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Above MCRIT in the transonic speed regime, at a constant angle of attack, CL will...
  • A
    remain constant throughout this regime.
  • B
    always increase as the Mach number increases.
  • C
    always decrease as the Mach number increases.
  • D
    vary depending on the Mach number.

Refer to figure.
At low subsonic speeds, the lift coefficient CL is assumed to be constant at a given angle of attack. With increasing Mach number, however, it will vary as shown in figure 1.
At high subsonic speed CL increases. This is the result of the changing pattern of streamlines.
At low speeds, the streamlines begin to diverge well ahead of the airfoil. At high subsonic speeds they do not begin to deflect until closer to the leading edge, causing greater acceleration and pressure drop around the leading edge. This phenomenon causes the stall speed to increase at high altitudes.

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