# How a sky serpent shaft can scale up?

I think, like I said earlier, that that does not hold for thin-walled structures.

For a thin-walled structure, most of the extra volume is air, which is of course neutrally buoyant in air.

Related, from the YouTube comments under the video I linked earlier in this thread:

I took that into account. When scaling linearily, the torque capacity increases more than linearily. Still all in all its not all rosy and not impossible

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As blade material scaling is also discussed, here is a link of a manufacturer of blades for wind turbines:

ATV 03m00 http://atv.tm.free.fr/FT_ATV_03m00.pdf compared to ATV 28m30 http://atv.tm.free.fr/FT_ATV_28m30.pdf seems indicate that the mass of these blades scales lower than a cubed rate.

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Searching for “turbine blade size logarithmic” gives this for example, which I think might also be relevant for a drive shaft:

Upscaling_Wind_Turbines.pdf (1.1 MB)

See: 2. THEORETICAL UPSCALING

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IMHO a blade could scale like a drive shaft for its hollow part towards the root.

I experimented about ten 0.2 m diameter rotors with a 5 m rope as axis. The rope transmitted only about 1/10 power, because the tension prevents rotation as shown on

And when the rope is not stretched the torque is also not well transmitted.

So a special rope which can fully rotate by using small torque, or a rigid or semi-rigid shaft is needed. If a one km long and light shaft can rotate 1 m diameter rotors without too much loss, SuperTurbine ™ could scale in any dimensions by the high number of rotors.
Scaling in numbers rather than size looks to be an option for how a sky serpent shaft can scale up.

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A lot of the rotation resistance is dynamic bending forces as the shaft is continuously re-bent during rotation.

The simplest way to test AWES scaling performance is to build a larger prototype of the same type and measure the loss of power-to-weight performance. The projected scaling loss between those two data points is roughly linear for the next up-scaling loss. Many teams up-scaling efforts reveal this. Makani’s M5 5MW concept died quietly as they learned the M600 was already too big for its premature architectural down-select.

KiteLab Group and kPower have long used many such “quick and dirty” empirical methods at small scales for insight into AWES principles. Compare such practice against narrow numeric calculations based on poor heuristics prevailing in other circles (GIGO effect).

If a one kilometer thin shaft (not a current rope but a special rope or semi-rigid shaft) can rotate multi 1 m diameter rotors without too much loss, that by being stretched, Serpentine could be a winner.

The problem is that torque sums toward the bottom. The top of the shaft is lightest-loaded and the bottom is totally loaded.

The high-stretching means more kite lift. There are more efficient ways to use kite lift for power than to make the ST workable.

Low angular velocity is also bad for shaft performance, and many other critical drive-shaft and ST rotor problems documented on the Old Forum, like safety for example, or how the 1km shaft is to be handled on the ground and launched and landed. Cost and lifecycle issues count as well.

It should be tested at larger scale to remove any doubts.

A long shaft with 1 m rotors leads to a very high angular velocity. And also the lift of tilted rotors compensates the loads. The main question is the material for the long shaft.

Which ones?

Carbon fiber composite is the current standard for high-performance drive-shafts. Expensive and brittle.

Greater than 10,000 rpm is high angular velocity for high power-to-weight. A large ST shaft might turn at ~200 rpm and only be comparatively very weak.

Pure power kite lift is a better by power-to-weight, if one accepts the great engineer Payne was right about the potential to convert traction to rotation in USP3987987. Classic rope-driving is industrially proven and lightest weight mechanical transmission over distance. Long drive shafts are necessarily massive.

Fiber glass shaft with high tension could work. And as I explain some R&D could be done to find the appropriate material for the long shaft.

I spoke about 1 m diameter rotors as I indicated several times. The tip speed being about 60 m/s (65 m/s for HAWT), the rpm is 1200, not 200. And 1200 rpm is a high angular value in wind energy industry.

Pure power kites make lift, force, but don’t rotate. Without any rotating device, reel-out/in method (yoyo) is used. Several years after R&D and funding there is none workable unity.

