KiteLab Ilwaco proposes that optimal crosswind sweep cycles of either
loops or figure-of-eights will generally follow principles noted below:
Nearly constant tension can be maintained throughout a sweep cycle
to minimize cyclic airframe fatigue.
Time spent in low-power phases of sweep is best minimized by speeding
For tether, kite, and load to approach steady-state loading, electrical
capacitance or mechanical buffering of the dynamic bidirectional forces,
at the ground interface, is useful.
Climb and zenith sweep phases can maintain kite speed and tether tension
by energy-return from winch retract, motoring, or tuned elasticity.
Elasticity nicely protects kite and tether from tensile spikes, and
then returns the energy.
High-Q elastic return is far less lossey than flygen motoring
energy return. A carbon or spring-steel boom, acting just like a
fishing-rod, is the lowest-loss solution.
Flight is sustainable by sweep in no-wind by "tow" during climb phase and
"play-out" during dive phase. One can actually fly a kite into modest
head-wind by aggressive powered-sweep.
Figure-of-eight and looping are roughly comparable-power modes, but with
major design and operational trades. Active-control looping is the most
risky aerobatic. Passive looping under a pilot lifter is far
In autonomous control, figure-of-eights are advantaged over loops by the
smooth state transition to-and-from "parked" flight. In the
passive-control case its just a matter of tuning in any desired amount of
Dutch-Roll oscillation. Looping is an all-or-nothing state for an
autopilot, with an abrupt qualitative change from parked. A passive
advantage is that a looping kite will naturally start and run lifted by a
pilot, and naturally stop before a soft landing in a lull.
Cyclic surge loads are not completely avoidable in a real-world system.
Soft structure and polymer tether will tend to tolerate lifetime cyclic
loading better than composite wings and complex conducting tethers,
~Dave Santos 12Nov2010
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