|Earlier draft:||Later, February 28, 2010 version:|
The magic of a tether is how great
tensile force is transmissible between points with minimal mass and
most AWE————critically depend on tethers
to convey reaction between the surface and the wind flow-field.
(virtual particle) theory is an atomic-scale quantum explanation of how
kiteline "tug" transmits force over distance. Model a single bond in a
kiteline molecule can quantify how much tug is phonon transfer and how
much energy dissipates as convective and radiative energy for total
thermodynamic transmission efficiency. At macroscales most kiteline
behavior is better characterized by classical mechanics of waves in a
physical medium (stress-waves) although atomic properties still influence
critical behavior. In fact, phonons and stress waves are merely
different views of the same solid-state physics. Electron transmission,
the basis of many AWE schemes, is discussed downpage.
Electrical failure mode of a tether is important:
If the circuit opens, then the load gets spiked and the turbine might
overspeed. If the circuit is shorted, then the flygen will brake
suddenly, possibly burning out and breaking turbine blades. Conductive
tethers are considerable hazards around power lines and lightning. In
non-saline atmosphere, polymer tethers are not direct shock/short hazards
when downed on power lines and are not a major lightning risk.
|Tethers are like powerful magic
for transmitting great tensile force between points with minimal mass &
cost. AWE kites depend on tethers to convey Newtonian reaction
between the ground & wind. Most kiteline dynamics is well described by
classical mechanics of waves in a physical medium (stress-waves). Phonon
(virtual particle) theory is the atomic scale quantum explanation of
kiteline "tug". In fact phonons & stress waves are merely different views
of the same solid-state physics. Electron transfer is the added basis of
conductive AWE tethers.
A kite & its tether must dynamically adapt to wind, which is often chaotic. Kiters select the weight of a tether & set its length to fit along the windspeed gradient with altitude. A dancing kite feels tether-force as quasi acceleration, a dynamic virtual gravity much like a science fiction "tractor beam". Unlike gravity, tether-force is capricious, itself a chaotic source, with slack & jerk in odd directions. Tunable critical-damping of the kite's inputs across all conditions is a key to reliable flight. Tether tuning is the easiest most flexible adjustment.
Since aero-towing (powered kiting) began almost 100 years ago, glider pilots (& later hang-glider & paraglider pilots) have dreaded "lock-out", where control is mysteriously overwhelmed & the towed aircraft hooks & dives into the ground. Pilots learned to mostly avoid lock-out without well understanding the phenomenon. Towing Aloft, by Dennis Pagen, is a good reference for taming lock-out. Its taken a couple of years of further study to formally identify multiple causes for lock-out emerging from the complex harmonic interaction of wind, kite, & tether. An common example is when a short tether's harmonic period roughly matches a kite's yaw period; wild instability ensues, much as a double pendulum acts freaky. Previous posts have detailed the tether harmonic issue.
Modern polymer tethers are reliable when well chosen & cared for. Some sport fliers almost never replace line & get away with years of avid use. The key is to start with good line slightly over specified for conditions, trading reliability for a bit extra drag. KiteLab flys the same tethers for hundreds to thousands of hours to see how they age & only once in recent years has a tether parted unexpectedly, burnt out in electric hail. As Mario Milanese has observed, flying multilines is pretty much guaranteed to prevent runaway.
Conductive tethers suffer from higher mass & aerodrag than polymer tethers of equivalent power transmission, but may be favored on small scales & where mechanical transmission seems less practical. One overlooked problem with high altitude conductive tethers is enhanced corona discharge at lower atmospheric pressure. This limits transmission voltage or entails more insulation mass. Electrical failure modes of a tether matter: If the circuit opens the load is spiked & the turbine overspeeds; If shorted the load still sees an open circuit, but now the flygen will brake suddenly, possibly burning out &/or snapping turbine blades. Conductive tethers are considerable hazards around powerlines & lightning. Conductor heating can melt the polymer load bearing part of a tether. Conductor hazard mitigation is by such means as bypass conductors, fuses/cicuitbreakers, varisters, UPSs. etc. In non-saline conditions, polymer tethers are not direct shock/short hazards when downed on power lines & are not a major lightning risk.
