I mean, when I think about it, I come to the conclusion that you
always have to (and can only) optimise your speed through the airmass
by flying MC (putting aside what that actually means in reality). If
the resultant XC speed given by MC theory is less than the windspeed,
well, you're stuffed.

Dan

That's correct. The assumption is that you're flying in thermals, and
thermals drift with the airmass. Actually thermals usually drift a bit
less than airmass, and if you've got ridge or wave, those are
stationary with respect to the ground. But otherwise, by definition
there is no way to fly faster than the optimum speed through the
airmasss -- the fact that the ground is moving around below you is
irrelevant.

John Cochrane