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A normal shockwave begins to form very shortly after the aircraft passes Mcr. Once the normal shockwave forms there is a dramatic increase in drag. Therefore, this speed is called Mach Drag Rise Number Mdr.
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In a mixed airflow situation, where some of the airflow is subsonic and some is supersonic all the pressure waves will build up, adding to each other to form a shock wave, at the boundary between the supersonic and subsonic flow. This is shown in the diagram to the left.
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The shock wave between the supersonic and subsonic flow will always form at right angles to the airfoil surface. Therefore, it is known as a Normal Shock Wave. We can state two simple rules of thumb:
A shock wave is a line along which many smaller pressure waves pile up. They add to each other through constructive interference to create a very dense high pressure region. The shockwave causes drag is two ways. First, its very existence requires energy to be taken out of the air flowing through it. This drag is called "Shockwave Drag." The second form of drag, due to the shockwave is unique to the normal shock wave. The shock wave tends to stop the boundary layer, causing the flow to separate and thereby producing effects almost the same as a stall. Thus, the term "Mach Stall" has been coined.
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The diagram to the left shows a wing with the lift being disrupted and the drag greatly increased by a Mach Stall. In a mach stall the aircraft experiences a loss of lift, increase in drag and usually a tendency to pitch nose down. In other words it has most of the same symptoms as a low speed stall except the angle of attack is quite small and the TAS is very high. Drag due to flow separation is a form of pressure drag, as discussed earlier. It may be minimized by strategically placing vortex generators at the point where the shockwave first forms. These vortex generators will delay flow separation, but will not prevent it if the aircraft continues to accelerate well beyond Mcr. |
As the aircraft accelerates toward the speed of sound the region of supersonic flow gets greater and greater. Eventually a supersonic flow region forms on the bottom of the airfoil as well.Once the wing reaches Mach 1 all the airflow will be supersonic. The movie below shows how the normal shockwave moves during the speed range from Mdr to M=1.0.