Abstract
Most foot-propelled swimming birds sweep their webbed feet backwards in a curved path that lies in a plane aligned with the swimming direction. When the foot passes the most outward position, near the beginning of the power stroke, a tangent to the foot trajectory is parallel with the line of swimming and the foot web is perpendicular to it. But later in the stroke the foot takes an increasingly transverse direction, swinging towards the longitudinal axis of the body. Here we show that, early in the power stroke, propulsion is achieved mostly by hydrodynamic drag on the foot, whereas there is a gradual transition into lift-based propulsion later in the stroke. At the shift to lift mode, the attached vortices of the drag-based phase turn into a starting vortex, shed at the trailing edge, and into spiralling leading-edge vortices along the sides of the foot. Because of their delta shape, webbed feet can generate propulsive forces continuously through two successive modes, from drag at the beginning of the stroke, all the way through the transition to predominantly lift later in the stroke.
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References
Blake, R. W. Mechanics of drag-based mechanisms of propulsion in aquatic vertebrates. Symp. Zool. Soc. Lond. 48, 29–52 (1981)
Braun, J. & Reif, W.-E. A survey of aquatic locomotion in fishes and tetrapods. N. Jb. Geol. Paläont. Abh. 169, 307–332 (1985)
Johansson, L. C. & Lindhe Norberg, U. M. Asymmetric toes aid underwater swimming. Nature 407, 582–583 (2000)
Johansson, L. C. & Lindhe Norberg, U. M. Lift-based paddling in diving grebe. J. Exp. Biol. 204, 1687–1696 (2001)
Ellington, C. P., van den Berg, C., Willmott, A. P. & Thomas, A. L. R. Leading-edge vortices in insect flight. Nature 384, 626–630 (1996)
Katz, J. & Plotkin, A. Low-speed Aerodynamics (McGraw-Hill, New York, 1991)
Lee, M. & Ho, C.-M. Lift force of delta wings. Appl. Mech. Rev. 43, 209–221 (1990)
Thomas, A. L. R. On the aerodynamics of birds' tails. Phil. Trans. R. Soc. Lond. B 340, 361–380 (1993)
Norberg, R. Å. Swallow tail streamer is a mechanical device for self-deflection of tail leading edge, enhancing aerodynamic efficiency and flight manoeuvrability. Proc. R. Soc. Lond. B 257, 227–233 (1994)
Walker, J. A. QuickSAND. Quick Smoothing and Numerical Differentiation for the Power Macintosh 〈http://www.usm.maine.edu/~walker/software.html〉 (1997).
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We thank B. Svensson for building the mechanical experimental device.
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Johansson, L., Norberg, R. Delta-wing function of webbed feet gives hydrodynamic lift for swimming propulsion in birds. Nature 424, 65–68 (2003). https://doi.org/10.1038/nature01695
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DOI: https://doi.org/10.1038/nature01695
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