Extracting Energy from Atmospheric Turbulence

Birds frequently use the energy present in atmosphe ric 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 ha s not been utilized to its full potential. The emer gence of ultra-light sailplanes has opened up the possibi lity of utilizing this form of ‘gust-soaring’. This paper deals with the design of simple control laws to ext ract energy from atmospheric turbulence and their application to small manned as well as unmanned air craft. A simulation-based optimization procedure to design control laws for energy extraction from real istic turbulence was developed, leading to about 36 % average energy savings for a ‘bird-sized’ glider. F light test results are presented to demonstrate the energy extraction concept and validate the predicte savings. Nomenclature bref Reference span cref Reference chord CD Coefficient of drag CDp Coefficient of parasite drag CL Coeffienct of lift CLmax Maximum coeffienct of lift D Drag E Total energy with respect to the atmosphere eAR Effective aspect ratio g Acceleration due to gravity h Height He Total energy per unit mass with respect to an inert ial frame of reference Kx Feedback gain, where x = 1, 2, 3, p, and d L Lift Lw Length scale of vertical turbulence m Aircraft mass Sref Reference area t Time U Aircraft speed Vair Airspeed Vref Reference speed wg Vertical gust velocity {x, z} Horizontal and downwards inertial axes {u, w} Components of inertial velocity along {x, z} axes δflap Flap deflection γ Flight path angle λ Wavelength of a sinusoidal gust σw Intensity of vertical turbulence