If you think you can start with this
…and apply scaling laws to arrive at an optimal design for AWE, then you also could have started with this:
…to arrive at this:
I think perhaps you are putting too much value into the scaling laws. Sure scaling laws are there, but these are two different structures alltogether…
Dave, I can agree this is an important concern but, Tallak, I am not sure the question is properly asked as an AWES is a wing + a tether. If we take the kite alone, it can scale while being rigid like airliners or wind turbines do. In return a tether is a “flexible” very long element transmitting huge forces. So a rigid AWES is in fact a rigid element + a soft element.
The question could yet be someting like: is the tether preventing the rigid kite scaling up? If yes why? For example is the crash risk higher with a tethered rigid wing in regard to a free rigid wing?
Its true: I find scaling laws answer the “soft v rigid” question in favor of “soft”. Highest power-to-weight performance of wings predict the same outcome (1kg paraglider handles far more power than 1kg of rigid glider). Let others continue to doubt scaling laws clearly predict what sort of wings will scale the most.
It was Dave Culp who long ago taught me how scaling laws affect different kinds of kite, and how his OL ship-kite design resulted from that logic.
By this reasoning 1 kg soft blade handles far more power than 1 kg rigid blade. However current wind turbines use rigid blades.
The same logical flaw would apply in wrongly claiming 1kg of soft sailboat sail is the right comparison.
Its best to reason from power kites, where rigid wings cannot offer higher power-to-weight, due to the parasitic effect of greater mass on flying power.
A wind turbine bolted to a pole is a very misleading model for AWE. The soft power kite not only reaches far higher, but at far higher power-to-weight.
However current airliners and numerous other planes use rigid wings.
Only aircraft needing to operate at higher velocities (>Mach ~0.3) require rigid wings. Real world power kites operate at <Mach ~0.1, that rigid wings are extremely poorly suited for.
Great to constantly review of “Aerospace-101” class lessons, for those new to these facts, about why kites are soft in order to scale to ship-kite dimensions, and rigid wings can’t scale as well.
I disagree with you here
Having used both soft and rigid form kites
Rigid wing Power to weight is far superior
Yes rigid unit scaling is not so impressive.
Just have lots of smaller, faster rigid wings for the best performance.
And despite protestations… small rigids survive crashes really well in my experience
Rigid has some other benefits also such as minimum added weight in rain. A nylon or other weaved skin kite will gain significant weight in rain… it would seem non-weaved sails fare better in this regard (windsurf sails)
Thought experiment-
Take a soft polymer wing of 1kg, like the lightest paraglider, at its maximum working load. Lets roughly assign this as ~10kW model.
Now take the same wing and add resin mass to make it a rigid composite wing. Its now brittle, heavier, and not hardly stronger because resin is weak. It now must sustain flight with greater mass. Its no longer safe at 10kW of working load, because forces now concentrate in the rigid wing, in buckling stresses, rather than distribute. Even more polymer fabric and resin must be added just to make it workable.
Its now a far worse wing, with obviously lower power-to-weight, and less robust. It needs more wind to just minimally fly as well. If it crashes once, its destroyed.
Only a pure soft polymer wing achieves highest power-to-weight and at maximum robustness.
Agreed that small rigid wings do work well at “toy” kite scale, and do not scale so well from there. A key reason is that a given AWE wind itself is constant, and does not scale up along with a wing. Bigger wings need faster wind for equivalent performance by unit mass. Such increased wind progressively becomes too improbable for economic duty.
Rain is a minor performance factor. Look at water kites. They fly just fine. Where are the rigid versions?
You advocate in favor to soft wings since years and years, repeating the same arguments thousands times unconvincingly, just in order to have the last word.
For only one time you could experiment both soft and rigid wings, using a wattmeter and a peson, then transmit the results.
Pierre, Its not necessary to be factual to try for the last word. Nobody really gets the last word anyway, nor is that a proper technical concern, but an emotional issue. Go ahead and complain without addressing the factual basis of the topic here. Maybe you’ll get the last word, that hardly matters.
I am getting practice arguing for soft kite power-to-weight. Its not the same argument, it gets better.
The power to weight of a modern power kite is incredible, roughly equivalent to the Space Shuttle Engine, but without fuel (!)
Another metric is sink rate. A 1kg paraglider can have an equivalent sink rate to a hot glider of 100x mass. Both are doing equivalent work of sustaining the pilot’s mass in sink.
If you need speed, a rigid wing is fine. AWE needs masssive truck-like performance, “grunt”, more than it needs speed limited to small scale.
The answer is not yet known and perhaps will not be under the form you assign as “soft vs rigid”.
Imho in the current state of the art both don’t scale well in regard to AWE requirement.
A new paradigm should be studied. I don’t think a successful AWE wing would be something intermediate between soft and rigid. I rather think about how the rigid and soft parts should be distributed. A rigid AWES has already a rigid part (the wing) and a soft part (the tether). A soft or rigid network of soft or rigid parts can be a possibility,
Pierre, The answer was known since Dave Culp worked out kite scaling in the last century.
