Exceeding Critical Mach (M_crit)
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
- 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).
- 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.
- Loss of Lift: Due to flow separation behind the shock.
- 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).
- Control Problems: The shock wave can form over control surfaces (ailerons/elevators), making them ineffective or causing "Buzz".