There are long thin ropes. It could be the same for drive shafts.

And also the great engineer and expert Douglas Selsam is among us and can advice you.

As Wubbo suggested, make any kind of AWES you want, even if its not most economic.

1m diameter rotors are pretty small and the blades should be hinged for decent autogyro performance. Surface based HAWTs are not a high standard of shaft efficiency, but fortunately their heavy drive shafts are very short and sit on towers.

Power kites are ahead of STs in turning a generator. Compare Doug’s best effort with SkySails’ proven 100kW land unit, and 200kW unit upcoming. This is more real than ST progress.

Vestas engineers are the great HAWT engineers of our time, but only with tower-based turbines. No, Doug cannot advise anyone how to scale up an ST economically; he is no aerospace expert. Where is his large shaft? He has not developed it. Go ahead and take Doug’s advice, and I will follow Payne and the ship-kite power-kite, like Wubbo suggests we all do, to each of us follow our dreams.

Good luck following Doug’s advice. AWEurope should fund you.

Dave, keep in mind that Wubbo was famous as an astronaut, but also known with Laddermill of which the principle was already registred by Doug. So if you follow Wubbo, you also follow Doug.

100 kW? Have you a link towards reliable measures for a sufficiently long time?

Doug is our master of all and an inspiration: Daisy, Rotating Reel, even recently skewed HAWT. All roads come from SuperTurbine ™. You have much to learn from him.

Both Wubbo and Doug rejected the LadderMill, and Wubbo went on to the SpiderMill. Its a logical fallacy to think Wubbo must be an ST advocate, or to blame Doug or Wubbo for the LadderMIll false-start.

SkySails has done years at sea. Ask Doug to show “reliable measures for a sufficiently long time” better than SkySails.

It likely won’t be me learning from Doug. You would have to be very specific about the knowledge he has added to AWE. I can answer about ST in relation to aviation scaling laws. Doug cannot. If Doug has taught you something great, then go forth and triumph.

Wubbo beat 8000 other Dutch competitors to become an astronaut, not someone who sold speakers from his van. I wish you had known Wubbo in person. I met Doug at HAWPcon09. He is not on my long list of kite teachers, for lack of being able to say what he has ever taught that I can use. Wubbo taught me a lot. I owe him deeply.

Dave, SpiderMill is seen as an improvement of Laddermill. So rejecting Laddermill is a nonsense when SpiderMill is chosen.

So you have not any reliable curve for the 100 kW kite system.

To be specific, the AWE knowledge Doug brought is summarized into a single word: Watt (and not “what?” as you suggest).

You will progress by listening his teaching.

No, ask any engineer, the SpiderMill is a different principle of operation. Nor is the ST “an improvement of Laddermill”. Do you think Doug likes the SpiderMill? I do.

I trust the size of the SkySails ship-kite to have a power curve that beats Doug. If I had not worked for KiteShip and done Mothra, at 300m2, I would not be confident. You and Doug do not have this background, so go ahead and doubt.

I do listen to Doug. Test me on any of his “teaching”. Even he admits “All roads…” is more a commercial slogan than engineering knowledge. I don’t think its true. Quote him technically if you can.

I am not sure any engineer even knows the SpiderMill as it is not well documented. Both Laddermill and SpiderMill have a common principle of operation. SpiderMill adds a crosswind component.

So you have not any reliable curve for the 100 kW kite system.

You could try to understand what (watt) is behind the words like “All roads…”.

Tell them the Spidermill is not a loop of kites but a train of kites pulling the line into a zig-zag. I got it from Wubbo himself. Do not just guess they are the same principle-of-action.

I think SkySails has a more reliable curve than Doug’s Sky Serpent, which I saw in action. Ask them both if you are doubtful. I am a third party.

Everybody knows making power is the challenge. Saying “Watt” as one word is your idea of great, not mine. I would say “Terra-Watt”, in answer to Doug, and in relation to the world need, but that’s still just one word, not mastery.

“Lift” as JoeF says. AWE is flight.

So you have not any reliable curve for the 100 kW kite system.