When is the critical Mach number relevant to aircraft performance?

The critical Mach number is a vital concept in aerodynamics that directly relates to an aircraft's behavior as it approaches the speed of sound. This particular speed represents the point at which the airflow over some parts of the aircraft begins to reach supersonic speeds, leading to changes in aerodynamic characteristics and overall aircraft performance.

As an aircraft approaches its critical Mach number, compression waves start to form, which can result in increased drag and potentially lead to flow separation. This phenomenon is particularly significant because it is closely associated with the transition from subsonic to transonic flight and is crucial for understanding how the aircraft will handle at these speeds.

In contrast, engine start-up, landing and approach, and slow flight conditions typically do not involve speeds that are close enough to the speed of sound for the critical Mach number to have an impact. Instead, these phases of flight are characterized by different aerodynamic behaviors where other performance factors, such as lift efficiency and stall characteristics, are more relevant. Thus, the context of high-speed flight approaching transonic levels solidifies the importance of the critical Mach number in assessing aircraft performance.