Delta-wing function of webbed feet gives hydrodynamic lift for swimming propulsion in birds

Christoffer Johansson, Åke Norberg

Research output: Contribution to journalArticlepeer-review


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.
Original languageEnglish
Pages (from-to)65-68
Issue number6944
Publication statusPublished - 2003
Externally publishedYes

Bibliographical note

The information about affiliations in this record was updated in December 2015.
The record was previously connected to the following departments: Theoretical ecology (Closed 2011) (011006011)

Subject classification (UKÄ)

  • Ecology


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