Death to "Soft vs rigid"

[tallakt]

I find the distinction soft vs rigid kites too polarized and it is taking nuances out of discussions about AWE. I hope it could end.

I can see that in the early days of AWE, one would like to take the camp of one of the extreme designs, either completely soft or completely rigid. It is not unlikely that one of these will end up being the most used technology for AWE.

I would like this thread to celebrate all the design options in between these two extremes. When building a new kite, we have old ideas and new ideas as our building blocks. I won’t go into details about the pros and cons about each design, but I hope it will be clear that kite/wing/blade design for AWE is a lot deeper than just soft vs rigid.

This is one of my favorite kites, the Peter Lynn Single Skin Single Line. Peter Lynn is devoting a lot of his time to making this kite fly reliably. It does not leave a lot of design room for other stuff though, as just making it is current state of the art wrt complexity. This kite has a bridle and only one skin. Definitely a soft kite in any understanding of the term.

The second kite is also a soft kite, but this time with two layers upper and bottom skin to create a complete airfoil. This opens up design possibilities but adds to the weight. These kites will also maintain their shape a lot better in a lull.

The third kite is a NASA wing derived kite, which has only one skin, and no bridle. Rather the top skin provides forces necessary to keep the kite open.

The fourth kite is a single skin kite in a sense, with thin spars to keep it from collapsing. The spars also provide depowering ability.

The fifth kite is the Wipika Classic, a kite featuring a single top skin, and an frame to provide an accurate and lightweight shape.

The seventh image is not a kite rather a sailboat. The main sails are supported by a stiff beam (mast) while the spinnaker is just supported by three lines. Notice stiff members at the trailing edge of the sail to (probably) reduce flutter.

The next image is a windsurfing sail which has a lot in common with the sailboat. There is a single stiff mast, but depower is arranged by spars that “give” in high winds.

The ninth image is a model airplane that has a wooden support structure covered by a thin plastic film. This is probably a transition to fully “rigid” wings, as the skin does not deform much during flight.

The tenth image comes from Makani. They have a fully composite wing. To support the large wingspan they have added a Y shaped fixed bridle. This kite is not free to rotate in roll direction, only pitch and yaw.

The eleventh image is an Ampyx prototype. It only has a single attachment point for the tether. So it may rotate in roll, pitch and yaw. This has the benefit that the orientation is towards the airflow rather than the tether. The tether will in steady state have a predefined angle to the wind, but not in transient response.

The twelth image is a rotating blade. I would probably say this is a multiwing design. Definitively the rotating part would be a rigid kite, though few similarities with the Makani kite. This one has a soft lifter kite, but this one may in theory be removed (not that there is a good reason to).

The thirteenth image is a KPS rendering of a rigid kite. The wing has more bridles than Makani, and also “anti-bridles” (lacking a better word) to support the bridle while still being able to have a single attachment point for all three free axes. Personally I’d like to credit KPS for the design that fuses the rigid wing with the bridle of the soft kite, bringing to some degree “the best of both worlds”. I have no idea if it works though.

(Thanks @Tom for the pointer to KPS, I had forgotten where I saw that image)

(My normal disclaimer - these are my personal views, even though I am currently working at Kitemill I am not speaking on behalf of Kitemill here. This is also why I have left Kitemill out of this)

[tallakt]

Now, to bring the discussion forward: I gather a wing is a device that utilized relative airspeed over it to provide a lift force (useful) and a drag force (losses, to be avoided). The ratio of losses to generated power is given by the glide numbe G_e, taken at the operating point when the energy kite is producing full power or producing at cut-in.

We will assume that the kite may generate the vertical force throughout the flying pattern (not true, but a good first approximation).

Next it is plain to see that a kite flying overhead may more easily translate lift force to keeping afloat, while a kite flying in the power zone has an “opposite cosine loss” with regard to keeping the kite afloat. Still, if the kite is generating a large tether force, the vertical component needed to stay afloat is mostly quite small in relative terms. Flying horizontally in the power window, the change in roll to maintain constant altitude is likely quite low. Lets call the weight force G and the lift of the wing L, the tether tension T.

A simple approximation of the usable tether force after accounting the loss because of G is (we are assuming production downwind, and L \approx T, then applying pythagoras):

T’ = T_0 \sqrt{1 - \frac{G^2}{T_0^2}} = T_0 K_G

T_0 is tension on the tether for the reference case with zero mass.

