The boundary layer is a very thin layer of air lying over the surface of the wing and, for that matter, all other surfaces of the airplane. Because air has viscosity, this layer of air tends to adhere to the wing. As the wing moves forward through the air, the boundary layer at first flows smoothly over the streamlined shape of the airfoil. Here the flow is called the laminar layer.
As the boundary layer approaches the center of the wing, it begins to lose speed due to skin friction and it becomes thicker and turbulent. Here it is called the turbulent layer. The point at which the boundary layer changes from laminar to turbulent is called the transition point. Where the boundary layer becomes turbulent, drag due to skin friction is relatively high. As speed increases, the transition point tends to move forward. As the angle of attack increases, the transition point also tends to move forward. With higher angles of attack and further thickening of the boundary layer, the turbulence becomes so great the air breaks away from the surface of the wing. At this point, the lift of the wing is destroyed and a condition known as a stall has occurred. In Figure 3-58, view A shows a normal angle of attack and the airflow staying in contact with the wing. View B shows an extreme angle of attack and the airflow separating and becoming turbulent on the top of the wing. In view B, the wing is in a stall.
Boundary Layer Control
One way of keeping the boundary layer air under control, or lessening its negative effect, is to make the wing’s surface as smooth as possible and to keep it free of dirt and debris. As the friction between the air and the surface of the wing increases, the boundary layer thickens and becomes more turbulent and eventually a wing stall occurs. With a smooth and clean wing surface, the onset of a stall is delayed and the wing can operate at a higher angle of attack. One of the reasons ice forming on a wing can be such a serious problem is because of its effect on boundary layer air. On a high-speed airplane, even a few bugs splattered on the wing’s leading edge can negatively affect boundary layer air.
Other methods of controlling boundary layer air include wing leading edge slots, air suction through small holes on the wing’s upper surface, and the use of devices called vortex generators.
A wing leading edge slot is a duct that allows air to flow from the bottom of the wing, through the duct, to the top of the wing. As the air flows to the top of the wing, it is directed along the wing’s surface at a high velocity and helps keep the boundary layer from becoming turbulent and separating from the wing’s surface. Another way of controlling boundary layer air is to create a suction on the top of the wing through a large number of small holes. The suction on the top of the wing draws away the slow-moving turbulent air, and helps keep the remainder of the airflow in contact with the wing.
Vortex generators are used on airplanes that fly at high subsonic speed, where the velocity of the air on the top of the wing can reach Mach 1. As the air reaches Mach 1 velocity, a shock wave forms on the top of the wing, and the subsequent shock wave causes the air to separate from the wing’s upper surface. Vortex generators are short airfoils, arranged in pairs, located on the wing’s upper surface. They are positioned such that they pull high-energy air down into the boundary layer region and prevent airflow separation.