Just for interest. I eventually got an answer from Boeing.
Critical Mach number is a term aerodynamicists use to quickly compare the "drag rise" or "compressibility drag" of several wing design alternatives. At Boeing, we define critical Mach number as the Mach number, at a constant lift coefficient, where the drag coefficient is 20 counts greater than the drag coefficient at the incompressible Mach number.
Typically, a wing does not have a critical Mach number. Rather, critical Mach number of a wing varies with lift coefficient. Generally, as lift coefficient increases, critical mach number decreases.
Wing designers can change critical Mach number by changing wing sweep, airfoil thickness or airfoil technology.
There are two other speeds that characterize the integrated airplane (as opposed to just the wing). These speeds are "Max Range Cruise speed" and "Long Range Cruise speed".
They continue on about the calculations for Max range cruise speed and LRC speed. Basically they are more worried about MCDR as that affects their economy, and as you see they refer to that as “critical Mach number” As they allow a margin of safety when they calculate their Max range cruise speed, which is calculated at a given weight and therefore lift co-efficient, I would estimate the “aerodynamic” MCRIT to be in the vicinity of M 0.80 under the same conditions on a B737.
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