Quote:
Originally Posted by tgrayson  Only true for slow airplanes that can't fly very high. For turbine powered airplanes, however, as the mach number goes up, the maximum lift coefficient decreases, which increases the stall speed. |
As the airflow around the aircraft becomes transonic the pressure distribution changes, which changes the location of where flow could separate from the lifting surfaces, which changes the stalling characteristics.
The start of the amber tape ("Vmin") is usually the "1.3g buffet onset speed." I.e., the speed at which the stall buffet will occur when the aircraft is experiencing a 1.3g normal load factor (~40° level bank).
Vmin is primarily affected by the weight of the aircraft (heavier is a faster minimum), the CG location (forward is a faster minimum), and the altitude (higher is faster).
Technically speaking, the Coffin Corner is where the minimum and maximum operating speeds converge to be the same (usually at high altitudes). On both sides you're squeezed by the minimum and maximum buffet onset speeds approaching the same value, and is a particularly bad place to be in an aircraft with big floppy wings.