Page 97 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 84

84 CHAPTER THREE

Form Drag

We should look at drag a little more carefully in order to better under-
stand stalls. Parasite drag is composed of two parts. In Chapter 2 the
effect of friction was discussed. Also mentioned was form drag. Form
drag is the drag associated with moving things like antennae and
wheels through the air. Form drag can be thought of as the drag asso-
ciated with pulling the wake of the airplane along with it. Form drag
plus skin drag make up parasite drag.
Form drag is what people usually associate with aerodynamic
shapes. People often look at cars and instinctively know which are
better aerodynamically. We tend to look at how streamlined the car is.
What usually distinguishes a streamlined car from an unstreamlined
car is the apparent wake resulting from the rear of the car. A truck
clearly has a large wake and high form drag, as illustrated in Figure
3.23. It is desirable to reduce form drag on an airplane as much as
possible. This means reducing the wake. Any part of an airplane
where the air separates from the surface produces a wake. Even a
small, cylindrical antenna will produce a significant wake. Therefore,
protuberances such as antennae are encased in aerodynamic
The wing of a Boeing 747 has
fairings.
the same parasitic drag as a
Until about 1920, airplane designers thought wings had to be
1 2-inch cable of the same
thin. To create an efficient structure, there was much use of
length.
external wire bracing. Ironically, the external bracing resulted in
much higher drag than what was saved by making the wings
thin. Late in WWI the Germans discovered that the wire bracing was
adding too much drag and started to use fatter wings that could hold
more structure.

Fig. 3.23. Form drag illustrated by the wake of a truck.