# Influencing Critical Mach (Increase M_crit)

> Design features used to delay the onset of shock waves and increase cruising speed.

- **Subject:** 081 — Principles of Flight
- **Section:** 081-02 — High Speed Aerodynamics
- **Topic code:** 081-02-05
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/principles-of-flight/high-speed-aerodynamics/means-to-influence-critical-mach-number-mcrit/

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## Goal
To fly faster without incurring the penalties of Drag Divergence and Shock Stall. We want to **increase** $M_{crit}$.

## Design Features
1.  **Sweepback (Flecha)**: The most effective method.
    *   Decomposes velocity into a spanwise component (doesn't accelerate much) and a chordwise component (accelerates flow).
    *   Effective Mach number seen by the aerofoil is lower ($M_{eff} = M \times \cos(\text{sweep angle})$).
    *   Disadvantage: Poor low-speed lift, tip stall tendency.
2.  **Thin Wings (Low Thickness/Chord Ratio)**:
    *   Less curvature means less acceleration of airflow over the wing.
    *   The peak velocity is lower, delaying the formation of a shock.
    *   Disadvantage: Less fuel volume, heavier structure needed.
3.  **Supercritical Aerofoil**:
    *   Flattened upper surface: Reduces velocity peak.
    *   Cambered rear section: Recovers lift at the rear.
    *   Result: A weaker shock wave that forms further aft.
4.  **Area Rule (Waisting)**:
    *   "Coke bottle" fuselage shape. Smooths the cross-sectional area distribution of the aircraft to reduce wave drag.
5.  **Vortex Generators**:
    *   Small vanes that re-energize the boundary layer. They don't increase $M_{crit}$ itself, but they delay **Shock Stall** (separation behind the shock).

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