AWE physics is powered aviation "run backward" using
well known aircraft flight dynamic modes to extract energy.
Sweeping Figure-Eights is but vigorous Dutch Roll. Phugoid
mode is the tug & furl pumping cycle idealized. Short Period
Instability is the membrane wing-mill's bread & butter. Looping
kites tap powerful Spiral/Spin Instabilities. These
are minimal energy geometries, basic Lissajous patterns, the
simplest harmonics of mixed pitch, roll, yaw, heave, surge, &
Change one motion anywhere in the system & the effects
propagate in odd ways to all the motions, exciting or dampening
adjacent or remote oscillators. Kites are thus revealed
as harmonic systems of linked cycles or oscillators dynamically
similar to orbital-mechanics, electronics, & life science.
Every part of a multi-body kite system is tunable, an
essential exploration path.
The aerospace aeroelasticity revolution is underway
& might just as well be called the aeroinertial revolution, for it
is the interaction of aerodynamic, elastic, & inertial forces that
determine flight. Controlled Aerodynamic Instability has
been identified by AWE watcher Cristiano Trein, et al , as
a prime wind energy mechanism. For useful oscillations merely
reverse Eddy's kite train formula of dancing diamond kites over stable
Hargrave box kites. Bondestam's passion for the dancing kite
is paying off; the dance is self-organized control suitable for AWE
Single-line kite dynamics long resisted adequate
explication. Ito & Komura, after years of
admirable focused study, admitted defeat . After thirty
years of trying Peter Lynn, the world's leading
all-around kite designer, still struggles to wrap his mind
around the subject. . Van Veen hit on a fundamental insight, that
kite flight is a quintessentially chaotic regime
. Windfields are chaotic, the kite itself is a fine
source of chaos, & such combinations are called
hyperchaos. Science maps deterministic structure in
chaos. Gran-pa's Statistical Mechanics & Nonlinear
Dynamics evolved into Dynamical Systems Theory illuminating
problems in many fields, including kite dynamics. As
formal numerical methods initially prove inadequate to predict
exotic combinatoric dynamics, empirical & heuristic tinkering
advances, as high art.
Small perturbations couple or uncouple critical system
oscillators in often unpredictable ways &
the system crashes. On the other hand tiny well-timed
& nicely calibrated tunings, "chaotic control", can greatly
enhance AWE system stability.
Close observation of kite dynamics reveals a
banding effect or interference pattern in many tuned
parameters. There are dimensions where you tune in stability,
keep tuning & lose it, continue & find a new stability,
& so on. A classic case is bridling up or down to improve
stability in changing wind. Some kite fliers do
one & some do the other as increasing wind detunes.
Both succeed albeit at different elevation angles. A suggestion
is to up-bridle a heavy kite & down-bridle a light one. As
wind lightens a heavy kite will need downbridling. A wide unswept
wing is particularly upsettable as large span
sections experience sudden major flow separation.
Secondary elements like kiteline & tail contribute
their own complex motions. The effect of a dynamic tether acts like an
added variable acceleration source. The sequential tensioning
of bridle & the roving CE/CP really makes things
dynamic. Simple analysis is confounded. A thick kiteline or
long tail usefully dampens higher frequency motion but a
rare unlucky helical wind input can still drive
the kite, dampeners & all, into a 1st mode spiral
wave that hooks the kite into the ground. Events in transparent
air are often mysterious & hard to troubleshoot.
A kite's yaw frequency at a given windspeed is limited by its
periphery mass & drag, as it flywheels against its pendulum
stability. Keelless flat kites dampen yaw with long tails
&/or a beard of fringe. Fighter kites use a rapid yaw
rate to select a course & a tug on the line pulls a keel into
shape & the kite tracks. Box kites & 3D kites generally have
highly dampened yaw.
In hand launching there is a moment when the short tether
roughly matches yaw frequency, the two oscillators
then interact as a double pendulum & chaotic
gyrations ensue. Jerky motion spikes the line's frequency.
