An excellent way to gain an understanding and a feel for
the interaction of
forces
on an object is to
fly
a kite.
Kite flying is fun when done
safely
and you can study many of the fundamentals of airplane
aerodynamics
because a kite works very much like an airplane.
There are, however, some important differences in the response of
a kite to external forces that do not occur in an airplane. An airplane
in flight rotates about its
center of gravity which is the
average location of the weight of all the parts of the airplane.
A kite in flight does not rotate about its
center of gravity because
it is pinned by the
bridle
to the
control line.
In flight, the kite rotates about the
**bridle point**
which is the place where the line is
attached to the bridle.
*(A kite in flight
is more closely related to a hinged door than to an airplane in flight.
The center of gravity of a hinged door is in the center of the
door, but the door rotates about the hinges.)*

The location of the bridle point relative to the center of gravity (cg) and
center of pressure (cp)
determines the balance of
torques
on the kite and the
trim angle
at which the kite flies. On this page we show how to compute the
location of the bridle point.
We have an installed an X-Y co-ordinate system on the kite with
the origin at the bottom of the kite; the Y-axis is along the
height **H** of the kite, the X-axis is perpendicular to the Y-axis
and goes through the bottom of the kite. The X-axis is used as the
reference line in the computation of cg and cp.
The length of the bridle is called **B** and is measured from the
origin to the top of the kite along the bridle string.
The bridle is attached to the control line with a knot. The distance from
the knot to the origin is length **K**.
The knot (bridle point) is inclined at an angle to Y-axis which is
called the knot angle **A**.
The co-ordinates of the bridle point are **Xb** and **Yb** and from
trigonometry:

Xb = K * cos(A)

Yb = K * sin(A)

To determine the angle **A**, we need another formula from trigonometry.
This formula relates the sides and angles of any general triangle. We will
apply the formula to the triangle formed by the bridle and the height of the kite.

cos(A) = [K^2 + H^2 - (B - K)^2] / (2 * K * H)

The KiteModeler computer program can be used to calculate the various geometric variables described on this page and their effects on kite performance.

Navigation...

- Beginner's Guide to Aerodynamics
- Beginner's Guide to Propulsion
- Beginner's Guide to Model Rockets
- Beginner's Guide to Kites
- Beginner's Guide to Aeronautics

Go to...

- Beginner's Guide Home Page

*byTom
Benson
Please send suggestions/corrections to: benson@grc.nasa.gov *