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
A force may be thought of as a push or
pull in a specific direction.
A force is a
so a force has both a magnitude and a direction. When
we have to specify both the magnitude and the direction.
This slide shows the forces that act on
an airplane in flight.
Weight is a force that is always directed
toward the center of the earth. The
of the weight depends on
the mass of all the airplane parts, plus the amount of fuel, plus any
payload on board (people, baggage, freight, etc.). The weight is
distributed throughout the airplane. But we can often think of it as
collected and acting through a single point called the
center of gravity.
In flight, the airplane
center of gravity.
Flying encompasses two major problems; overcoming the weight of
an object by some opposing force, and controlling the object
in flight. Both of these problems are related to the object's weight
and the location of the center of gravity.
During a flight, an airplane's
constantly changes as the aircraft consumes fuel. The distribution of
the weight and the center of gravity also changes. So the pilot
must constantly adjust the controls to keep the airplane balanced, or
To overcome the weight force, airplanes generate an opposing force
called lift. Lift is
generated by the motion of the airplane through the air and is an
"Aero" stands for the air, and "dynamic" denotes motion.
Lift is directed perpendicular to the flight direction.
The magnitude of the lift depends on several
velocity of the aircraft.
As with weight, each
part of the aircraft contributes to the aircraft lift force.
Most of the lift is generated by the wings. Aircraft lift acts
through a single point called the
center of pressure.
The center of pressure is defined just like the center of gravity, but
using the pressure distribution around the body instead of the
The distribution of lift around the aircraft is important for solving the
control problem. Aerodynamic surfaces are used to control the aircraft in
As the airplane moves through the air, there is another aerodynamic
force present. The air resists the motion of the aircraft and the
resistance force is called drag.
Drag is directed along and opposed to the flight direction.
Like lift, there are many
that affect the magnitude
of the drag force including the
shape of the aircraft,
"stickiness" of the air,
velocity of the aircraft.
Like lift, we collect all of the individual components'
drags and combine them into a single aircraft drag magnitude.
And like lift, drag acts through the aircraft center of pressure.
To overcome drag, airplanes use a propulsion
system to generate a force called thrust.
The direction of the thrust force depends on how the engines are attached
to the aircraft. In the figure shown above,
two turbine engines
are located under the wings, parallel to the body, with thrust acting
along the body centerline. On some aircraft, such as the Harrier,
the thrust direction can be varied to help the airplane take off in a
very short distance.
The magnitude of the thrust depends on many factors associated with
the propulsion system including the
type of engine,
the number of engines, and the
For jet engines, it is often confusing to
remember that aircraft thrust is a reaction to the hot gas rushing
out of the nozzle. The hot gas goes out the back, but the thrust
pushes towards the front. Action <--> reaction is explained by
Newton's Third Law of Motion.
The motion of the airplane through the air depends on the relative
strength and direction of the forces shown above. If the forces are
balanced, the aircraft cruises at constant
velocity. If the forces are unbalanced,
the aircraft accelerates in the direction of the largest force.
Note that the job of the engine is just to overcome the drag
of the airplane, not to lift the airplane. A 1 million pound airliner
has 4 engines that produce a grand total of 200,000 of thrust. The
wings are doing the lifting, not the engines. In fact, there are
some aircraft, called
that have no engines at all, but fly just fine.
Some external source of power has to be applied
to initiate the motion necessary for the wings to produce lift.
But during flight, the weight is
by both lift and drag.
Paper airplanes are the most obvious
example, but there are many kinds of gliders. Some gliders are
piloted and are towed aloft by a powered aircraft, then cut free to
glide for long distances before landing. During reentry and landing,
the Space Shuttle is a glider; the rocket engines are used only to
loft the Shuttle into space.
You can view a short
of "Orville and Wilbur Wright" explaining how the four forces of weight,
lift, drag and thrust affected the flight of their aircraft. The movie file can
be saved to your computer and viewed as a Podcast on your podcast player.
Forces on an Airplane:
- Beginner's Guide Home Page