Scientific Principles of Kites

A flying kite is a demonstration of the opposing work of gravity and aerodynamic forces.

As enthusiasts elevate their kites high into the sky they are demonstrating the scientific principal of torque. In order for a kite to fly, the torque lifting the kite from the wind's aerodynamic force must exceed the gravitational torque pulling it down.

The Kite System

Kites come in a variety of shapes and sizes, but all contain the same key elements that allow the kite to eventually take flight when pulled through the air. All kites feature a fabric material that catches the air to provide upward force. The fabrics are attached to a sturdy frame, commonly made of plastic or wood, and the frame is attached to a line which is pulled by the individual flying the kite to provide the forward motion required to generate the upward force.

Gravity Forces

The enemy of flight, gravity is constantly working to pull the kite out of the sky and back down to the earth. It succeeds when the wind passing over the kite is significantly reduced. Gravity acts straight down on the kite, from the center of the kite's gravity, creating a force equal to the mass of the kite multiplied by approximately 9.8 meters per second-squared.

Aerodynamic Forces

The aerodynamic force is the upward application of force provided by wind hitting the fabric materials of the kite. The angle and magnitude at which the aerodynamic force is applied on the kite is dependent on the shape of the kite, as different shapes will catch the wind in different ways. As the height and amount of line let out changes, so too does the relative angle of the force to the angle of the line, which changes how effectively the force keeps the kite aloft.

Flying the Kite

The force of gravity and the aerodynamic force oppose each other as forces of torque, looking to rotate the kite about the pivot point of the line tied to the kite. However it is not a simple matter of the aerodynamic force being larger than the force of gravity. Torque is measured by the force applied multiplied by the length of a line from the pivot point to a line showing the angle of the force, drawn perpendicular to the force's direction. Changes to the line being led out, and the speed being pulled, can adjust the relative angle of the forces to the pivot point, thus deciding whether gravity wins and lowers the kite, or aerodynamic forces win and raise the kite.

References

• NASA: Torques on a Kite