Since the reduction in tether force may be seen as similar to a reduction on the lift coefficient of the wing, we see that:

C_l' = K_G C_l

Again, since genreated maximum power is (with x \propto y meaning x is proportional to y):

P \propto C_l'^3 \Rightarrow P \propto K_G^3 C_l^3

So, for cut-in, I’ll assume that the for a fully rigid wing we have:

• G_e = 10 (must include tether drag in addition to kite drag)
• G = 1000\, \mathrm{N} (approx 100 kg)
• T = 5000\, \mathrm{N} (approx 500 kg, assumed around 5 m/s)
• \frac{G}{T} = 0.2
• A = 5\,\mathrm{m}^2

The loss of power due to mass is approx 11.5% (K_G^3 \approx 0.885).

Next consider a single skin kite generating the approximate same power. To compensate for the much lower efficiency, the area must be increased by a factor of 16 (as power P \propto G_e^2)

• G_e = 2.5
• G = 150\, \mathrm{N} (approx 15 kg)
• T = 5000\, \mathrm{N} (approx 500 kg)
• \frac{G}{T} = 0.03
• A = 80\,\mathrm{m}^2

The loss of power is approx 0.27% (K_G^3 \approx 0.9973).

So with these numbers it seems rational to choose either solution. Increasing G_e seems important as is reducing mass. But even with 11.5% losses to mass, it may be preferable operational wise to have a fully rigid wing relative to a much larger kite. Increasing the wing area by 11.5% should compensate for the loss due to masses.

[tallakt]

Theres always something to be learnt by speaking with one’s peers. Even when it’s kind of a monologue like in this case.

In this case I was triggered to go a bit deeper understanding that the soft vs rigid debate still (in my understanding) is a fully open question. And I still think its not the most useful thing to discuss

[dougselsam]

Well I would think it’s a perfectly valid topic for AWE, but to approach it like some sort of religion, something to “prove”, insisting people “believe in” soft versus hard is just annoying. He’s certainly just beaten the topic to death for many years now. But from a wind energy standpoint, most everything he says is usually wrong. Sometimes he backs off the single-skin theme, and goes hog-wild defending fabric use in general. People will mention UV and wear, and he will point out all the planes made that way sitting on the tarmac. Blah blah blah, it’s like a script - or maybe a repeating bad dream that makes no sense - when you have a fever. For a few years, he kept referring to a highly-discussed supposed GE grant-funded project to make blades like a piper cub - rigid frame covered with fabric. Knowing a bit about how these things work, I told him it was never even going to happen, let alone be successful. Meanwhile, I happened to have had a conversation with the Director of Research at GE, at a wind energy trade show in Las Vegas, just a few hours across the desert from here, an easy drive. We talked about a lot of things. When I asked him about the fabric-covered blade effort, he was quite dismissive. It was just some girls that had managed to win a grant. On paper. He didn’t seem to think one would ever get built. I would guess more effort went into writing articles about the phantom project than ever went into the project itself. But my actual conversation with the director of research at GE meant nothing to Santos. His “off-the-cuff-opinion” was far more valuable than my actual information, which was completely ignored, as he continued to “promote” this abandoned project for years anyway. In his mind, fantasy rules. Facts are to be ignored. The key question is, hard or soft, do you have a way to turn it into electricity, as in, spin a generator? The problem with discussing stuff like this is, it’s like the 3 blind men trying to reverse-engineer the elephant, but without having an actual elephant there to touch. How does anyone know what could turn out to be an answer without seeing it in operation? Now this guy has quite a few of these themes he likes to periodically harp on, but if you ask him HOW to use any of the things he keep yelling about, he can’t tell you. He just cannot seem to do it. He just likes to fly kites. He can easily obtain soft kites, as anyone can. He’d have to make his own hard kites, but mostly he just likes to fly kites, and post misleading, intimidating, often provably false crapola on the internet. You guys weren’t around one fine summer when he kept saying he was hosting an AWE-powered concert - with all the details, even claiming to have the band lined up. Except it never happened. He missed the little step of having AWE to power anything. Doesn’t matter, you can’t get him to even admit it. But one must wonder what mental process could lead someone to go into so much detail about an “AWE-powered” concert, since the whole idea would be to demo your AWE system, while having no AWE system. It never made any sense at all. Completely illogical. But see, that’s a simple situation that any person can flag as complete nonsense. What you non-wind energy people don’t realize is, if you HAVE a good working knowledge of wind energy, almost NOTHING he says makes any sense. To me it’s the ravings of a madman trying to see how annoying he can be. I guess it must be entertaining or something because we all keep putting up with it. Maybe it’s just comforting to know there is someone you can always win an argument with. Except he can never understand that he loses his arguments. He carries on as though he’s proven his point and if worst comes to worst he can usually get old JoeF to put in a good word for his insanity. Maybe point out how if a few word definitions were changed, daveS MIGHT make sense on SOME level. Just more crapola. Every time I think he might someday actually put something together that makes some power, it never happens. So I’d say if he can’t show you a way soft kites, single-skin kites, “kite-matter”, “metamaterials”, “bose-einstein kite-condensates”, “flip-wings”, “power-kites” magically spinning TeraWatt generators, "kite networks over New York City that workers climb around on all day, “for the honor” (unpaid), or whatever else he’s promoting on any given day, could actually work, it’s just an empty message - noise basically. It’s daveSantos entertaining himself. And you get to be the latest victim - er um I mean beneficiary, of his daily dose of genius. Who knows, he might spark an idea at some point, like the “monkey-at-the-typewriter” that might accidentally type a phrase that inspires a Shakespeare play. “Test everything”! he says. Or at least he did for a year or two. Then he goes and just flies off-the-shelf kites.every so often and pretends he is conducting AWE research, well, because as he and Joe say, just flying a kite actually IS AWE, since it takes energy to keep it loft, so no need to try to make any power. Turns out, the idea the whole time was just to play word games, not solve a problem. Inmates running the asylum.

