[Ref. ACE The Technical Pilot Interview 699/6496, Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 5-44]
Mcrit is the (Mach) speed at which the airflow over any part of the aircraft (e.g a wing) first reaches but does not exceed Mach 1.0 (become sonic)
Mcrit = M 1.o
What’s the significance of Mcrit?
During flight, a wing produces lift by accelerating the airflow over the upper surface. This accelerated air can reach sonic speeds even though the aircraft itself may be flying subsonic (reaching Mcrit).
Above 5-10% Mcrit (critical Mach number), compressibility effects begin – shock waves, buffet, and airflow separation will occur.
The result is that aircraft experience a rapid rise in drag and because the aircraft’s engines do not have the available power to maintain its speed and lift values under these conditions.Finally, the aircraft suffers a loss of lift.
“drag divergence” is defined as the point (above 5-10% Mcrit), where drag begins to rise sharply and cause buffet. In turn, it causes trim and stability changes and a decrease in control surface effectiveness.
Notes: The Mach number in which above 5-10% Mcrit is known as “force divergence Mach number”. At this speed, the airflow separation induced by shock wave formation can create significant variations in the drag, lift, or pitching moment coefficients.