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The aerodynamic characteristics of various wing planforms at low Reynolds numbers (about 1 10 4 ) were studied by conducting wind-tunnel tests. These low Reynolds numbers correspond to the flights of small creatures, such as insects. Elliptical, rectangular, and triangular planforms with various aspect ratios were used in this study, as well as a swept rectangular (parallelogram) wing with an aspect ratio of four. The wing sections of all models were thin rectangular airfoils. The aerodynamic forces (lift and drag) and the pitching moment acting on the wing were measured for a wide range of angles of attack (including the maximum of 90 deg). Nonlinear characteristics of the lift coefficient were obtained, even at low angles of attack for high-aspect-ratio wings, whereas a small lift slope and a large maximum lift coefficient were obtained for low-aspect-ratio wings. The drag and the pitching moment coefficients also exhibited nonlinear characteristics. The effect of the Reynolds number based on the wing chord was comparatively small, but a distinctive phenomenon in low-Reynolds-number flow was observed in flow visualization using oil-film and smoke-wire methods. Nomenclature AR = aspect ratio, b 2 =S a = lift slope b = span length c = chord length C = coefficient of forces and moment related to ( ) c = mean aerodynamic chord D = drag D i = induced drag D p = pressure drag D 0 = minimum drag e = span efficiency factor f = friction drag, frequency k = proportional constant defined by Eq. (9) K p = lift slope given by lifting surface theory K V = constant value given by leading-edge suction force K W = constant value about aspect ratio of wing in blockage correction factor L = lift M = pitching moment q, q c = dynamic pressure and its corrected value R = resultant aerodynamic force, L 2 D 2 p r = distance between feathering axis and center of gravity Re = Reynolds number based on c S = wing area St = Strouhal number S ts = cross-sectional area of test section t = wing thickness t= c = thickness ratio U = wind velocity = angle of attack = proportional constant defined by Eq. (9) x = chordwise distance between quarter-chord and measurement point z = thicknesswise model distance between measurement point and wing chord " = wall blockage correction factor = leading-edge sweep angle = air density Subscripts M = measured value max = maximum value min = minimum value ST = strut ts = test section

The aerodynamic characteristics of the wings and body of a dragonfly and of artificial wing models were studied by conducting two types of wind-tunnel tests and a number of free-flight tests of gliders made using dragonfly wings. The results were consistent between these different tests. The effects of camber, thickness, sharpness of the leading edge and surface roughness on the aerodynamic characteristics of the wings were characterized in the flow field with Reynolds numbers (Re) as low as 103 to 104.