The strongest fibers enable the thinnest & lightest tethers. Highest performers suffer a lower ROI if too pricey. Primary qualities are yield/breaking strength, elasticity, aerodrag, & mass. Other factors include UV/abrasion resistance & melting point. The best performing standard kiteline is Ultra High Molecular Weight PolyEthylene (UHMWPE; Dyneema/Spectra) which rivals in strength early carbon nanotube samples. Polyester & Nylon are cost-to-performance competitive with UHMWPE when stretch & thicker cross-section is allowable. Nylon is favored when some stretch is desirable but has low UV resistance. Elastomer, usually synthetic or natural rubber, is used for shock absorption & compliance. For terrain enabled applications, where weight limits are relaxed, wire rope (galvanized steel) is hard to beat. Its possible biomaterials like silk, hemp, linen, & cotton will find a place in advanced kiting due to aesthetic or environmental grounds. There are many curious tether interactions, for example, weaker cotton will saw, or rather melt, a stronger UHMWPE tether by greater friction & heat resistance.
Like tether weight, aerodrag fundamentally limits performance. Line rake is a great drag reduction mode, the more rake angle the more simple round line wins by unbeatable strength-to-drag. Crosswind tethers are high drag. No tether is truly worst-case crosswind as some catenary always rakes in. Angled upward downwind tethers generate downforce & must be longer for a given altitude. Angling a conductive tether upwind against a downwind angled polymer tether may help flygen applications. Faired tethers is a well known & obvious idea. Good data has existed for nearly fifty years since MIT first did experiments. Faired tethers do have considerably less drag but the balsa TEs MIT tested don't survive normal usage & suitable material hardly exists to this day. Line twist & strumming drag is uncontrollable without weight & complexity penalties. Faired line entails handling & wear issues with pulleys, fairleads, & reels. Round line of the same strength has less drag than ribbon sectioned line due to lessened strum. Twisted fibers with a fine enough texture have an aero advantage, by shedding strum canceling vortices & acting like golf ball texture, postponing detached flow.
A graded tether made up of stronger lower sections & thinner upper sections, often with swivels in between, can out-perform a long monotether. Assembling a tether in sections with hardware is classic art from the Victorian Era. High wear sections are sleeved or overspecified without much added weight. Multiple kites often promptly saw each others lines if allowed to cross. A multi-tether is runaway resistant, but many lines adds snag risk & other operational challenges. A kitefield is best purged of all snags. Knots are well known weak points in a line. The basic cause of weakness is looping line under stress around a tight radius. Nicks & abrasion are far more common failure modes. One can usefully assume a common knot to be within a tolerance for replacing worn line. Previous posts have detailed the rigging & flying of complex train, arch, & mesh tether/kite arrays.
Dipping booms (think fishing-pole) & elastic sections ("snubbers") absorb peak loads to maximize tether reliability & performance. One use of a snubber is at a pilot-lifter bridle to insulate it from yanking by a power element down-line, like a looping foil.
An inclined tether that slacks suddenly at the kite (usual cause- a reverse eddy "pocket") creates a transverse wave that moves quickly downward (faster & at higher amplitude the more massy the tether). As such a wave races groundward its hard to retract fast enough to prevent tether touchdown. KiteLab often observes this experimentally, most dramatically in heavier lines like electrical conductors. Call it Sudden Tether Sag (STS). An engineer at HAWP09 discounted STS & particularly questioned the massy faster sag observation invoking Galileo, to wit, a heavy tether must sag as fast as a light one. A less elastic & less draggy (by strength growing faster than cross-section) tether sags a bit faster, but this is not the main sudden sag effect. Maybe its that a heavy tether has more self-tension & thus a higher internal "speed of sound". Dave Lang, AWE's tether guru, has seen similar sag effects in his simulations. STS is a major challenge in large high-altitude flygen schemes, as McNaughten & Co. suggest.
Power kite tethers are quite hazardous. They lasso & pull aloft kiters & drop them as did Osborne's Monster to Eideken. When that tether parted it snapped back like a cannon shot & would have killed more had not the human ants at the anchor point scattered. A moving AWE tether can suck you into machinery. Thin tethers easily cut flesh & dejoint like a sword, where a thicker line might only burn. Joe Hadzicki taught me at NABX that when all hell breaks loose & killer kiteline is slithering or whistling about, you should crouch slightly, chin tucked, cupping hands over ears with elbows tucked. Thus poised jump-rope nimbly over any ground sweeping lines, a hazard Peter Lynn describes well. The idea is to protect form garroting the throat, jugular, & all other major arteries & veins; dangly bits & ears too, since having them sewn back on is a hassle. Better for a line to saw at bone than slice right thru a joint. To handle fast moving tethers under high tension; wear gloves with gauntlets, carry a hook knife, dress to minimize snags, & carry an organ donor card.
Thanks to Dave Lang for having answered many tether questions over time & for helpful input to this overview.