[dougselsam]

There is no certainty kites will be used at all. And yet people argue about what KIND of kite… Then others argue that they’re tired of the argument. More blind men arguing about the elephant that doesn’t exist.

[PierreB]

@dougselsam

Well I would think it’s a perfectly valid topic for AWE, but to approach it like some sort of religion, something to “prove”, insisting people “believe in” soft versus hard is just annoying. He’s certainly just beaten the topic to death for many years now. But from a wind energy standpoint, most everything he says is usually wrong. Sometimes he backs off the single-skin theme, and goes hog-wild defending fabric use in general. People will mention UV and wear, and he will point out all the planes made that way sitting on the tarmac. Blah blah blah, it’s like a script - or maybe a repeating bad dream that makes no sense - when you have a fever. For a few years, he kept referring to a highly-discussed supposed GE grant-funded project to make blades like a piper cub - rigid frame covered with fabric. Knowing a bit about how these things work, I told him it was never even going to happen, let alone be successful. Meanwhile, I happened to have had a conversation with the Director of Research at GE, at a wind energy trade show in Las Vegas, just a few hours across the desert from here, an easy drive. We talked about a lot of things. When I asked him about the fabric-covered blade effort, he was quite dismissive. It was just some girls that had managed to win a grant. On paper. He didn’t seem to think one would ever get built. I would guess more effort went into writing articles about the phantom project than ever went into the project itself.

This looks to be a relevant argument in regard to what is done in wind energy. So if the whole message is flagged later, we can keep this part.

[kitefreak]

My kite-expert preference for Single-Skin Power-Kites comes from Dave Culp, Peter Lynn, NASA Power Wings, and the last ten years of fresh SS progress. Its necessary to appreciate SS engineering advantages by higher power-to-mass, greater scalability, and so on. Non- kite-experts can only guess about kite design.

Perhaps the easiest way to distinguish soft v. rigid is the addition of resin to stiffen fabric until its subject to brittle failure modes. The engineering choice between these two different structural approaches is stark. Choose wrong, and AWES success may be out-of-reach.

Doubt whether “soft vs rigid” is an important AWES engineering trade will be settled. “Death” is quite likely for ventures that choose wrong and cannot pivot to the better choice. Only AWE ventures driven by investment hype can ignore critical design factors, for a time.

[PierreB]

Also a relevant argument. The question “Soft vs rigid” is a key question. Soft wings can prevail, but only for boat translation like sails do. If AWES is successful in electricity production, it could be with rigid wings, in a similar way as rigid blades for windmills or current wind turbines.

[Windy_Skies]

How do you make an as light as possible, as strong as possible, low drag, blade or plane, that uses appropriate, not too expensive, materials, is not too labor-intensive to make, and has good durability.

So you spend 99 percent of your time optimizing your design for all of that, and then the other 99 percent on improving your ease of manufacture of product and manufacturing process so that you can deliver on customer orders, so that your customer can offer electricity at a tenth of the price of the competition, for example.

I’m not interested in soft vs rigid. I’m interested, as a disinterested outsider, in material properties, manufacturing processes, stuff like that. If you could make a lightweight low drag plane that’s cheap and durable, that would be best, so how do you make one, or thousands?

[Tom]

This deserves a mention in this topic:

Inflatable kites using the concept of Tensairity | Request PDF

[kitefreak]

EMPA’s super-pressure tensarity R&D seems to have stopped, for specific shortcomings predicted on the Old Forum. Kites using the tensairity principle passively, at lower pressure, have been around since the 80s, in the form of the classic soft Morse Sled’s whisker-sparred tubes.

Kite pros would all use rigid wing kites, if they were practical. No one in AWE or the kiting world is able to make large rigid kites workable, because Galileo’s classic square-cube scaling law sets such severe limits.

