Hi,

I wonder whether the lift over resistance coefficient diagram (aka polar
diagram or lilienthal diagram) of a given airplane changes much with
altitude.

The rationale behind the question is, that I read somewhere, that commercial
aircraft are flying in the so called "coffin corner", that is so high they
can't go faster because of compression and can't go slower because of stall.

Why should they do this, when they could well be a little lower and
comfortly fly at the point of optimal angle of access, well away from stall.

Do optimal point and stall point come closer together with increasing
height?