  Using nonlinear vortex-sheet simulations, we determine the region in
parameter space in which a straight flag in a channel-bounded inviscid flow is
unstable to flapping motions. We find that for heavier flags, greater
confinement increases the size of the region of instability. For lighter flags,
confinement has little influence. We then compute the stability boundaries
analytically for an infinite flag, and find similar results. For the finite
flag we also consider the effect of channel walls on the large-amplitude
periodic flapping dynamics. We find that multiple flapping states are possible
but rare at a given set of parameters, when periodic flapping occurs. As the
channel walls approach the flag, its flapping amplitude decreases roughly in
proportion to the near-wall distance, for both symmetric and asymmetric
channels. Meanwhile, its dominant flapping frequency and mean number of
deflection extrema (or "wavenumber") increase in a nearly stepwise fashion.
That is, they remain nearly unchanged over a wide range of channel spacing, but
when the channel spacing is decreased below a certain value, they undergo sharp
increases corresponding to a higher flapping mode.
