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
Weight is the force generated by
the gravitational attraction of the earth on the airplane.
We are more familiar with weight than with the other forces acting on
an airplane, because each of us have our own weight which we can
measure every morning on the bathroom scale. We know when one thing
is heavy and when another thing is light. But weight, the
gravitational force, is fundamentally different from the aerodynamic
forces, lift and drag.
Aerodynamic forces are mechanical forces and the airplane has to be
in physical contact with the the air which generates the force. The
gravitational force is a field force; the source of the force does
not have to be in physical contact with the object to generate
a pull on the object.
The nature of the gravitational force has been studied by
scientists for many years and is still being investigated by
theoretical physicists. For an object the size of an airplane, the
descriptions given three hundred years ago by Sir Isaac Newton work
quite well. Newton developed his theory of gravitation when he was
only 23 years old and published the theories with his laws
of motion some years later. The gravitational force between two
objects depends on the mass of the objects and the inverse of the
square of the distance between the objects. Larger objects create
greater forces and the farther apart the objects are the weaker the
attraction. Newton was able to express the relationship in a single
Weight is a force, and a force is a
having both a magnitude and a direction associated with it.
For an airplane, weight is always directed
towards the center of the earth. The magnitude of this force
depends on the mass of all of the parts of
the airplane itself, plus the amount of fuel, plus any payload on
board (people, baggage, freight, ...). 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
about the center of
gravity, but the direction of the weight force always remains toward
the center of the earth. During a flight the aircraft burns up its
fuel, so the weight of the airplane constantly changes. Also, the
distribution of the weight and the
center of gravity
can change, so
the pilot must constantly adjust the controls to keep the airplane
Flying involves 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.
The dream remains that, if we could really understand gravity, we
could create anti-gravity devices which would revolutionize travel
through the sky. Unfortunately, anti-gravity devices only exist in
science fiction. Machines like airplanes, or magnetic levitation
devices, create forces opposed to the gravitational force, but they
do not block out or eliminate the gravitational force.
You can view a short
of "Orville and Wilbur Wright" discussing the weight 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.
Forces on an Airplane:
Forces on a Glider:
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