Nasa Kite modeler
Kite modeler is a open software from NASA which allow you to understand the physics and math behind the flight of a kite and you can use it to design your own kite.
This program is easy to use and you just need to change the values of different variables until you have a Torque = +/-0.
Forces on a kite : http://www.grc.nasa.gov/WWW/k-12/airplane/kitefor.html
Torques on a kite : http://www.grc.nasa.gov/WWW/k-12/airplane/kitetor.html
Aerodynamics of kite : http://www.grc.nasa.gov/WWW/k-12/airplane/kiteaero.html
With this program you could set the wind speed, the altitude, the payload and even the type and the shape of the kite etc...
It is possible to use it on-line or off-line.
for more information follow this link http://www.grc.nasa.gov/WWW/k-12/airplane/kiteprog.html