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Rod Machado’s Private/Commercial Pilot Handbook
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Parasite Drag

The following three types of drag fall under the catego-
ry of parasite drag.
Form drag is that portion of parasite drag that’s gener-
ated by the shape of the airplane and the flow of air
around it. The fact is that no matter how sleek, skinny or
slick you make an airfoil (Figure 39), it still generates
parasite drag because air must flow over, under, and
around it. Wings, antennas, the engine cowling, wheel Form drag is a result of the entire body of the
pants, and so on all generate form drag. The degree of airplane as it moves through the air. Streamlined
form drag present depends on how quickly and smoothly aircraft such as a glider has much less form drag. Fig. 39
the displaced air rejoins itself after displacement.
Interference drag results from the eddy currents and Interference drag develops
turbulence created when airflow from different surfaces when two aircraft surfaces
join each other, such as the
interact. For instance, air flowing over the fuselage inter- wing and the fuselage.
feres with air flowing over the wing generating a small
degree of turbulence as a result. This disruption of the air-
flow creates drag. The most interference drag, for exam-
ple, is created when two perpendicular surfaces meet,
such as the fuselage and the wing (Figure 40). Of course,
fairings (blending of metal shapes) reduce interference
drag (Figure 41). Separating lifting surfaces and external
airplane components also helps reduce interference drag.
Skin friction drag results from the aerodynamic resis-
tance that occurs when moving air contacts an airplane’s
surface. You can spend all day dusting and polishing your
Fig. 40
airplane but there will always—always—be some skin
friction drag present, no matter how smoothly you polish The turbulence, pressure disruption, and airflow separation
your bird. Why? Because no surface is perfectly smooth. causing interference drag can be minimized to a degree by
blending these shapes as shown here.
Air molecules that have direct contact with the wing’s
surface become essentially motionless, which is quite
undesirable. Ideally, we want those molecules touching
the surface to flow without any resistance. That, howev-
er, can’t happen given that any surface, no matter how
smooth it is, is still rough on a molecular scale. So skin
friction drag is always present, but it can be minimized to
a degree by using flush-mounted rivets, reducing surface
irregularities, and keeping the airplane’s surfaces cleaned
and waxed.
By moving away from the wing’s surface, the layers of
air molecules move slightly faster (Figure 42) until these Fig. 41

Fig. 42