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Optimal Crosswind Sweep Cycling

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 through them.

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 more robust.

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,

CoolIP                       ~Dave Santos             12Nov2010        M2525

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