Aerodynamic drag is the resistance (retarding effect) applied to
a body as it passes through the air. It is made up of two
components, caused by pressure and by friction. Resistance
caused by pressure occurs because there is a difference in air
pressure between the front and rear surfaces of the body. This
has to be overcome to keep the body moving through the air, and
is particularly significant in the case of relatively flat-
surfaced, bulky objects such as motor vehicles. If the body
moving through the air is slim and streamlined, however, like a
modern aircraft, resistance due to friction represents a higher
proportion of total aerodynamic drag. It is caused by the
airflow rubbing against the surfaces as it passes over them.
Aerodynamic drag increases at the same rate as the square of the
air speed. In other words if the speed doubles, the drag
increases fourfold; if it quadruples, it increases sixteen-fold.
See also: cd value.