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Consider an aircraft equipped for high speed flight. As the weight (1) _____, MCRIT will be (2) _____.

  • A

    (1) decreases; (2) decreased

  • B

    (1) increases; (2) unchanged

  • C

    (1) increases; (2) decreased

  • D

    (1) increases; (2) increased

Refer to figure.
MCRIT is the Mach number where the airflow, somewhere on the aircraft, first becomes supersonic. As the airflow over the wings is accelerated relative to the free-stream airflow, the air here usually reaches supersonic speeds first.

In straight and level flight, the lift produced by the wings must balance the weight of the aircraft. If weight increases, lift must increase. This is normally achieved by slightly increasing the angle of attack, to increase CL.

Increasing the AoA increases how much the air over the wing is accelerated, which increases the pressure difference above and below the wing in order to increase lift. Therefore for a given free stream Mach number, the local Mach number over the wing will be higher. It will become supersonic at a lower flight Mach number ⇒ So MCRIT has decreased

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