Birds are a primal model for our concept of flight. Ornithic influences on Lilienthal, the Wrights and the Gossamer Condor, and the aerodynamics of those configurations, are discussed, with historical anecdotes. Leonardo da Vinci and Lord Raleigh's comments on energy extraction by birds from natural winds are noted. A video of actual flight of the reconstructed Leonardo 1504 Fiesole flyer is shown. Because of low flight speeds, birds and small Remote Piloted Vehicles can extract energy from naturally occurring spatial and temporal wind speed variation. This has been spectacularly exploited in mountain wake cycling with model sailplanes. Videos of gliders and albatrosses exploiting dynamic soaring are shown. The fundamental equations of motion for a flight vehicle are developed for an arbitrary wind field. Using a normalizing speed, all vehicles of any size and performance can be collapsed into a general form. Exact analytical solutions are obtained for some restricted maneuver cases, while the general case is solved numerically. Optimal conditions for finite shear fields, as utilized by the Southern Albatross in the marine boundary layer, and infinite shear cases, as for building and obstruction wakes as utilized by starlings and model gliders are developed. Variational techniques for solution of various practical cases are derived. The natural constant shear layer cannot realistically sustain flight cycles, but the typical marine boundary layer, of varying shear, is habitually exploited by cruising and migrating pelagic birds for non-stop flights of many thousands of kilometers, as observed by Rayleigh. Energy extraction from other natural wind variations, like the wakes of hills and structures, is used by birds and was described by Leonardo in 1500. This strategy can significantly extend the performance of small mechanical flight vehicles.
