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A shock wave on a lift generating wing will:
  • A
    move slightly forward in front of a downward deflecting aileron.
  • B
    reach its highest strength when flying at the critical Mach number.
  • C
    move aft as Mach number is increased.
  • D
    be situated at the greatest wing thickness when the aeroplane reaches the speed of sound.

Refer to figure.
Tuck under, or Mach tuck, is an effect where the aircraft tends to pitch downward as the airflow around the airfoil reaches supersonic speeds.
It occurs well before reaching Mach 1 speed.

Mach tuck can be caused by two things; a rearward movement of the center op pressure (CP) and a decrease in wing downwash at the tailplane.
Both of these conditions cause a pitch down effect, as they generate a (bigger) positive moment aft of the CG.

When the airflow around the airfoil starts reaching supersonic speed (this can happen before the aircraft flies at Mach 1 speed),
small shockwave forms over the upper side of the wing, generating a large amount of lift.
If speed increases, this shockwave will move backwards to the mid chord position, causing the CP to move backwards.
The adjusted flow over the wings can reduce downwash on the tailplane, contributing to Mach tuck.

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