Fundamentals of Energy Extraction from Natural Winds

Abstract

When a bird or vehicle is oriented with a horizontal component of its lift vector aligned with the natural wind work is done on the flight system. Consequently, by suitable trajectories, variations in wind speed can be used to add energy to the system.  These maneuvers are used by albatrosses and many other birds.  An energy neutral cycle, defined as one where an unpowered vehicle returns to initial velocity and height, depends on the maximum lift/drag ratio of the vehicle and the wind speed variation.  The minimum speed difference for a neutral energy cycle occurs for a vertical or horizontal step in wind speed.  The equations of motion can be normalized by the minimum drag speed and have been solved numerically for an arbitrary wind profile.  This solution is complicated and not provided here, but the results are discussed.  In this paper a physical interpretation of the energy extraction mechanism is provided.  Birds and small UAVs, with flight speeds comparable to atmospheric wind variations, can profit from wind energy extraction.