Kinematics of free-flight ornithopters

Ornithopters are flapping-wing aircraft that mimic natural fliers; their wings flap, twist and deform during flight. This study was conducted to improve the kinematic understanding of such vehicles. Three experimental models were used with wingspans of 15, 25 and 74 cm. The wings consist of a sheet of material connected to a span-wise leading edge spar. The sheet of material is supported by two to four ribs on each side of the wing. Free-flight and ground tests were conducted using reflective markers whose positions were tracked using a Vicon™ system. In total, ten ground and thirteen free-flight tests were conducted using various flapping frequencies ranging from 5 to 25 Hz. The study found that flapping-induced oscillations exist for all measured body and wing kinematic parameters, and those oscillations occur at the same frequency as flapping. Roll oscillation amplitude is significantly greater than that of either pitch or yaw due, in part, to the low roll-inertia of small flapping-wing vehicles. For a flapping membrane-batten wing with chordwise ribs, pitching angle phase is virtually independent of spanwise position and pitching angle amplitude increases with spanwise distance from the wing-root.