British Young Pilots · Principles of Flight · Module POF-2.1
An aerofoil is a surface designed to help lift, control or propel the aircraft by using the airflow over and around it. The wing is the famous one, but the tailplane, the fin and the propeller blades are aerofoils too, and the control surfaces (ailerons, elevators, rudder) form part of them.
Moving a control surface changes the aerofoil's shape and so changes the forces the airflow generates; that is how the aircraft is manoeuvred. Flaps change the wing's shape for better low-speed controllability at take-off and landing.
Source: Pooleys Vol 4 (10th ed.) p.9.
Lift production is explained by Bernoulli's Principle: high flow velocity gives low static pressure, also called the venturi effect. The airflow around an aerofoil behaves like airflow through a venturi. And only the relative velocity between aeroplane and air matters: aeroplane through air or air past aeroplane, both views give the same answers.
FOUNDATIONSA river speeds up where the banks squeeze together. Air does the same over the curved top of a wing, and faster-moving air pushes less.
Streamlines squeeze together over the top of the aerofoil like the throat of a venturi. Source: Pooleys Vol 4 (10th ed.) p.9–10.
The layer of air nearest the surface is the boundary layer. Friction slows the layers closest to the skin, and the air actually touching the surface may have a relative velocity of zero. The whole slowed layer is typically only several millimetres thick.
Along the surface the smooth (laminar) boundary layer eventually turns turbulent and thickens significantly. That point is the transition point.
Source: Pooleys Vol 4 (10th ed.) p.10.
In streamline flow, succeeding molecules follow the same steady path: flow runs along the lines, never across them. In turbulent flow, molecules take paths quite different from the ones before. Streamline flow is what you want in most phases of flight.
Where the boundary layer separates from the surface and the main flow breaks away turbulent is the separation point. Keep wings (and helicopter rotors) free of frost and dirt: contamination encourages turbulent flow.
STRETCHFrost as thin as sandpaper matters on a wing metres long. Why would such thin roughness trip the flow early? Think about how thin the boundary layer is.
The separation point returns with force in the stalling module. Source: Pooleys Vol 4 (10th ed.) p.10–11.
Streamline diagrams are data. Wider spacing between streamlines means the flow has slowed; closer spacing means it has sped up.
The energy trade. A fluid in motion carries static pressure energy and dynamic pressure energy (kinetic energy due to motion). Bernoulli's principle is the trade between the two.
Limits of the model. PPL flying mainly concerns 200 knots or less. At higher speeds air compressibility can no longer be disregarded, and that is CPL territory. Airflow character also depends on aircraft size, wing shape, and the air's density and viscosity.
Source: Pooleys Vol 4 (10th ed.) p.9–11.
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