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The type of operation for which an airplane is intended has a very important bearing on the selection of the shape and design of the wing for that airplane. Boundary layer effects play a very important part in determining the drag for the aircraft. Thus, the wing should be designed to minimize the drag.
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 (Fig. 3). 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.
Various methods have been developed to control the boundary layer in order to reduce skin friction drag.
Suction Method. One method uses a series of thin slots in the wing running out from the wing root towards the tip. A vacuum sucks the air down through the slots, preventing the airflow from breaking away from the wing and forcing it to follow the curvature of the wing surface. The air, which is sucked in, siphons through the ducts inside the wing and is exhausted backwards to provide a little extra thrust. The laminar flow airfoil is itself a structural design intended to make possible better boundary layer control. The thickest part of a laminar flow wing occurs at 50% chord. The transition point at which the laminar flow of air breaks down into turbulence is at or near the thickest part. The transition point at which the laminar flow of air becomes turbulent on a laminar flow airfoil is rearward of that same point on a conventional designed airfoil.
Vortex generators are small plates about an inch deep standing on edge in a row spanwise along the wing. They are placed at an angle of attack and (like a wing airfoil section) generate vortices. These tend to prevent or delay the breakaway of the boundary layer by re-energizing it. They are lighter and simpler than the suction boundary layer control system described above.
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Updated: March 12, 2004