What is the free stream MACH Number commonly referred to as?

The free stream Mach number is commonly referenced as the critical Mach number because it represents the speed at which an aircraft starts to encounter notable changes in aerodynamic properties. This is the point where airflow around the aircraft begins to transition from subsonic to supersonic conditions, typically over the wings or other surfaces.

Understanding the critical Mach number is essential for aircraft design and operation, as it indicates the maximum speed at which the aircraft can fly without experiencing shock waves that can affect performance and control. As the aircraft increases its speed beyond this point, the airflow starts becoming supersonic, leading to increased drag and potential issues such as loss of control or increased fuel consumption.

The other terms mentioned refer to broader flight regimes but do not specifically define the free stream conditions in the way that critical Mach number does. The transonic regime suggests speeds close to but not exceeding the speed of sound; supersonic indicates speeds above the speed of sound, and subsonic refers to speeds well below it. Thus, the term "critical Mach number" directly describes the transitional nature of flow characteristics encountered at the free stream Mach number when an aircraft is approaching its maximum efficiency at subsonic speeds.