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Abstract:
The wing beat of tiny insects has attracted considerable interest because conventional aerodynamics predicts a reduction of flight efficiency when aerofoils are comparatively small and slow. Here, two approaches are reported by which we investigated the dynamics of the wing beat of tethered flying Drosophila melanogaster. First, the forces acting on the moving wing were calculated from three-dimensional kinematic data, following the blade-element theory which assumes quasi-steady aerodynamics. Under these conditions, the flight force is directed upwards, relative to the longitudinal body axis, during the second half of the downstroke; it is oriented forwards and downwards during the upstroke. The time average of the force generated according to this theory does not correspond to the direction and magnitude of the actual average force of flight. The expected force is directed forwards, along the body’s longitudinal axis, and is too small to keep the fly airborne. Secondly, an attempt is made to measure the timecourse of flight forces by attaching the fly to along the body’s longitudinal axis, and is too small to keep the fly airborne. Secondly, an attempt is made to measure the timecourse of flight forces by attaching the fly to a string, the displacement of which is monitored by means of laser interferometry. A sharp lift-pulse is observed when the wing is rapidly rotated during the ventral reversal of the wing-beat cycle. A second lift maximum of variable strength seems to be associated with the squeeze-peel events during the dorsal reversal. These results support the notion that flight in small insects might be dominated by unsteady mechanisms.
