Wave drag

Now let us return to stronger shock waves across which a noticeable change in flow properties occurs. The changes which take place in the shock wave as the air compresses are extremely rapid, taking place in a distance not much greater than the average distance between impacts of air molecules (approxim­ately 6.6 x 10-5 mm at sea level). Because of this a great deal of the mechanical energy in the flow is converted into thermal energy which is paid for in terms of a large drag force acting on the aerofoil or aircraft. Because this drag is solely associated with the existence of the shock wave systems within the flow, it is known as ‘wave drag’, and one of the main aims in the aerodynamic design of high speed aircraft is the reduction of this drag.