Naturally, rigid wing developers are in a fatal venture trap, to test constantly only to establish predictably poor survival over time. All along, none of the rigid players have ever been able to bear open crash reports and complete power curves. Power kite pros are not fooled. The soft power kite is dominant for known advantages, and only getting better.

No rigid winged AWES program will be able to reverse the scaling and robustness barriers. No matter how many more millions they raise, the results will only make power-kites more clearly superior. Rigid wings are great outside of AWE to fly really fast, in flight applications where inherently higher mass is not a critical flaw.

For the aerodynamically-minded, soft wings do develop quite high L/D at lowest wing-loadings unattainable by rigid wings. Also, most AWES rigid wings so far lack enough control surface area for difficult slow speed states, and often not enough vertical area forward for balanced lift at the edge of the kite window. Let them learn by crashing then.

Time and testing will soon enough settle the soft vs rigid divide in AWES design. I am sorry so many naïve AWE developers must fail in order to prove predicted rigid scaling and safety barriers are real, but some are getting rich from rigid-wing AWE venture hype, so no one should be too sorry for them.

[Rodread]

Well to counter the argument. I’m an AWES developer with a budget of zero, testing rigid wing AWES. I share my crash data. They have survived repeated smashing into the ground quite well, still using my first set.

[kitefreak]

Rod,

Try to scale larger with rigid wings, but be careful. You should notice scaling laws kicking in severely, sooner than soft kites experience. We are talking about Makani and Ampyx as having scaled rigid wings beyond what a proper kite can be. Lets hope they share crash data based on your noble example,

[Rodread]

No, I’m only going to scale the physical size of unit rigid wings where that creates a suitable improvement in crossover of parameters including power to weight ratio, L/D, wing loading, survivability, controlability , cost/W…
Much more interested in scaling top performance by number.

[PierreB]

For now Makani’s wing is a step. But a soft set of rigid small wings could be a purpose.
Beside the torque transfer concern, @Rodread looks to develop means to scale a rotor.

[Rodread]

Maybe the real soft VS rigid discussion,
isn’t a kite manufacture tech and performance choice…
It’s a torque transfer method performance choice.
Soft torque can scale given the right kites according to a bunch of simulations I’ve done…
It can also fail given the wrong conditions according to a bunch of simulations I’ve done.
Death to “Soft vs Rigid”

[kitefreak]

M600 is the needed step to prove rigid limits compared to soft ship kites already of higher inherent power for over a decade.

Rod is wise not to scale his rigid blades up, unless that would allow him to directly grasp the scaling crisis.

Torque transfer faces the same scaling effects as rigid wings, if it uses rigid spars. Galileo’s Scaling Law applies in both cases. Torque’s only big scaling chance is carousel dimensions at the surface, using the ground as the spreader medium.

“Death” to inherent Scaling Law outcomes is the forlorn desire of this topic. Makani’s M600 hope has always been a denial of scaling law advantages of soft kites. None of founders knew to fear scaling law predictions, as my presence in their circle, at the behest of KiteShip, discovered. They all reasoned that L/D of rigid wings was the key factor, nevermind Galileo.

[Rodread]

Really like the direct and clean approach @tallakt has taken to the argument.
And the answer - Choose to go either way - seems sensible as every wing has its sweet spot.
Many companies have now run both soft and rigid wings for similar missions, whether pumping, flygen drag, or mechanical drag.
Further definition of nomenclature and explanation of derivations will further aid accessibility.

I’m sure Antonello Cherubini has done extensive analysis into various configurations and comparative metrics of AWES classes.
He reported on soft vs rigid in https://www.sciencedirect.com/science/article/pii/S1364032115007005
A more pictorial presentation here …
https://www.antonellocherubini.com/uploads/4/5/7/1/45719075/awe_fundamentals_cherubini_ppt.pdf
And video from his lessons on fundamentals of AWES… http://www.antonellocherubini.com/awe-course.html

[kitefreak]

Tallak does not seem to know whether rigid or soft scales greater in hoping rigid scales comparably, and one need only choose a preference. Rod does not require a definite answer, a “clean” answer will do, whatever the factual complexities of scaling…

Ship Kites are not a fluke, meanwhile the AP3 is far smaller and not even flying. Dave Culp found that soft kites scale better decades ago, and founded KiteShip. He pioneered >300m2 ship kites. Let anyone doubt soft kites scale better. The real world may already have settled this question.

Rigid advocates seem shy about applying exponential scaling factors to predict their scale limits. Soft advocates depend on the scaling math to their advantage. Crashworthiness and insurability are related factors. Its a great debate; only one side can prove best; there is no tie.

[tallakt]

I have updated the equations about power loss due to weight. The original post was bith wrong factually and also impossible to understand. Sorry about this