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While flying slightly above the critical Mach number (MCRIT), a control surface is deflected down by a pilot input. What could happen?
  • A
    The control surface’s efficiency might be reduced due to shock-induced separation.
  • B
    The shock wave will disappear due to the downwards deflection of the control surface.
  • C
    The control surface’s efficiency might be increased due to the shock wave itself.
  • D
    Low speed buffet could take place due to shock-induced separation.

Refer to figure.
When shock waves form on the aircraft's wing, the shock induced separation behind them can lead to vibration and control surface ineffectiveness.

The disturbed airflow over the control surfaces may cause uncommanded erratic movements, although this will not directly affect the air ahead of the shock wave, because the resulting pressure disturbances are prevented from travelling forward. The pressure distribution over the front of the wing is, however, altered, which varies the position of the wing’s center of pressure and its overall pitching moment. This alters the wing’s angle of attack and results in rapid backward and forward movements of the shock waves.

A kind of instability is set up, and the rapid changes in the pressure distribution result in vibration of the whole aircraft. This is primarily due to the distributed airflow behind the shock wave hitting the tail plane. If shock waves form on the control surfaces, it will also affect the stick forces by altering their hinge moments.

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