Authors: Dr. Paul B. MacCready
Date Submitted: May 31, 2006
Article Type: Journal

There are kite challenges in our planning, specifically a) a device that can be induced to stay aloft indefinitely at 700 to 1,000 feet above ground, in all wind and weather conditions, to monitor video and meteorogical and acoustic signals and telemeter the information elsewhere, b) the use of a kite to extract energy from the winds in the atmosphere, the kite moving back and forth crossways to the wind so as to extract extra energy until the wind slows to below half the ordinary stall speed of the kites (at ever slower wind the kite can be kept aloft by energy from the ground, but does not acquire energy from aloft), c) various kites to move nearly crosswind and provide thrust in almost all directions to land or water vehicles, and d) making small versions of such kites serve as a propeller for vehicles which can go downwind or upwind faster than the wind speed.

Techniques With Coupled Devices

If we consider a kite on a string, we realize the kite can be aloft and interact with a stronger relative wind than is available were it to be affixed onto the surface of a slow vehicle. The kite can be used to tow a ground or water vehicle. The kite can also be guided to pull at an angle nearly 80 degrees from the relative wind at the kite altitude, and can provide a good force to the ground vehicle that is pulled with say 80 degrees of the kite line direction. Energy for the ground vehicle comes from the pull component in the direction of motion of the vehicle. Alternatively, the kite can extract energy by a propeller mounted on it. This decreases its performance, but provides an independent source of energy (which presumably is brought to the ground via two insulated wires that comprise the line to the kite).

An advantage of the kite is that it can be controlled to move fast: left and right, up and down. Its speed becomes much faster than the wind speed, and the power it derives can be far larger than with the motionless kite. To some extent it operates like the whirling blade of a wind turbine, its effectiveness relating to the swept area. Further, when the wind becomes somewhat too light to support the stationary kite, the moving to and fro can provide line tension and thus allow it to remain aloft.