  We give an explanation for the onset of wind-induced flutter in a flag. Our
theory accounts for the various physical mechanisms at work: the finite length
and the small but finite bending stiffness of the flag, the unsteadiness of the
flow, the added mass effect and vortex shedding from the trailing edge. Our
analysis allows us to predict a critical speed for the onset of flapping as
well as the frequency of flapping. We find that in a particular limit
corresponding to a low density fluid flowing over a soft high density flag, the
flapping instability is akin to a resonance between the mode of oscillation of
a rigid pivoted airfoil in a flow and a hinged-free elastic filament vibrating
in its lowest mode.
