Low-Speed Aerodynamics

In the history of aviation, flight speeds have constantly increased due to the development of more powerful engines and the improvement of aerodynamic properties. Especially, the introduction of jet engines and sweptback wings made it possible to approach and even exceed the speed of sound. This sonic speed, the propagation speed of very small pressure disturbances in the atmosphere, plays an important role in determining which flow phenomena will occur. In the low-speed range, all flow velocities around the aeroplane are significantly smaller than the speed of sound. The pressure disturbances caused by the aircraft can propagate forward – in a way the air is “warned of” the oncoming aircraft – and the air particles recede for the leading edge of a wing or the nose of a body. This is no longer the case when the flight speed exceeds the speed of sound. Then the flow pattern is greatly changed by the occurrence of shock waves, nearly discontinuous pressure changes. These are caused by the compressibility of the atmospheric air, that is, the ability of air to change specific volume and density with increasing pressure.