Drag is the aerodynamic force that
opposes an aircraft's motion through the air. Drag is generated by
every part of the airplane, even the engines.
How is drag generated?
Drag is a mechanical force. It is generated by the interaction and
contact of a solid body with a fluid (liquid or gas). It is
not generated by a force field, in the sense of a gravitational
field or an electromagnetic field, where one object can affect
another object without being in physical contact. For drag to be
generated, the solid body must be in contact with the fluid. If there
is no fluid, there is no drag. Drag is generated by the difference in
velocity between the solid object and the fluid. There must be motion
between the object and the fluid. If there is no motion, there is no
drag. It makes no difference whether the object moves through a
static fluid or whether the fluid moves past a static solid object.
Drag acts in a direction that opposes the motion. (Lift acts
perpendicular to the motion.)
While many of the factors that affect lift
also affect drag, there are some
additional sources of aircraft drag.
We can think of drag as aerodynamic friction, and one of
the sources of drag is the skin friction between the molecules
of the air and the solid surface of the aircraft. Because the skin
friction is an interaction between a solid and a gas, the magnitude
of the skin friction depends on properties of both solid and gas. For
the solid, a smooth, waxed surface produces less skin friction than a
roughened surface. For the gas, the magnitude depends on the
viscosity of the air and the relative
magnitude of the viscous forces to the motion of the flow, expressed
as the Reynolds number. Along the solid surface, a
of low energy flow is generated and the magnitude of the
skin friction depends on the state of this flow.
We can also think of drag as aerodynamic resistance to the
motion of the object through the fluid. This source of drag depends
on the shape of the aircraft and is called form drag. As air
flows around a body, the local velocity and pressure
are changed. Since pressure is a measure of the momentum of the gas
molecules and a change in momentum produces a force,
a varying pressure distribution will produce a force on the body. We
can determine the magnitude of the force by
integrating (or adding up) the local pressure times the surface area
around the entire body. The component of the aerodynamic force that
is opposed to the motion is the drag; the component perpendicular to
the motion is the lift.
There is an additional drag component caused by the generation of
lift. Aerodynamicists have named this component the induced
drag. This drag occurs because the flow near the wing tips is
distorted spanwise as a result of the pressure difference from the
top to the bottom of the wing. Swirling vortices are formed at the
wing tips, and there is an energy associated with these vortices. The
induced drag is an indication of the amount of energy lost to the tip
vortices. The magnitude of induced drag depends on the amount of lift
being generated by the wing and on the wing
geometry. Long, thin (chordwise) wings have low induced drag;
short wings with a large chord have high induced drag.
All of the factors
that affect aircraft drag can be combined into a single mathematical
which can be used to predict aircraft performance.
At the time of the Wright brothers, a slightly different version of the
The Wright brothers learned about induced drag the hard way. Following
their first glider flights of
they knew that they had to increase the size of their wings to allow flight in
reasonable winds. For the
1901 aircraft they increased the chord of the wing but
kept the span nearly the same. This produced a wing with high induced drag. The
brothers had made mathematical predictions of the performance of their aircraft.
But the 1901 aircraft did not meet their range predictions because of lower than
expected lift and higher than expected drag. During the winter, with the aid of their
wind tunnel, they began to understand the role of
high induced drag on their aircraft's poor performance. They then designed the
1902 aircraft to have a longer span and shorter chord
than the 1901 aircraft. The 1902 aircraft was able to meet their performance goals
and paved the way for the
You can view a short
of "Orville and Wilbur Wright" discussing the drag force
and how it affected the flight of their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page