Powered animal flight has evolved separately and quite independently in insects, bats, pterosaurs and birds, and has, in many cases, led to spectacular success in allowing broad geographic distribution of species, together with the colonisation of new habitats. As mentioned in the introductory paragraphs, flapping wings operate in regimes where unsteady aerodynamic effects can be of overiding importance, and many simple analyses from the textbooks break down when applied to flying animals -- the (probably apocryphal) story of proof by engineers at Douglas that bumblebees almost certainly could not fly, being a case in point. The degree to which unsteady mechanisms need to be invoked in order to account for the lift and drag forces on animal wings has been a controversial topic for about 40 years. Fortunately, current models and understanding are a little more sophisticated and quantitative than in the times when these controversies were first spawned, and some of these advances may have practical application beyond their initial domain. The steady/unsteady debate is reviewd in more detail in [Sp92] and [Sp93].

Just as in standard aeronautics practice, the action of the wings can be inferred to some extent by studying the wake structure that results when the fluid is disturbed. If the patterns of vortex lines in the wake are in some relatively simple arrangement, then this can be used as a basis for modeling efforts. Indeed, early theoretical predictions that vortex rings would be found in flapping wing wakes were confirmed in [SRP84, Sp86], but insufficient momentum was measured to apparently support the weight of the animal in air! This paradoxical result, reminiscent of the early 'engineering estimates', persists to this day, although some possible explanations can be offered (see refs above). It transpires that this wake momentum deficit is observed only in low speed flight, but investigations of flight at higher speeds, more typical of cruising or commuting flight (and corresponding to a lower value of the reduced frequency), revealed a new surprise. The trailing vortices are now continuous, with no measurable concentrations of spanwise vorticity. They undulate up and down (in space), following the path left by the wing tip, and are shed by an asymmetric wingbeat with a reduced span on the upstroke. The flight efficiency is greatly enhanced by a reduction in the downwash associated with vortex shedding at the trailing edge and the circulation of the wake vortices is constant [Sp87b, Sp92]. This variable-span or constant-circulation (VS/CC) concept allows a new generation of simple models to be contructed for forward flight at moderate speeds, where calculations require not one Cray-month, but only paper and pencil.

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