realistic turbulence was developed, leading to about 36% average energy savings for a ‘bird-sized’ glider. Flight test results are presented to demonstrate the energy extraction concept and validate the predicted savings.