The Kinematics and Aerodynamics of the Free Flight of Some Diptera

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Abstract
Seven representative species of the order Diptera were filmed in free flight using high-speed cinematography. Insects were killed after filming, and morphological measurements were made in the manner of Ellington (1984b). The detailed kinematics of selected sequences were then found using frame-by-frame digitization, followed by computer reconstruction of the third dimension. Kinematics were qualitatively similar to those observed by Ellington (1984c), though in three species the wings often underwent ventral flexion near the base at the end of the downstroke. For aerodynamic analysis of hovering flight, modified forms of the equations of Ellington (1984e,f) were used. Forward flight was analysed by a novel method, which assumes that an equal but opposite circulation is built up for each half-stroke and allows linear equations to be used. The lift coefficients calculated for hovering were commonly well above those possible by quasi-steady mechanisms, but rotational coefficients were within those that could be achieved by the unsteady lift mechanisms: clap-and-fling (Weis-Fogh, 1973) and flex (Ellington, 1984d). The lift and rotational coefficients of the two half-strokes were often unequal. In forward flight, the equal circulation assumption often led to an incorrect estimation of the aerodynamic force vector, showing that the circulations during the two half-strokes were unequal. It is suggested that flies manoeuvre largely by altering the unsteady circulations produced at stroke reversal via alterations in the speed and timing of wing rotation. The differences in the mechanisms used by different fly species are related to their flight behaviour in the field.