# Exceeding Critical Mach (M_crit)

> Effects of shock waves on lift, drag, and stability.

- **Subject:** 081 — Principles of Flight
- **Section:** 081-02 — High Speed Aerodynamics
- **Topic code:** 081-02-03
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/principles-of-flight/high-speed-aerodynamics/effects-of-exceeding-the-critical-mach-number-mcrit/

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## 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".

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