What is critical Mach number ?
When there is an airflow over the airfoil, the gas expands around the top surface of the airfoil near the leading edge which rapidly increases the velocity and Mach number of the airflow. There are regions on the surface of the airfoil where the local Mach number is greater than the freestream Mach number. As the freestream Mach number increases, the peak local Mach number on the surface also increases. On increasing further the freestream Mach number, peak local Mach number on the surface of the airfoil can be 1.0, which is a locally sonic flow on the surface of the airfoil.
Therefore, flow can be locally sonic or higher, even if the freestream Mach number is subsonic. Therefore, a critical Mach number is a free-stream Mach number at which sonic flow is first achieved somewhere on the airfoil surface. For a thin airfoil, the expansion over the top surface is mild, there is only a slight increase in velocity. For a medium thickness, the flow expansion over the leading edge for the medium airfoil is stronger, the velocity increase to a large value. Since flow separation is stronger, sonic conditions are obtained sooner on the surface of the airfoil. Therefore, the critical Mach number of medium-thickness airfoil is less than the critical Mach number of a thin airfoil and thick airfoil has lower critical Mach number than thin airfoil and medium-thickness airfoil.
Swept wings have a greater critical Mach number than the straight wing.