Exceeding Critical Mach (M_crit)

081-02-03Pablo Asensio Martínez2026-04-011 min

Flow Separation

Behind a strong normal shock wave, the boundary layer may separate due to the adverse pressure gradient (sudden rise in pressure).

  • This is called Shock Induced Separation or Shock Stall.

Effects

  1. Drag Divergence: A sudden, rapid rise in drag ($C_D$) shortly after passing $M_{crit}$. The Mach number where this typically happens is $M_{DD}$ (Drag Divergence Mach).
  2. Buffet: Turbulent separated air hitting the tailplane causing vibration.
    • High Speed Buffet: Due to shock stall.
    • Low Speed Buffet: Due to normal low-speed stall (high angle of attack).
    • Coffin Corner: The altitude where Low Speed Buffet and High Speed Buffet speeds converge.
  3. Loss of Lift: Due to flow separation behind the shock.
  4. Mach Tuck: As the shock wave on the upper surface moves backward (approaching M 1.0), the Center of Pressure moves backward (up to 50% chord).
    • This creates a strong Nose Down Pitching Moment.
    • Also, downwash on the tail is reduced (less tail down-force -> nose drops further).
  5. Control Problems: The shock wave can form over control surfaces (ailerons/elevators), making them ineffective or causing "Buzz".