Airflow & the Boundary Layer
Streamlines, continuity and Bernoulli, angle of attack, the laminar/turbulent boundary layer, separation and surface contamination.
Airflow & the boundary layer, summary notes
- 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.
- Continuity
- A₁·V₁ = A₂·V₂
- Bernoulli
- p + ½·ρ·V² = constant (along a streamline)
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.
Bernoulli & the venturi
Airflow concept map
Airflow & the Boundary Layer
Airflow & boundary layer quiz
Airflow & the Boundary Layer, quiz
1. In the aerofoil section shown, the straight line joining the leading and trailing edges, marked A, is the:
Chord lineMean camber lineBoundary layer2. In the figure, the angle between the chord line and the relative airflow is the:
Angle of attackAngle of incidenceDihedral angle3. In the figure the boundary layer thickens along the surface, and the point marked T is where its character changes. Point T is the:
Transition pointStagnation pointCentre of pressure4. Bernoulli's principle states that where a streamlined flow speeds up, its static pressure:
FallsRisesStays the same5. The angle of attack is the angle between the:
Chord line and the relative airflowChord line and the horizonFuselage and the runway6. The boundary layer is:
The thin layer of slowed air next to the surfaceThe shockwave ahead of the wingThe wake far behind the aircraft7. Compared with a laminar boundary layer, a turbulent one has:
More skin-friction drag but resists separation betterLess drag and separates soonerNo difference in behaviour8. Flow separation at a high angle of attack causes:
The stall, loss of lift and a rise in dragA large increase in liftA reduction in total drag