Fig. 1. Schematic of a downward flicking flagellum rotating at angular velocity relative to the pivot point at its base in a fixed X–Y coordinate system. Relative to an x–y coordinate system fixed at point z along the flagellum's arc length, the approaching local fluid velocity normal to the secant R(z) is U_yf. This generates components U_nf=U_yfcos and U_tf=U_yfsin along the normal (n_f) and tangent (t_f) to the flagellum. Aesthetascs and hydrodynamic sensilla on the flagellum are not shown.