Short-Stroke Pumping Notes
Bob is right about how easy it is to include a
high-wear section in a short-stroke tether, so that groundgen tether
fatigue is not an issue. This can be just a thicker section of tether
replaced periodically. Bob's metal tape idea is an option, but there are
many others that may be better to grip a drive wheel of small radius. A
tooth belt or polymer webbing is good. Chain is pretty tough and can key
into a chain-wheel; the weight of a short chain section would not matter
much. Wire drive rope is tough in this sort of job. Overall, the polymer
webbing belt seems a good trade-off of availability, cost, and simplicity.
As for the best short-stroke pumping cycle, a normal
sawtooth wave, with a brief recovery phase after each ramp-up, would
be pretty efficient (so long as the sudden slacking does not go to far),
but so is a sine wave with elastic recovery. Either waveform might be
advantaged in a particular design, but an essential precondition seems to
be a minimum base tension on the tether. Too loose a tether with too wild
tugging cycles and the tether flops around in the air in transverse waves
There are complexities to bend the mind. Tensioned UHMWPE has a speed of
sound (stroking is acoustic) of over 10 km per second (comparable to
diamond Mach) that drops to zero with no tension. So the pumping cycle has
this fantastic variation in properties going on. The tether length of
course also determines the fundamental harmonics. The big open question is
how to find the optimal frequency and amplitude of pumping, and whether a
practical kite can output that signal. Presumably a suitable ground
transmission exists to condition most any input to the desired output.
Kicking a flywheel mass will be a common method to smooth output, but even
a section of twisted rope might do as a filter.
One experiment I intend is to connect a UHMWPE line to a speaker linear
motor to drive a remote speaker cone and send varied acoustic signals down
the line to hear how they attenuate and distort. This should give nice
clues as to how best to tune short-stroke AWES.
~Dave Santos 23March2012 AWES5875
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Metal tape is 99% efficient as a belt drive, so it would be a good
candidate. It might be clipped to the tether at intermediate points in
certain weather patterns. ~ Bob Stuart