  We have preformed experiments on a liquid curtain falling from a horizontal,
wetted, tube and lateraly constrained by two vertical wires. The fluid motion
nearly reduces to a free-fall, with a very low detachment velocity below the
tube. Thus, the curtain contains a large subsonic area, i.e. a domain where the
sinuous waves travel faster than the fluid. The upper boundary not being
constrained in the transverse direction, we have observed the appearance of an
up to now unreported instability when the flow rate is progressively reduced:
the top of the curtain enters a pendulum-like motion, coupled to a propagative
pattern of curtain undulations, structured as a chessboard. Measurements of the
phase velocity and frequency of this pattern are reported. Data are in
agreement with a simple dimensional argument suggesting that the wave velocity
is proportional to the surface tension divided by the mass flux of liquid per
unit length. This scaling is also that followed by the fluid velocity at the
transonic point, i.e. the point where the fluid velocity equals that of sinuous
waves. We finally discuss implications of these results on the global stability
of falling curtains.
