This page is intended for college, high school, or middle school students.
For younger students, a simpler explanation of the information on this page is
available on the
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 is a force and is therefore a
having both a magnitude and a direction.
Drag acts in a direction that is opposite to the motion
of the aircraft. Lift acts perpendicular to the motion.
There are many
that affect the magnitude of the drag.
Many of the factors also affect lift
but there are some factors that are unique to 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 conditions in the boundary layer.
We can also think of drag as aerodynamic resistance to the
motion of the object through the fluid. This source of drag depends
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.
Both the lift and drag force act through the
center of pressure
of the object.
There is an additional drag component caused by the generation of
lift. Aerodynamicists have named this component the
It is also called "drag due to lift" because it only occurs
on finite, lifting wings. Induced drag occurs because the distribution
of lift is not uniform on a wing, but varies from root to tip.
For a lifting wing, there is a
pressure difference between the upper and lower surfaces of the wing.
Vortices are formed at the wing tips, which produce a swirling flow
that is very strong near the wing tips and
decreases toward the wing root. The local
angle of attack
of the wing is increased by the induced flow of the tip vortex,
giving an additional, downstream-facing, component to the
aerodynamic force acting on the wing.
The force is called induced drag because it
has been "induced" by the action of the tip vortices.
The magnitude of induced drag depends on the amount of lift
being generated by the wing and on the distribution of lift across the span.
(chordwise) wings have low induced drag;
short wings with a large chord have high induced drag.
Wings with an elliptical distribution of lift have the minimum induced drag.
Modern airliners use
to reduce the induced drag of the wing.
Two additional sources of drag are wave drag and ram drag. As an
aircraft approaches the speed of sound,
are generated along the surface. The shock waves produce a change in
static pressure and a loss of total pressure.
Wave drag is associated with the formation of the shock waves.
The magnitude of the wave drag depends on the
of the flow. Ram drag is produced when
free stream air is brought inside the aircraft.
bring air on board, mix the air with fuel, burn the fuel, then
exhausts the combustion products to produce
If we look at the basic
there is a mass flow times entrance velocity term that is subtracted from the gross thrust.
This "negative thrust" term is the ram drag.
Cooling inlets on the aircraft
are also sources of ram drag.
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.
Sources of Drag:
Factors that Affect Drag:
Forces on an Airplane:
Forces on a Glider:
- Beginner's Guide Home Page