How a sky serpent shaft can scale up?

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.

Neither do you, for Doug or SkySails. I have to go on my ship kite experience and witnessing and inspecting the Sky Serpent. I do think SkySails has the technical data your require, but Doug does not. Ask them, its your issue.

The top ST number here is what a 1000m shaft rated Wattage is estimated to be(?) That will determine a minimum loading to estimate what sort of shaft cross-section for the Base, in fiberglass or whatever is wanted. The top rotor will determine the loading there. The whole shaft will taper between these two numbers.

After specifying what the power output will be, to design from there we add in all the higher order factors, like safety margins, one-by-one. I have done this heuristically, analytically, and don’t see much power for a lot of cost, bother, and danger. Let anyone please prove me wrong.

The 1000m ST shaft would be akin to the triple the size of the greatest trees on earth, and be proportioned similarly. 1m diameter rotors will not not be optimal at 1000m. Across the wind gradient, its a complex problem what design each rotor should have. In modern aerospace, foil sections and AoA vary continuously across a platform. Aeroelasticity is critical complexity.

Nice setup! Can you share a bit more details? What swivels did you use? What exactly is the “rope” made of? Where do you think is friction?

My experiment tended to show that the rope does not transmit the torque well. The rotors rotate fast but the torque is not well transmitted. I measured roughly the same power with a multi-meter for almost ten rotors and for only one rotor, and using the same small motor-generator. At the end of the rope I used a swivel for kite like on https://www.amazon.com/EMMAKITES-Snapped-Connector-Parafoil-Inflatable/dp/B01C2UMXIC.

As the video shows it the tension of the rope prevents a lot the transmission of the torque.

@dougselsam uses a transmission by rope on


I asked him what was the transferred power, but without reply. Perhaps a special rope is used. But I think a semi-rigid or a rigid shaft is needed.

Are you literally using rope? That’s not going to work well.^^
Doug has a very small motor attached in the video.
I recommend switching to a semi-rigid system with a decent radius. Maybe put a few universay joints inbetween:


You could also try corrugated electrical conduit for a low torque torsion transfer.
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Yes, it was a rope. A rope doesn’t transmit much torque. Perhaps an elastic rope would be better. But I don’t think it would be enough for very long multi-rotors.

I had also a small motor I used for measures. But for the video the motor was not needed.

I would think since this is slightly borderline infeasible to begin with it doesnt make sense to test anything other than carbon fiber in well engineered dimensions and weave layup. Better to start with some calculations (like I did), then prototype something that might work.

I also did some tests with spinning tethers and swivels, seeing like you did that the amount of torsion propagated in a string is quite small

I suggested using this for @Rodread ‘s stuff but he advised against it, so probably a bad idea, but still here it is, again:

This stuff could act as a torque transferring tether if drag is of lesser concern. You could add fringes or a tail for lesser drag, and get the most advanced tether design ever (in a bad way) :slight_smile:

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