What is Mach number?
Mach number (MN) is a true airspeed indication, given as a percentage relative to the local speed of sound; e.g., half the speed of sound = 0.5 Mach.
What is the critical Mach number (Mcrit)?
Mcrit is the aircraft’s Mach speed at which the airflow over a wing becomes sonic—critical Mach number. The aircraft’s Mach speed is lower than the airflow speed over a wing. A typical Mcrit speed of 0.72 M experiences sonic Mach 1 airflow speed over the upper surface of the wing. Subsonic aircraft experience a rapid rise in drag above the critical Mach number, and because the aircraft’s engines do not have the available power to maintain its speed and lift values under these conditions, the aircraft suffers a loss of lift.
Characteristics of critical Mach number (Mcrlt)?
1. Initial Mach buffet, caused by the shock waves on the upper surface of the wing as the aircraft approaches Mcrit, is usually experienced.
2. An increase in drag because of the breakdown of airflow causes the stick force to change from a required forward push to a neutral force and then a required pull force as the aircraft approaches and passes Mcrit.
3. A nose-down change in attitude (Mach tuck) occurs at or after Mcrit.
4. A possible loss of control.
Changes in the center of pressure as an aircraft speed increases past the critical Mach number (Mcrlt). The center of pressure moves rearward on a swept wing as the aircraft passes its Mcrit for two reasons:
The center of pressure moves rearward on a swept wing as the aircraft passes its Mcrit for two reasons:
1. The shock waves on the wing’s upper surface occur toward the leading edge because of the greater chamber, which creates the greatest airflow velocity to be experienced at this point. This upsets the lift distribution chordwise and causes a rearward shift in the center of the lift (center of pressure).
2. The swept wing tends to experience the shock-wave effect at the thick root part of the wing first, causing a loss of lift inboard, and therefore, the lift force now predominantly comes from the outboard part of the swept wing, which is further aft because of the wing sweep.