Hovering Flight of the Dragonfly Aeschna Juncea L., Kinematics and Aerodynamics

Abstract

The kinematics of free unimpeded hovering flight of Aeschna juncea L. was analysed from films taken in the field with 80 frames sec-I, and from still pictures taken with a motorized camera. The body is held almost horizontal, and the wing stroke plane is tilted 60 0 relative to the horizontal. In these respects the dragonfly differs strongly from most other hovering animals. The wing beats essentially in the same plane on the downstroke and upstroke. All wings are strongly supinated (pitched-up) during the upstroke. The stroke angle is ca. 60 0 and the wing beat frequency ca. 36 Hz. Average, minimum force coefficients were calculated with use of steady-state aerodynamic theory. Calculations were made under several alternative assumptions and gave lift coefficients of 3. 5 to 6. 1, which are all far too large to be explainable with steady-state aerodynamics. At least 60% of the force generated in hovering flight are due to non-steady-state aerodynamics. The pitching rotations of the wings at top and bottom of the stroke are believed to contribute much force, although the exact mechanism is not clear. At the leading edge of the wing of dragonflies there is a unique morphological arrangement, the node. It permits elastic tension of the leading edge and seems to be an adaptation permitting strong wing twistings. The node may also function as a shock absorber. 763