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The enhanced aerodynamic performance of insects results from an interaction of three distinct yet interactive mechanisms: delayed stall, rotational circulation, and wake capture. Delayed stall functions during the translational portions of the stroke, when the wings sweep through the air with a large angle of attack. In contrast, rotational circulation and wake capture generate aerodynamic forces during stroke reversals, when the wings rapidly rotate and change direction. In addition to contributing to the lift required to keep an insect aloft, these two rotational mechanisms provide a potent means by which the animal can modulate the direction and magnitude of flight forces during steering maneuvers. A comprehensive theory incorporating both translational and rotational mechanisms may explain the diverse patterns of wing motion displayed by different species of insects.
Insects were the first animals to evolve active flight and remain unsurpassed in many aspects of aerodynamic performance and maneuverability. Among insects, we find animals capable of taking off backwards, flying sideways, and landing upside down (1). While such complex aerial feats involve many physiological and anatomical specializations that are poorly understood, perhaps the greatest puzzle is how flapping wings can generate enough force to keep an insect in the air. Conventional aerodynamic theory is based on rigid wings moving at constant velocity. When insect wings are placed in a wind tunnel and tested over the range of air velocities that they,encounter when flapped by the animal, the measured forces are substantially smaller than those required for active flight (2). Thus, something about the complexity of the flapping motion increases the lift produced by a wing above and beyond that which it could generate at constant velocity or that can be predicted by standard aerodynamic theory.
The failure of conventional steady-state theory has prompted the search for unsteady mechanisms that might explain the high forces produced by flapping wings (3, 4). The wingstroke of an insect is typically divided into four kinematic portions: two translational phases (upstroke and downstroke), when the wings sweep through the air with a high angle of attack, and two rotational phases (pronation and supination), when the wings rapidly rotate and reverse direction. The unsteady mechanisms that have been proposed to explain the elevated performance of insect wings typically emphasize either the translational...