High-speed flight and compressibility effects
Overview
High-speed flight and compressibility tests critical Mach, shock waves, Mach tuck and high-altitude envelope limits.
Core rules
- Mach number is TAS divided by local speed of sound; compressibility effects appear as local airflow reaches Mach 1, even below aircraft M = 1.
- Critical Mach (M_CRIT) is the free-stream Mach at which airflow somewhere on the aircraft first reaches Mach 1. Higher C_L or thicker airfoils lower M_CRIT.
- Beyond M_CRIT, shock waves form, causing wave drag, buffet and possible separation.
- Mach tuck: shock-induced rearward movement of the centre of pressure pitches the nose down.
- Sweep, thin supercritical airfoils, area ruling and vortex generators raise M_CRIT or reduce wave drag.
- Coffin corner is the narrow band between high-speed buffet (M_MO) and low-speed stall buffet at high altitude.
Worked example
At FL400 in ISA, a ≈ 573 kt. If M_MO = 0.85 and the low-speed buffet occurs at M 0.78, the usable band is only 0.07 M, about 40 kt TAS - hence the need to fly accurately.
Traps
- Compressibility is about Mach, not IAS.
- Mach buffet and stall buffet feel similar but have different causes.
- M_MO is a Mach limit; V_MO is an IAS limit - one may be reached before the other.