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The critical Mach number of an aerofoil is the free-stream Mach number at which

  • A.A normal shock first stands at the trailing edge
  • B.The drag coefficient reaches its maximum
  • C.The flow first reaches sonic speed somewhere on the surface
  • D.The whole flow field first becomes supersonic

Answer: C

As the free stream speeds up, the acceleration over the upper surface makes some point reach Mach one before the free stream does. That free-stream value is the critical Mach number. It is not itself where drag rises sharply; the drag-divergence Mach number lies a little above it.

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