The use of oscillating flexible fins in propulsion has been the subject of several studies in recent years, but attention is rarely paid to the specific role of stiffness profile in thrust production. Stiffness profile is defined as the variation in local chordwise bending stiffness (EI) of a fin, from leading to trailing edge. In this study, flexible fins with a standard NACA0012 shape were tested alongside fins with a stiffness profile mimicking that of a Pumpkinseed Sunfish (Lepomis gibbosus). The fins were oscillated with a pitching sinusoidal motion over a range of frequencies and amplitudes, while torque, lateral force and static thrust were measured.
Over the range of oscillation parameters tested, it was shown that the fin with a biomimetic stiffness profile offered a significant improvement in static thrust, compared to a fin of similar dimensions with a standard NACA0012 aerofoil profile. The biomimetic fin also produced thrust more consistently over each oscillation cycle.
A comparison of fin materials of different stiffness showed that the improvement was due to the stiffness profile itself, and was not simply an effect of altering the overall stiffness of the fin. Fins of the same stiffness profile were observed to follow the same thrust-power curve, independent of the stiffness of the moulding material. Biomimetic fins were shown to produce up to 26% greater thrust per watt of input power, within the experimental range.