See also for this topic note:  Latest 3r Variation: Legacy Power Plant Reuse  

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Often overlooked:

• - Greater area of a lower L/D wing will beat higher L/D in max-power, by equivalent mass.
• - Low wing-loading of a cruder wing boosts L/D, while max Cl of a high L/D wing drastically lowers its working L/D.
• - High L/D wings are not crashworthy. High-mass-density and high-velocity are less safe.
• - Tether drag most harms high L/D performance. Thinner tethers with less safety margin are trap.
• - Max scaling goes to highest power-to-mass of single-skin.
• - The performance limit is pure polymer kept at working load, not a hotter wing with mass-parasitic resin and avionics.
• - There is no existence proof of a practical high L/D kite (>10).
• - Lower L/D has comparable or better turning rate, for better airspace utilization.
• - Most probable wind velocity is low, while a high L/D wing min velocity is high. Very easy to stall, harder to launch, etc.
• - Low L/D ship kites have already shown ~2MW unit-power. High L/D wing data not impressive, before game-over crashing.

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July 9, 2020, Max Langbein wrote:
- however,  L/D should be high enough. as power goes approx L³/D² rho A v².  A first simplified UFO sail I simulated only achieved max 1.4 as L/D, (2.4 L³/D²) while a simple paraboloid triangle achieved up to 3.8 L/D  and up to 4 L³/D² (surprisingly not much worse than a simple leading-edge-inflatable parafoil).
Of course, these were very crude simulations with no intense error-checking, so the reality will be different.


=======July 9, 2020, post by Dave Santos
= July 10, 2020, post by Dave Santos

Some L/D numbers to inform estimation:

• Classic NPW best NASA-measured models- 3.2
• Frisbee Disc- ~5
• Aerobie Flying Ring- >10
• Advanced SS paragliders- ~9

I looked at paraglider data, and 9 is low-end, with reviewer claims that new SS wings are equivalent performers now. The flying disc/ring cases are hard to nail down since they draw on kinetic energy until a final glide slope. Ring wing well known to have higher L/D than disc wing. Vortex Lift is another complication for these low AR planforms.

L/D5 seems like a reasonable rounded estimate for a flattened SS ~ring-wing (center hole), lacking measured data. That's plenty for effective crosswind load motion, as the NPW is quite good crosswind at only 3.2. Crosswind 3r does not depend on close upwind progress, and ~L/D10 seems eventually feasible by matured optimization

Min Sink Rate is critical for kites, and a low L/D wing has a sink-rate closely comparable to the highest L/D gliders. Lowest Min Velocity is desirable for zero-point flight energy station-keeping (no free-energy for generation). Sweeping to maintain flight is called "clawing"; very vulnerable to turbulence ("wind "pockets").

High L/D kites also are vulnerable to Over-flying Zenith and snubbing-up on-tether; very troublesome states. Low L/D kites cannot easily overfly zenith, nor snub-up violently.

Survival-to-payback is another key factor. Cheap soft wings pay back in weeks, but fine wings need ~5years (Makani estimate), yet currently crash "totaled" in hours or days. SS kites are uniquely crashworthy:
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== July 10, 2020, post by Max Langbein
Thank you for your detailed arguments.
Yes, you convinced me that (in the area of AWE) a minimal L/D is sufficient.

In the single-line setups it is quite obvious, as the crosswind movement only serves to increase drag in the pull-out phase, which can be also achieved by just a higher area of the sail then having to cope with less area load making it lighter, however pure drag operation is with < 4/27 far off the Betz limit of 2/3 of the kinetic energy flowing through a disk-shaped area being harvested (which modern wind turbines almost reach, of course, causing much higher turbulences and larger vortices than kite networks).

For 3r network setups working in periodic operation one needs a minimal crosswind movement, so an L/D of at least two may be needed.

An argument *for* higher L/D may be the "visual clutter" of the power-sail network covering a large part of the sky and by that decreasing public acceptance.
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==July 20, 2020, post by Dave Santos
Thanks, Max, for reviewing the "detailed arguments". Joe's AWES forum has reached around 30k postings, so virtually all the major issues questions have been exhaustively explored. A few more chestnuts-

There is no evidence anyone is offended by the appearance of kites in dense formations at kite festivals or kite sport competitions. Most observers instead find kite spectacles picturesque, even fascinating. "Painting the sky" with kites is even a serious art form. Its been theorized that wind farms are most offensive visually due to permanent transformation of natural landscapes, especially ridgelines. Kite advocates argue for light shifting ephemeral visual impact, until perhaps fusion comes along. Kite formations passively decommission compared to wind towers. This is just a summary overview of years of detailed discussions.

High L/D kiteplanes and turbines by their high-velocity mass are tremendously destructive of birds and bats; a far more significant and definite negative than slow ponderous low L/D kites, known not to kill birds and bats. This is a tremendous ecological advantage of low L/D kites over all other wind tech.

Betz has been put to rest by its unrealistic disc-model assumption poorly applying to kites. In fact, hobby-version Archimedes Screw kites can beat Betz by arbitrarily entraining more kinetic energy along their long axis than their frontal projected disc sees. Further, Betz himself was not even the originator of the century-old (obsolete) disc model, and was an over-promoted National Socialist. The sky is so vast, Wubbo Ockels calculated you could power EU by the area behind your extended hand held up above the horizon. Berlin could be powered by a frontal area barely larger than 100m wide by 10km tall. By the established dictates of HTA flight, power-to-mass of a kite is the greater figure of merit.

Thanks for thinking boldly. There are two important limits at present, the practical unit-scale of 2000m2 per kite (1000m2 would be handier), and the near-term FAA/ICAO regulatory allowance of 2000ft service ceiling, permitting AWE as a simple obstruction rather than the full impact of FARs (aircraft regulations). Many-connected topological stability is huge 3r safety advantage the starting paper did not identify. There is really nothing to stop this approach from fast tremendous progress.