High Speed Flight
Introduction to High Speed Flight
High speed flight can be put into 4 categories:
- Subsonic (less than 0.8 mach).
- Transonic (between 0.8 mach to 1.2 mach).
- Supersonic (between 1.2 mach to 5 mach).
- Hypersonic(more than 5 mach).
Transonic flow is where the airflow is part subsonic and part supersonic. Supersonic and Hypersonic have an airflow is always supersonic.
Speed of sound: A stationary object vibrating at certain frequencies causes a continuous series of pulses through the air. The pressure waves look similar to the reaction you see when you throw a stone into a river, ripples come out from were the stone dropped. The ripples move outwards at a speed determined by the ambient temperature. As an aircraft approaches the speed of sound, the pressure ripples or disturbances would accumulate in front of the aircraft to form a continuous wave. Once the speed of sound is reached the aircraft and the sound waves are travelling together. As the aircraft continues to accelerate it is travelling faster than the rate of propagation of the pressure disturbances thus producing an infinite number of pressure waves would be produced. These form a mach cone towards the back of the aircraft. The angle of the mach cone decreases as speed increases, there is also a continuous wave in front of the cone is known as the mach wave.
A mach number is a measurement of speed which is about 1100 feet per second, or most commonly 760 miles per hour at sea level and a temperature of 15°C. The speed of sound (760 mph) is 1 mach number. To determine the mach number the ratio of 'Speed of Flight of Aircraft at a Particular Height' to the 'Speed of Sound at the Particular Height'. The speed of sound is affected by the temperature of the air. The speed of sound will decrease as the temperature falls (as altitude rises). In normal conditions the speed of sound is 760 mph but at -56.5°C which is at about 36,000 ft the speed of sound is now 660 mph.
Shock Wave Development
During subsonic flight the air experiences small changes in pressure, when related to density changes are negligible. The airflow is considered to be incompressible. As the speed of the aerofoil approaches the speed of sound the pressure disturbances can't propagate faster than the speed of sound. This causes the air to become compressible. The molecules of air are not being pushed out of the way and a compression wave forms on the aerofoil leading edge. This is what is known as a shock wave.
Shock waves can also develop in subsonic airflow, this is known as an incipient wave. Incipient waves are caused by the airflow over the upper surface of an aerofoil reaching the speed of sound at the maximum curvature point of the aerofoil. At this condition both transonic and supersonic flow are being created at a high mach number. This suggests that although the aircraft isn't moving at the speed of sound, parts of the aircraft have an airflow that will reach or exceed the speed of sound.
When an airflow becomes supersonic a shock wave is formed. This will lead to an increase in drag, a decrease in lift, a movement of the centre of pressure and a high level of buffeting. This is a shock stall condition which will be discussed later. The speed at which shock stall occurs is known as the critical number. The amount of turbulence behind the shock wave is large, which happens because of a sudden increase in pressure and density and a decrease in velocity.
During high speed flight there are more disturbances than normal, these are detailed in another hub which you can access by the link below.