Boundary layer
The thin layer of air adjacent to the surface transitions from laminar to turbulent. Turbulent layers are thicker and produce more friction drag but resist separation better — which is why vortex generators are deliberately placed ahead of control surfaces.
Compressibility
Above about Mach 0.3 density changes matter. As the aircraft nears its critical Mach number, local flow over the wing reaches Mach 1 and shock waves form, causing a sharp drag rise and possible Mach tuck.
Designing for speed
Swept wings delay the drag rise by reducing the chordwise Mach component. Supercritical aerofoils flatten the upper surface to weaken the shock. Winglets and raked tips reduce induced drag by moving the vortex outboard.
Key points
- Turbulent boundary layers resist separation — sometimes desirable.
- Critical Mach number is where local flow first reaches Mach 1.
- Wing sweep and supercritical aerofoils delay the transonic drag rise.
Knowledge check