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
kite. You can compare these
forces to the
on the aircraft when flown as a piloted
glider 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
use kite experiments to determine the aerodynamic
unpowered aircraft from 1900 to 1902.
They also used kite experiments to
learn the basics of flight
You can also learn the basics of
by flying a kite. The forces
on your kite are
exactly the same as the forces on the Wright brothers' kite.
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. 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, an airplane rotates about the center of
but a kite rotates about the connection point of the control wires.
A kite's motion is confined or pinned like a door which rotates
on its hinges. The kite's weight is always directed toward the center of the earth.
To make a kite fly,
we must generate a force to overcome the
weight. This force is called the lift and is
generated by the motion of the air over the kite. Lift is an
aerodynamic force ("aero" stands for the air, and
"dynamic" denotes motion). Lift is directed perpendicular
(at right angle) to the wind direction. As with weight, each
part of the kite contributes to a single lift force. Most of the lift of
the Wright 1900 kite 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 air moves past the kite, the
kite resists the motion of the air. This
resistance force is called the drag of the
direction of the drag force is always in the direction of the wind.
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 kite.
Engineers break this force into lift and drag to more easily explain the
motion of an object along its flight path.)
To keep the kite at a fixed location,
a pair of control lines are attached to the kite.
The control lines generate a force called tension
which is used to overcome the drag.
Without the control lines, the kite would move in the
direction of the wind and there would be no
between the wind and the kite. The lift would go to zero and the
kite would fall to the ground because of gravity.
For convenience, the tension force is often broken into two components, one vertical
and one horizontal.
When the kite is in stable flight, the lift is equal to and opposes the
combination of the weight and the vertical pull of tension. The drag is equal to and
opposes the horizontal tension. Compared to the forces on an airplane,
the horizontal pull on a kite plays the roll of the thrust.
The vertical pull of the line tension is mainly the weight of the line;
the kite must lift its own weight and the weight of the line.
The relative strength of the forces determines the motion of the
kite as described by Newton's
laws of motion.
If the wind velocity increases,
the lift increases and exceeds the weight of the kite. The kite
then moves vertically and the tension force increases because of increased
drag. The vertical component of tension increases because of the
change in angle that the tension force makes with the vertical. A new balance
point is established and the kite achieves a different stable condition.
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page