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
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
while hop-scotching across fixed groundgen fields.
~Dave Santos , 2010
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