A principal characteristic of “flight” is that a significant part of the aerodynamic force is needed to cancel the weight of the organism. Thus, certain features of flying apply to buoyant fish. In forward flight such a force can be obtained by creating horizontal bound vorticity, this being the main purpose of the lifting surfaces of the body. The soaring and gliding of birds provides a familiar example where the classical aerodynamics of fixed-wing aircraft can be applied at once, a problem we consider in Section 11.1. It is worth mentioning that there is evidence that observations of birds led Lanchester to the notions of circulation and induced drag of finite wings [see the engrossing historical summary in Durand (1963)].

However, gliding, insofar as it may be taken to be stationary in time, is a rather special instance of animal flight. The common maneuvers of natural flight – takeoff and landing, flapping flight, and hovering – are fundamentally time-dependent phenomena (see the estimate of the frequency parameter σ given in Chapter 1). In effect, natural fliers, in particular the hummingbirds and certain of the hovering insects, explore the problem of lift generation in a very different parameter range from that conventionally exploited in aeronautics. In later sections we examine some of these time-dependent problems in the context of unsteady airfoil theory.

For general references on this subject, see Gray (1968) and the review articles by Lighthill and Weis-Fogh in Wu et al. (1975, vol. 2). For well-illustrated popular accounts of insect flight, see Nachtigall (1968) and Weis-Fogh (1975b).