What defines the critical Mach number (Mcrit) of an aircraft?

The critical Mach number (Mcrit) of an aircraft is defined as the speed at which airflow over the wing becomes sonic, meaning that the local airflow reaches the speed of sound. This is a crucial concept in aerodynamics because as an aircraft approaches this speed, it begins to experience significant changes in its aerodynamic characteristics.

When the airflow over the wing reaches Mach 1, it can lead to the formation of shock waves and phenomena such as increased drag and potential adverse effects on control surfaces. Understanding Mcrit helps designers and pilots know the limits of safe operation at high speeds to prevent performance issues associated with transonic flight, including drag rise and loss of lift.

The other options do not reflect the definition of critical Mach number. The first option relates more to maximum speed, while the third option speaks to stall conditions, which are not directly connected to sonic airflow. The fourth option refers to level flight conditions, which are independent of sonic airflow characteristics. Therefore, option B accurately describes the point at which the wing encounters transonic conditions.