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Principles of flight
5.3 Airflow around the aircraft
Air consists of molecules. Therefore, the denser it is, the more resistance it
provides against an object moving through it. There are more molecules that
need to be displaced in dense air.
5.3.1 Airflow
When an object moves through the air, it pushes the air molecules, causing them
to change direction and speed.
5.3.1.1 Boundary layer
When an object moves through the air (or the air moves around a profile, like a
wing), the air speed decreases in what we call the boundary layer.
The boundary layer is
approximately 0.1 to 5-10 mm
thick.
Close to the profile, the air speed is nearly zero! The airflow speed decreases in
the boundary layer.
5.3.1.2 Laminar flow, transition point and turbulence flow
Air can flow smoothly (laminar) in the boundary layer, or it can form small
vortices (turbulent).
When the airflow is
smooth, there is the
least resistance.
Where the flow transitions from laminar to turbulent is called the transition
point.
Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 446
