The amount of drag generated by an object
depends on the size of the object.
Drag is an aerodynamic force
and therefore depends on the
pressure variation
of the air
around the body as it moves through the air. The total aerodynamic
force is equal to the pressure times the surface area around the
body. Drag is the
component
of this force along the flight direction.
Like the other aerodynamic force, lift, the
drag is directly proportional to the area of the object. Doubling the area
doubles the drag.
There are several different areas from which
to choose when developing the reference area used in the
drag equation.
If we think of drag as being caused by friction between
the air and the body, a logical choice would be the total surface
area (As) of the body. If we think of drag as being a resistance to
the flow, a more logical choice would be the frontal area (Af) of the
body which is perpendicular to the flow direction. This is the area
shown in blue on the figure.
Each of the
areas are proportional to the other areas, as designated by
the "~" sign on the figure. Since the drag
coefficient is determined experimentally, by measuring the drag
and measuring the area and performing the necessary math to produce
the coefficient, we are free to use any area which can be
easily measured. If we choose the surface area, the computed coefficient
has a different value than if we choose the crosssectional
area, but the drag is the same, and the coefficients are related by
the ratio of the areas. In practice, drag coefficients are reported
based on a wide variety of object areas. In the report, the
test engineer must specify what area is used and when using the data
the reader may have to convert the drag coefficient using the ratio
of the areas.
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