| I used a dynamically scaled mechanical model of the fruit fly, Drosophila melanogaster, to study how changes in wing kinematics influence the production of unsteady aerodynamic forces in insect flight. I examined 191 separate sets of kinematic patterns that differed with respect to stroke amplitude, angle of attack, flip timing, flip duration, and the shape and magnitude of stroke deviation. Instantaneous aerodynamic forces were measured using a two-dimensional force sensor mounted at the base of the wing. The influence of unsteady rotational effects were assessed by comparing the time course of measured forces with that of corresponding translational quasi-steady estimates. For each pattern, I also calculated mean, stroke-averaged values of the force coefficients and an estimate of profile power. The results of this analysis may be divided into four main points. First, for a short, symmetrical wing flip, mean lift was optimized by stroke amplitude of 180° and an angle of attack of 50°. At all stroke amplitudes, mean drag increased monotonically with increasing angle of attack. Translational quasi-steady predictions better matched the measured values at high stroke amplitude rather than at low stroke amplitude due to the increasing importance of rotational mechanisms in kinematic patterns with low stroke amplitude. Second, for 180° stroke amplitude and a 45° angle of attack, lift was maximized by short duration flips occurring just slightly advanced of stroke reversal. Symmetrical rotations produced similarly high performance. Wing rotation that occurred after stroke reversal, however, produced very low mean lift. Third, the production of aerodynamic forces was sensitive to changes in the magnitude of the wing's deviation from mean stroke plane and the shape of its wing tip trajectory. However, in all examples, stroke deviation lowered aerodynamic performance relative to the no deviation case due in part to a trade-off between lift and a radially directed component of total aerodynamic force. Although I found no evidence that stroke deviation can augment lift, it may be used to modulate forces on the two wings. Thus, insects might use changes in wing kinematics during steering maneuvers to generate force moments. Finally, while quasi-steady estimates failed to capture the time course of measured lift for nearly all kinematic patterns, they did predict with reasonable accuracy stroke-averaged values for the mean lift coefficient. (Abstract shortened by UMI.)... |
