S = reference area (projected wing area) s = wing semispan t = time U = horizontal velocity w = width of test section x = axial body-fixed coordinate y = spanwise coordinate z = out-of-plane deflection of apex, Fig. 17 a = angle of attack A = increment or amplitude Af = time lag £ = dimensionless z coordinate, z/c() 17 = dimensionless y coordinate, yls() 0A = apex half angle, ir/2 — A A = leading-edge sweep f = dimensionless x coordinate, p = air density a = inclination of the roll axis 4> = roll angle a) = angular frequency, 27rf