Influencing Critical Mach (Increase M_crit)
Goal
To fly faster without incurring the penalties of Drag Divergence and Shock Stall. We want to increase $M_{crit}$.
Design Features
- 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.
- 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.
- 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.
- Area Rule (Waisting):
- "Coke bottle" fuselage shape. Smooths the cross-sectional area distribution of the aircraft to reduce wave drag.
- 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).