Wind Energy Extraction by Birds and Flight Vehicles

Abstract

When a bird or flight vehicle is oriented with a component of its lift vector aligned with the natural wind work is done on the flight system. Consequently, by suitable maneuvers, variations in wind speed can be used to add energy to the system.  These procedures are used by albatrosses and many other birds.  The equations of motion are simplified by normalizing using the minimum drag speed and integrating numerically for the control cycles involving angles of attack and bank.  An energy neutral cycle by which an un-powered vehicle returns to initial velocity and height depends only upon 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.  For a continuous wind profile a variational method is used to find the minimum gradient for a neutral energy cycle.  Simple expressions are derived for the minimum wind variations for these two cases.  The oceanic boundary layer and the shear layer downwind of a ridge are studied, and neutral energy wind criteria derived for them.  Birds and small UAVs, with flight speeds comparable to atmospheric wind variations, can profit from wind energy extraction. Presented at the 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno NV, January, 10-13, 2005: Lissaman, P.B.S., “Wind Energy Extraction by Birds and Flight Vehicles,” AIAA Paper 2005-241, AIAA Annual Meeting, Reno, Jan. 2005.