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LAUNCH and RETRIEVAL

Kites may be more difficult to deploy and retrieve than conventional sails, but not as difficult as carrier based aircraft. One alternative is to keep the kite aloft between towing jobs by using a small engine. The cost of keeping a UAV aloft in still air or while in port may be less than the cost of launch and retrieval onboard the ship. Kite sail systems based on Condor technology might stay aloft for periods up to a year, landing only for maintenance. The tether would be disconnected from the ship as the ship steamed into port, then dropped to the deck of another ship leaving port for another towing job at sea. Another technique32 may be to make the kite lighter than air by inflating all or part of it with helium or hydrogen.

FLIGHT CONTROL

As mentioned by Duckworth and others, the task of controlling a high performance kite can be daunting. However, skilled oriental kite flyers have developed techniques for controlling unstable single line kites that would boggle the minds of most kite sail critics. Simply stated, the technique is to take in line when the kite is headed in the right direction, and to pay out line when it is not. This is a technique that must be seen to be believed. Most of us are so convinced of the virtue of multiple line stunt kites that we cannot conceive of a single line kite that might be even faster and more maneuverable.

A related technique is used by two line stunt kite experts during periods of light air in competition. By pumping energy into the system by moving both hands together, the kite can be kept aloft in below minimum wind conditions. Both of these techniques could be used by commercial kite sailors during periods of light air, but radio control techniques based on modern model and UAV technology probably hold more promise.

While prior research26 has shown that "application of parachute kites to large ships of the BP fleet is uneconomic," the possibility is left open. "Ram air wings should be considered as their increased complexity and cost might be offset by increased thrust and greater utilization than parachute sails." These conclusions are equally valid today as they were in 1985, although we would add the possibility that low cost, automatic flight controls derived from modern UAV technology might further increase the thrust and utilization, thereby improving the economics of the system. We showed in Fig 6 how flying patterns in the sky will improve wind power extraction on most points of sail by up to an order of magnitude. The price we pay for this increased performance is "increased complexity," including the need for sensors, processors, and servo controls.

Automatic flight control has become a way of life for large segments of the aviation community, and the cost is not always high. The Rutan Voyager could not have been piloted around the world on a single tank of gas without an autopilot to relieve the workload on the pilots. These general aviation autopilots use signals from flight instruments to maintain altitude and heading. Then in 1989 a remarkable new product became available, a full performance autopilot for model airplanes. This $300 electronic device uses static and dynamic pressure and a magnetic compass to maintain altitude and heading through elevator, rudder, aileron, and throttle servos. A similar device could be used to control a high performance kite during long ocean passages.

We found in 199248 that kites with L/D above 20 could be controlled by adding rate gyros and servo controls. Then in 199339 we showed how the autopilot and stability augmentation could be combined to provide completely autonomous flight operation, including navigation, for days and even weeks at a cost less than $3500. Both of these flight demonstrations were carried out at model scale with a wing span less than 10 feet and max. wing lift below 100 lbs. There is no reason the flight control task would be more difficult for much larger wings, and the cost may be even less if the ship's captain retains the job of navigation.

The proposed flight control system would include a rate gyro, pitot, echo altimeter, computer, data link, rudder, and elevator servos. The computer would also need data from the ship's wind speed and direction instruments in order to optimize the wind energy extracted. Tow line angle and force might also be useful, but the key will be development of the software that will keep the kite safely above the wave tops while flying patterns in the sky to maintain optimum performance. The complete system might include a weather station and some degree of control of the ship's rudder and engine to optimize the economics of the entire system.

Modern aircraft use over a million lines of code to handle flight control and flight deck status display for the flight crew. Much of this is devoted to failure status of the various systems. The cost of development of similar code will dominate early commercial kite sail systems, but it would become reasonable once the basic parameters are developed through fleet experience.

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