Lets agree the answer to soft vs rigid scaling is not known in AWEC’s venture circles, or if it is, its a secret.
kPower discovered that to scale maximally, no rigid sub-parts are wanted; consistent with DaveC’s findings, although he was not analyzing mixed design.
My Peter Lynn Arc would not fly well in rain being considerably heavier, even in fine wind. At the same time a LEI kite flew with little degradation in performance. Wet kites are real.
A soft cloth kite cannot be made to any shape and thus the design (wingspan, bridles, wing profile) is subject to considerable limitations in the design room. So your arguments are more or less moot by me.
Not to be a soft kite hater - if you look at the initial postings, I believe at this point in AWE, it is wasteful to argue whether one of the two is better. Just use tools from what you can use (rigid, soft or whatever) and go make the best AWE you can.
The technology «deathmatch» you are wanting for AWE will never happen, because even if you were to prove soft kite superiority, suddenly someone invents a new rigid card to put the results to shame
I question what you want to achieve with this argumentation. The above is only correct if seen by wearing the largest blinds you could find…
Tallak, I intend to warn, as best I can, all rigid-kite developers that aviation scaling laws will prevent them from meeting the criteria of unit-power on a par with large tower-based wind turbines (~8MW these days).
Sorry of the larger truck vs smaller sports-car (grunt vs zip) AWES analogy made you question what can be achieved by “this argumentation” (engineering metaphor). Provoking technical questioning is often considered good. That’s what I am trying for.
I missed the “new rigid card” you refer to. What does it scale to? Does it survive years to pay back? Is it the safest most insurable solution? Where are the rigid kites in Kite Sports? There are no large rigid power kites outside of AWE ventures who do not share dismal scaled-up power curves nor crash reports. Why not?
It does seem to me there is more to celebrate in ongoing power kite progress than rigid kite progress. Thanks for any cases or comparative data proving otherwise.
Here’s a link related to scaling from the critical context of regulated airspace-
http://www.energykitesystems.net/CoopIP/TetheredAviationConOps.html
Regulations are to me of minor interest. Show me a power plant to save the world from global warming, and I’ll soon show you a regulation to allow its use… its that simple.
In the meantime of course we must adhere to whatever regulations that exist. I would be more interested if this eg. was my day job, but as if right now I am not much involved. I know that Kitemill are free to test within limits that are not very confining to us so I dont see the problem. If regulations are different where you live you are free to join us for testing at our location (I guarantee anyone with such a wish would be welcomed with open arms)
For all your argumentation and my quite extensive experience with many types og kites, it is still not clear to me that either soft or rigid kites do not have their place in AWE. Eg: if you need a fast rotating rotor (ST?) you would be mad to consider rigid wings. If you need something to stay airborne in low winds and no active control, soft kites are probably the way to go. Its all just the tools of the trade we are in…
Now lets rather discuss how to make something that will actually produce power
Tallak, its true that to be more interested in airspace regulations is to best appreciate key aspects of rigid vs soft debate. Our liberal experimental permissions are not the same as taking to the sky on a massive scale. No we don’t need to come to Norway to fly as we need, as lifelong Aviation and Aerospace natives. TACO got exactly what was asked of the FAA, no new regulations on kites, as we self-regulate, until someone messes up.
There is a lot of AWE scaling knowledge in TACO that is not just applicable regulations. Its as if you thought regulations somehow do not reflect inherent domain physics and safety knowledge. They do in both Nuclear Power and Aviation, to the deepest degree, because lives matter. So does insurability, as a strong cost-driver.
It seems like you will not be studying many sources of AWE knowledge on similar grounds. You did ask for links, but then dismiss them by emotional pretext. No more links then. Heaven help you if such willful lack of curiosity contributes to loss of life.
Of course ship kites could already in principle power the world as TRL9 MW devices, but that’s not what KiteMill has chosen. Go ahead and fight all the other me-too EU companies for scale-limited capital-intensive remote-power markets. kPower will avoid that trap.
Sorry your kite gets soggy; sounds like bad fabric. Peter Lynn has sometimes been very uneven in fabric quality. You do know that bird poop ruins the performance of fine wings. Birds don’t perch on soft kites nor does poop affect them so. Consider a full scoring matrix of these details, and don’t depend only on your direct experience.
“Soft vs rigid” is about the wing. But an AWES has also a long tether which can be seen as a soft element. A contrario a rigid tether is… a mast. Thus current wind turbines have rigid tether and blades, so are fully rigid.
From this we can deduce an unity of AWES construction as being fully soft (both tether and wing) in Dave’s way. If the tether is rigid, this is presumed to not be an AWES, although it could partially be if some significant lift is produced, for example by using kites or/and tilted blades.
We can also deduce only the wing is rigid, extrapolating on what works in current wind turbines.
Perhaps this would lead to remark the main advantage of AWES is avoiding the cantilever effect of the mast as the forces comprising the lift are fully aligned on a tilted segment (usually the tether).