Follow the kite fast & loose with your hand as
you play more line out and the bad tuning suddenly goes away.
This instability is commonly attributed to surface layer turbulence,
but the kite is still in the surface layer, considered to
extend to about 25m. The risk here is that a surface vortex will now
couple with the line's (lowering) frequency & cause a first
harmonic mode "death dive".
Once clear of the surface a kite settles down
into static or dynamic stability; either "pasted to the sky",
or a happy dancing or waggling flight. A pasted kite is more prone
to 1st mode harmonic coupling into a death hook. A dancing
kite may loop intermittently, but generally dances out of a dive.
Tails are a valuable tuning parameter. Despite the
anti-tail prejudice common in "serious" kiting Marc Rickets
developed a variable tail system & drogues are a major category of
kite gear. A quiver of tails allows adaptation of a single kite to
a wide wind range. Recent KiteLab experiments have shown the value of a
bit of mass between tail & kite. As the kite leans this mass droops
& curves the entire kite/tail combination into a
prompt recovery. Another tail trick is to put an elastic section
before the tail so that momentary slack never lets kite yaw get
too far before correction.
Key long-period non-mechanical AWE cycles
are easily overlooked in the building of a
state-machine. One must consider life cycles
& maintenance cycles: diurnal, work-shift, weather pattern
(frontal, seasonal) cycles; & so on. It helps to imagine
the kite in super fast motion.
A powerful method of taming a wild kite is to increase
its "capture factor", to constrain it with added lines that dampen
or prevent unwanted oscillation & state changes. Despite the
potential added lines as new failure points overall
reliability is enhanced One can even drive a persistently positive
Lyapunov Exponent (instability) negative (stability). Cross-linked
kites cancel out noise, rogue waves, single failures, & can
even move in phased synchrony by simple tunings. The most
powerful stabilizer is the ground surface, "free structure" excellent
for staking apart unruly kite elements.
Kitelines are a study in themselves. As a line is stretched its
internal "speed of sound" increases. Many neat effects are
possible. A set of thin high tension lines in parallel with the thick
primary load lines might enable real-time control input to
far higher altitudes than otherwise. The aeolian music one hears
in kite lines are a good source of state information.
Natural windfields are weird & complex. Turbulence, the
famous nightmare of classic physics, has at least
grown familiar. Each kite/wind situation is an
animated fingerprint of mass-energy in space-time. Kite & wind
interfere harmonically in "hypercomplexity". There are
few quality data sets of specific windfields, so make your own.
There is hardly understanding of what to do with such
data. For kites to become reliable the knowledge gap must end.
AT least one aspect of Awe harmonics is simple; desired power
out. Driving a generator is nicely done with a triple phased
mechanical input, as Loyd proposed . Just as with electrical supply.
single. double, or higher polyphase inputs of tensile kite energy
will find specific uses.
Kite Dynamics Vocabulary
A crude partial list, but the flavor is evident-
Stability, Instability, Astable, Polystable, Unstable, Unistable, Bistable
Harmonics- 1st mode, etc.; Complex Harmonics
Chaos- Attractor; Hyperchaos
Phugoid & Short Period Instability Dutch Roll/Lazy Eight
Oscillator- Dampened, Excited, Driven, Critically Dampened
Cycle, Limit Cycle, Stable Cycle
Phase, 3-Phase (linear to rotary without dead band)
Multivibrator (dynamics borrowed from electronics)
Stocks & Flows (dynamics borrowed from economics)
Pitch, Roll, Yaw, Heave, Surge, Sway
Aeroelasticity- Aerodynamic, Elastic, & Inertial interaction
Dynamic Systems Theory, Nonlinear Dynamics, Statistical Mechanics
 Kites: The Science and the Wonder, Ito and Komura
 The Tao of Kite-Flying, Harm van Veen,
 Loyd's AWE patent shows three-phase string power
 Martin Bondestam, the great Finnish kite visionary