Robotic fin forces at different flow rates at a flapping frequency of 1.00 Hz. (A) Thrust (+) and drag (–) and (B) lift (+) and downward (–) forces produced by biorobotic fin 3 with a flexural rigidity 1000 times that of the biological fin. Robotic fin forces at different flow rates at a flapping frequency of 1.00 Hz. (A) Thrust (+) and drag (–) and (B) lift (+) and downward (–) forces produced by biorobotic fin 3 with a flexural rigidity 1000 times that of the biological fin. (C) The 2-D forces in the thrust–lift plane, with arrows representing magnitude and direction of average force during the outstroke and instroke. For clarity, standard error bars are shown only for the force traces when the fin was operated at 90 mm s–1. These errors are representative of the errors associated with each curve. For clarity, only every third error bar is shown. Note the two thrust peaks (corresponding generally to fin outstroke and instroke), and that at 0 and 90 mm s–1 flow speeds, the robotic fin generates thrust only, even during the reversal from outstroke to instroke. James L. Tangorra et al. J Exp Biol 2010;213:4043-4054 © 2010.