Many of you will have seen the following photo, some thing similar either in photo or in person at an airshow. What you are seeing is not a Mach cone, but a Prandtl-Meyer expansion fan. The angle of the fan can be used to calculate the Mach of the airflow producing the fan, but not the Mach of the aircraft - important difference. When an object moves faster than the speed of sound, and there is an abrupt decrease in the flow area, shock waves are generated. If the flow area increases, however, a different flow phenomenon is observed. If the increase is abrupt, we encounter a centered expansion fan. You can see an expansion fan on top of the canopy as the flow area suddenly increases. The expansion fan on the fuselage/wing is due once again to the flow area increasing. The word "expansion" denotes that the area is increasing. There are some marked differences between shock waves and expansion fans. Across a shock wave, the Mach number decreases, the static pressure increases, and there is a loss of total pressure because the process is irreversible. Through an expansion fan, the Mach number increases, the static pressure decreases and the total pressure remains constant.
