CoolIP index                                                          Most recent edit: Wednesday October 24, 2012

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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 dispersing energy.

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.

CoolIP*                      ~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      23March2012

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