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Chapter 2 - Aerodynamics: The Wing Is the Thing
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Spoilers Alert!

A Spoiler

Parasite drag can be a good thing, especially when you can
control it by using spoilers. Spoilers can be extended and
retracted from inside the cockpit. They project upward into the
airstream, destroying some of the wing’s lift. Higher perfor-
mance airplanes, because of their low drag profile, often find
these devices useful in aiding their descents.
Fig. 43
happens, drag increases dramatically and the wing stalls.
The accumulative effect of all three forms of parasite
The parasite drag vs. indicated airspeed graph above drag is seen graphically in Figure 43.
shows how dramatically parasite drag increases as indi- The most desirable type of airflow to have over the top
cated airspeed increases. It’s easy to see why doubling
the airplane’s airspeed doubles its parasite drag. of the wing is laminar airflow. This is smooth, high-speed
air that produces relatively little skin friction drag. Some
molecules move at the free airstream velocity (the unre-
airplanes with laminar flow wings can maintain laminar
stricted flow of air over the airplane). Engineers refer to airflow over half of the wing’s upper curved surface. Most
the layer of air in contact with and just above the airfoil airplanes, however, lose the laminar flow of air at a point
surface as the wing’s boundary layer. This layer of air is where the wing’s center of pressure is located (25% to 30%
surprisingly thin, perhaps about as thin as a business
card. Ultimately, the actual speed of the air molecules aft of the leading edge). At this point, pressure begins
flowing over a wing depends on the shape of the wing, the increasing, resulting in a loss of energy and an increase
in drag. To help prevent early airflow separation, engi-
viscosity (stickiness) of the air, and the air’s compressibil-
ity (how much it can be compacted). neers add vortex generators (Figure 44) to the top of the
wing which creates small vortices (thus, the word, “vor-
It turns out that the airflow above the boundary layer
reacts to the shape of the top of the boundary layer just tex”). These small vertical strips mix high-energy air
from the free airstream above the wing’s boundary layer
as it would react to any solid object. This results in the
wing’s boundary giving this airflow an “effective” shape with airflow in the wing’s boundary layer. The result is
that can be slightly different from the wing’s shape. That, higher-speed airflow near the wing’s surface. While this
of course, would be a problem if engineers didn’t consider airflow isn’t laminar, it does resist early airflow separa-
how this “effective” airflow shape might interfere with tion over the wing, reducing the airplane’s stall speed and
such things as the ram air input to the pitot tube or fuel skin friction drag. This is one reason why STOL (short
tank vent lines. Of course, the wing’s boundary layer takeoff and landing) airplanes often have vortex genera-
might also become detached from the wing. When this tors on their wings and tail surfaces.

Vortex Generators

Fig. 44