wing boundary layer
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
The boundary layer is a very thin layer of air
flowing over the surface of an aircraft wing, or airfoil, (as well as other
surfaces of the aircraft). The molecules directly touching the surface of the
wing are virtually motionless. Each layer of molecules within the boundary layer
moves faster than the layer that is closer to the surface of the wing. At the
top of the boundary layer, the molecules move at the same speed as the molecules
outside the boundary layer. This speed is called the free-stream velocity.
The actual speed at which the molecules move depends upon the shape of the wing,
the viscosity, or stickiness, of the air, and its compressibility (how much it
can be compacted).
Further, boundary layers may be
either laminar (layered), or turbulent (disordered). As the
boundary layer moves toward the center of the wing, it begins to lose speed due
to skin friction drag. At its transition point, the boundary layer changes from
laminar, where the velocity changes uniformly as one moves away from the
object's surface, to turbulent, where the velocity is characterized by unsteady
(changing with time) swirling flows inside the boundary layer.
The flow outside of the boundary
layer reacts to the shape of the edge of the boundary layer just as it would to
the physical surface of an object. So the boundary layer gives any object an
"effective" shape that is usually slightly different from the physical shape.
The boundary layer may also lift off or separate from the body, creating
an effective shape much different from the physical shape of the object and
causing a dramatic decrease in lift and increase in drag. When this happens, the
airfoil has stalled.
As well as the development of
airfoil stall, the details of the flow within the boundary layer are very
important for many problems in aerodynamics, including the skin friction drag of
an object and the heat transfer that occurs in high-speed flight.
The theory that describes boundary
layer effects was first presented by Ludwig Prandtl in the early 1900s. Prandtl
was the first to realize that the forces experienced by a wing increased from
the layer very near the wing's surface to the region far from the surface.
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