We show the the voricity distribution obtained by minimizing the
induced drag on a wing, the so called Prandtl-Munk vortex sheet, is
not a travelling-wave weak solution of the Euler equations, contrary
to what has been claimed by a number of authors. Instead, it is a weak
solution of a non-homogeneous Euler equation, where the forcing term
represents a "tension" force applied to the tips. This is consistent
with a heuristic arguement due to Saffman. Thus, the notion of weak
solution captures the correct physical behavior in this case.