Spontaneous Symmetry Breaking of a Hinged Flapping Filament Generates Lift

Elastic filamentous structures found on swimming and flying organisms are versatile in function, rendering their precise contribution to locomotion difficult to assess. We show in this Letter that a single passive filament hinged on the rear of a bluff body placed in a stream can generate a net lift force without increasing the mean drag force on the body. This is a consequence of spontaneous symmetry breaking in the filament's flapping dynamics. The phenomenon is related to a resonance between the frequency associated with the von Karman vortex street developing behind the bluff body and the natural frequency of the free bending vibrations of the filament.