Which of these statements about drag divergence Mach number are correct or incorrect?
1. M_{CRIT} is higher than the drag divergence Mach number.
2. Assuming a given angle of attack and for M < 1 the drag coefficient C_{D} decreases above the drag divergence Mach number.
Refer to figure.
The Mach number at which the aerodynamic drag begins to increase rapidly is called the drag divergence Mach number. The Drag Divergence Mach Number is usually close to, and always greater than, the Critical Mach Number. Therefore, statement 1 is incorrect.
As speed increases above M_{CRIT} shock waves begin to form and drag increases more rapidly than it would have done without the shock waves. The additional drag is called wave drag and is due to energy drag and boundary layer separation.
The change in drag characteristics is shown by the C_{D} curve for a basic section at a constant angle of attack in figure 1. The ‘hump’ in the curve from M 0.89 to M 1.2 is caused by:
 The drag directly associated with the trailing edge shock waves (energy loss).
 Separation of the boundary layer.
 The formation of the bow shock wave above M 1.0.
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