This Demonstration illustrates the flight angle of a cubic box kite and the tension force in the string. The tension and the angle are affected by the lift, drag, surface area of one side of the kite, wind velocity, kite mass and string length. The coefficient of drag is related to the aspect ratio (side A/side B) of the kite, which in this case is kept constant at 1. For simplicity, the coefficient of lift is assumed to be constant; normally its value would need to be found experimentally.

A force balance is done and the sum of the forces is zero so drag equals lift:

,

,

where and are the forces in the and directions (N), is drag (N), is tension (N), is lift (N), is kite mass (kg) and is the gravitational constant.

The lift and drag coefficients are:

,

,

where is air density (), is wind velocity (m/s), and is the area of one side of the kite ().

In this Demonstration the coefficients of lift and drag and are constant, which allows calculating the tension in the string using the equations above.