Aerodynamics of Gliding Flight in a Falcon and Other Birds

An interesting characteristic of avian gliders is their ability to soar or remain aloft without flapping their wings for long periods. There are two ways to do this. In static soaring a bird maintains or gains altitude by gliding in air that has an upward velocity component equal to or greater than the bird's sinking speed. In dynamic soaring a bird uses changes in the horizontal wind-velocity component to stay aloft, and no upward wind component is required. The ability of a bird to use either method depends on its aerodynamic characteristics and on atmospheric conditions. The aerodynamic characteristics of a gliding aircraft such as a bird can be determined from gliding performance, which includes a description of the glide path of the aircraft flying at constant speed through still air. This description may be obtained from direct measurements. Alternatively, gliding performance can be determined from aerodynamic characteristics, which usually are measured in a wind tunnel by mounting an aircraft, a model of it, or pieces of it, on a flight balance. Both sorts of measurements have been difficult to make on birds. Several estimates of gliding performance based on observations in nature have been published, but these do not have sufficient accuracy for aerodynamic purposes. Aerodynamic force measurements on stuffed, frozen, or sculptured models mounted in wind tunnels are questionable because the shapes and surface features of the models may deviate significantly from those of their living counterparts. Catalogued aerodynamic information used by aircraft designers is usually not applicable to birds because of the disparity in speed and size between birds and man-made aircraft. The gliding performance of a living bird may be measured accurately if the bird is trained to glide freely in a wind tunnel that can tilt along its longitudinal axis. By altering the tilt of the tunnel and the speed of air flow, one can reproduce the aerodynamic and gravitational forces that act on a bird gliding at various speeds in nature. This method has been used for the pigeon (Columba livia; Pennycuick, 1968), and we have used it to investigate the gliding performance of a laggar falcon (Falco jugger). Laggar falcons are native to India and are similar to the more familiar peregrine falcon (F. peregrinus) in size and proportions. Although falcons are usually thought of as active fliers, they also glide and soar (Beebe & Webster, 1964).