CoolIP index           

Spanning Atmospheric Waves
with Tethered Foils

This post continues examination of long horizontal trains of dual-mode (towed/towing) AWE kiteplane gliders. Cross-country transport was one application identified. Virtues and methods of vertical lines were recently explored and in a similar spirit one sees unique properties of horizontal tensile structure and motion. Horizontal is the general direction of both wind and human transportation, but is orthogonal to gravity. A tether run axial to apparent wind develops minimal drag of any tether orientation, and run crosswind, maximal drag.

An interesting possibility with horizontal glider trains is maintaining flight across wind "deserts", areas of calm that would otherwise force grounding. The basic idea is to transfer energy from windy to calm zones as needed by kite-elevated long-lines. Dale Kramer tells us that a high performance glider can be towed with just 25 kilos of tug (at about 120 km/h). This is a baseline model for long polymer train tethers with high L/D gliders spaced every few kilometers. The toy kite train that reached the stratosphere gives us a rough proportion for tether to kiteplane. Presume the toy kite model to be about 1/20th scale, so full-sized gliders flying about 4 km will be spaced about 8 km apart to carry the tether well off the ground. These are crude estimates and hopefully the ignored dimensions mostly cancel.

Wind blows in a spectrum of wavelengths by combined mechanisms. Small waves are everywhere and the immediate challenge for early AWE systems is to gracefully persist through common lulls between wind wave peaks, without constant forced motoring or landing and relaunching. The lightest kites are clearly favored. The biggest wavelengths are planetary waves and the lulls between peaks are typically thousands of kilometers across and commonly take several days to pass overhead or even park for a season as persistent high-pressure zones.

Its looks marginally feasible to span moving planetary calm zones with horizontal long-lines of towed tethered foils. One hundred gliders spaced at 8 km would span 800 km, with a towing force requirement of 2500 kilos, plus tether drag--which we can guess will approximate or well exceed 50% of total drag before scale-limiting. The largest continental summer highs are a tough challenge for AWE and are generally better suited for solar-power. Another post will consider migrating "clouds" of tethered foils, as enabled by horizontal trains to follow wandering jet streams, while hop-scotching across fixed groundgen fields.

CoolIP                       ~Dave Santos             , 2010           M2558

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