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The drag coefficient at constant angle of attack:

  • A

    starts to decrease rapidly above the drag divergence Mach number.

  • B

    starts to increase rapidly above the drag divergence Mach number.

  • C

    starts to increase rapidly above MCRIT.

  • D

    increases only when MCRIT is above unity.

Refer to figure.
The critical Mach number, or Mcrit, is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound (Mach 1),
but does not exceed it. At the lower critical Mach number, the airflow around the entire aircraft is subsonic.
At the higher critical Mach number, the airflow around the whole aircraft is supersonic.

Because of the critical Mach number, it is possible that shockwaves can occur at speeds even below Mach 1.
As the aircraft speeds up beyond this critical Mach number, drag increases dramatically.
If the aircraft is not designed for transonic or supersonic flight, this could lead to controllability problems.
The critical Mach number increases with increasing wing sweepback. Thinner airfoils also increase the value of Mcrit.

The drag divergence Mach number is the Mach number at which the aerodynamic drag on an airfoil begins to increase rapidly as the Mach number increases.
The drag coefficient can multiply to 10 times its size compared to its low speed value.
The drag divergence Mach number is a number bigger than Mcrit and smaller than Mach 1.
The drag divergence Mach number can be increased by increasing wing sweepback.

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