Translated by BabelFish
The boundary layer is the thin film surrounding a body
moving in a fluid (air for example). In this thin film, the
forces of viscosities are important and one notes important variations
speed when one moves away perpendicularly with the wall. The
boundary layer directly conditions the resistance of frictions of the
body moving in the fluid.
The thickness of the layer is reached when the speed measured in the boundary layer reaches 99% the speed of the true fluid. The boundary layer Laminaire (speed parallel with the flow) drag less than one turbulent boundary layer, but this one takes off more easily because it is energy than the turbulent boundary layer (because of the Flight Path Vectors of the fluids which is different). Thus one places Vortices which create turbulent air in front of the control surfaces (before the point of transition)