The Sound Barrier

Sound waves travel through air at about 340 metres per second (770MPH). Objects such as our aircraft's, moving faster than this are said to be 'Supersonic'.

When an aircraft approaches the speed of sound, the waves of pressure become compressed into a smaller area. This is because the air can not transmit the pressure change very far in front of the aircraft before it reaches that location.

• The speed of sound decreases with increasing altitude.
• The atmospheric pressure decreases with increasing altitude.
• The atmospheric temperature decreases with increasing altitude.

Mcrit or known as the ‘Critical Mach Number’ It is the lowest Mach number at which the airflow over some point on the aircraft reaches the speed of sound. A Mach number of 1.0 indicates an airspeed equals to the speed of sound in the air.

So each aircraft will have its own critical Mach Number depending on their structural design. But from what I know, I believe that the critical Mach Number is around 0.8Mach, this is the time when airflow at the speed of sound starts forming on such components such as the wings, rudder or elevators etc.

There are two main types of shock waves, these are:

1. A Normal Shock Wave: is created when a aircraft is entering supersonic flow.
2. A Oblique Shock Wave. This is a sharp edged shock wave that is formed when supersonic flow is turned on itself.

A shock stall is a stall caused by the airflow over an aircraft's wings being disturbed by shock waves when flying at or near to the aircraft's critical Mach number.

So in other words when an aircraft is flying near Transonic and Hypersonic so around 0.8Mach to Mach1 we know that the temperature, pressure, and density dramatically rises as its velocity falls this is why it is important that you get out of this as quick as you can as this is producing a lot of heat and drag which can lead to a shock stall. This is around 0.8Mach hence why the fluttering starts and this can also change the angle of attack hence causing a stall.