Streamlines, continuity and Bernoulli, angle of attack, the laminar/turbulent boundary layer, separation and surface contamination.
Notes
Airflow & the boundary layer — summary notes
Free
Main ideas
In a streamtube, continuity means A·V is constant — flow speeds up where the tube narrows; Bernoulli then says total pressure p + ½ρV² is constant, so where the air speeds up the static pressure falls. This pressure difference is the basis of aerofoil lift.
The angle of attack (AoA) is the angle between the chord line and the relative airflow — NOT the pitch attitude relative to the horizon; an aircraft can have a high AoA in a level or even descending attitude.
The boundary layer is the thin film of air next to the surface where viscosity slows the flow from zero at the skin up to free-stream speed; it starts laminar (smooth, thin, low skin-friction drag) and transitions to turbulent (thicker, more skin friction but better mixed).
An adverse pressure gradient (static pressure rising toward the trailing edge) thickens the boundary layer and can make it separate; separation destroys lift and adds pressure (form) drag — at high AoA this separation is the stall.
Surface contamination — ice, frost, snow, even heavy dirt — roughens the wing, trips the boundary layer early and promotes separation; a layer of frost no thicker than sandpaper can cut maximum lift by a third, which is why critical surfaces must be clean before flight.
⚠ Exam trap: a turbulent boundary layer has MORE skin-friction drag than a laminar one, yet it clings to the surface longer and resists separation — vortex generators deliberately trip it to delay the stall.
Key formulas
Continuity
A₁·V₁ = A₂·V₂
Bernoulli
p + ½·ρ·V² = constant (along a streamline)
Solved examples
Why do many wings carry small angled vanes (vortex generators) just aft of the leading edge?
They deliberately trip the boundary layer from laminar to turbulent. A turbulent layer carries more energy near the surface, so it resists the adverse pressure gradient and stays attached to a higher angle of attack — delaying separation and the stall at the cost of a little extra skin-friction drag.