A force may be thought of as a push or
pull in a specific direction. This slide shows the forces that act on
when flown as a piloted
glider. You can compare these
forces to the
on the aircraft when flown as a
kite and you will note only a few differences.
There are also a few differences from the
But the similarities are so great that the Wrights were able to
learn the basics of flight
by flying their
unpowered aircraft as a glider.
You can also learn the basics of
by flying a glider or paper airplane. The forces
on your glider are
exactly the same as the forces on the Wright brothers' glider.
Weight is a force that is always directed
toward the center of the earth. The
of the force depends on
the mass of all the parts of the aircraft including the pilot. The weight is
distributed throughout the aircraft, but we can often think of it as
collected and acting through a single point called the center
of gravity. In flight, a glider
about the center of
To make a glider fly,
we must generate a force to overcome the
weight. This force is called the lift and is
generated by the motion of the glider through the air. Lift is an
aerodynamic force ("aero" stands for the air, and
"dynamic" denotes motion). Lift is directed perpendicular
(at right angle) to the flight direction. As with weight, each
part of the glider contributes to a single lift force. Most of the lift of
the Wright glider was generated by the wings. The lift acts
through a single point called the
center of pressure.
center of pressure is defined just like the center of gravity, but
distribution around the body instead of the
As the glider moves through the air, the
air resists the motion of the glider. This
resistance force is called the drag of the
direction of the drag force is always opposite the direction of the motion.
Drag acts through the center of pressure in the same way that lift
acts through the center of pressure.
(In reality, there is only one aerodynamic force on the glider.
Engineers break this force into lift and drag to more easily explain the
motion of the glider along its flight path.)
In order for a glider to fly, it must generate
lift to oppose the weight. To generate
lift, the glider must move through the air. But the motion of the
glider through the air generates drag. In a powered aircraft, the
from the engine opposes drag. The glider, however, has no
engine to generate thrust.
So how does a glider generate the velocity needed for
The simple answer is that a glider trades altitude for velocity. It
trades the potential energy difference from a higher altitude to a
lower altitude to produce kinetic energy, which means velocity.
Gliders always descend relative to the air in
which they are flying.
In the case of the Wright aircraft, the glider was flown from the
top of a sand dune and was only airborne for a short time.
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page