Here Power Kites are defined conventionally as soft fabric wings of the technical lineage of Jalbert, Rogallo, Barrish, Culp, Lynn, etc… This topic concerns modern soft power kites as primary AWE WECS, with rigid wing kites established as conceptual and operational competition.
This is a especially an invitation for best evidence and analysis AGAINST established Power Kite capabilities and FOR theoretic Rigid Airframe Kites. Whatever the discussion outcome here, the Power Kite is a serious contender for standard AWES unit status.
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Rigid wing kites: the power generated depends on \left(\frac{C_l}{C_d}\right)^2 C_l. Rigid wings provide the best \frac{C_l}{C_d} by far. Rigid wings provide superior flying speeds and thus better high wind capabilities. The high glide numbers can be easily reduced to get huge depower ratios allowing for operation in a huge wind range. Rigid wings are easier to design as their shapes dont change until it fails, even if you decide eg. to place a servo on the wing itself. Finally rigid wings at the current technology level could be built to last 20 yrs+.
I find rigid vs soft classificstion a bit simplistic anyway. We have fully rigid wings, single skin, open cell kites, closed cell kites, inflated frame kites, rigid kites covered by a soft skin etc etc. And we might see networks containing both rigid and soft kites (@Rodread has demonstrated this several times)
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Tallak,
Thanks for answering for rigid kites, which are fine aircraft, just not very common.
Standard power kites are at the soft end of the design spectrum of soft-to-rigid kite structure, with helpful operational qualities, like being inherently crash resistant and storable in bags, which rigid airframes are not so apt at. Your starting equation will be more realistically predictive by including Kite Area as a power generation dependence.
What is your rough estimate of relative soft v. rigid wing Area, presuming equivalent flying Mass?
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Is that the case for all blades moving through the air, also for a wind turbine for example? And do you have a source handy? Sorry if it’s an ignorant question.
Windy Skies, Let Tallak focus on the key soft v. rigid question about Area and Mass. I think I have him checkmated.
If you like basic kite power equations, there are many papers. Read them. Tallak has neglected Area and Mass factors, so you’ll soon see if his starting equation stands as presented.
Rigid wing kites have a far higher lifetime. And wind turbines have rigid blades.
Pierre,
Where is actual reliability data for rigid kites to support your opinion? What if such kites cannot survive years to pay-back, but crash data is withheld by the stealth ventures? Don’t be fooled.
Large conventional wind turbine blades are very unsuited for flight. Large power kites are suited for flight.
I cant find a link to a proper explanation in a rush, but Argatov has written about this in a few papers, and calculating it is quite straightforward.
The following link at least contains more info to get started in a deeper search (eg eq 1,2,3)
I cant compare weight to power in a meaningful way. Weight seems most important in very low winds. A single skin kite can have huge lift at a large area and super liggtweight. But with a Glide number at 2.5 theres not much possibilities for depowering the kite considering the C_d of the area in fully dragging configuration is probably around 1.0. So a misbehaving single skin will probably pull enough to be a problem, while a high glide number rigid wing will not generate force unless it’s flying perfectly.
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Numerous observations rate soft kites or paragliders for 500 hrs, sometimes 2000 hrs for sails, compared to:
The lifetime of a passenger plane is 20 years according to this:
https://www.quora.com/When-does-an-airliner-get-retired-from-use
Im no expert in these questions, so thats about as far as I can defend my position on this.
A rigid energy kite will have far more hours flight time and higher wing loading in these 20 years, but then again there are no humans on board. I trust a person skilled in composite design could dimension a kite for a lifetime of 20 years, though Im not sure which weight penalty this would come at
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To be fair, nylon is probably not the ultimate material for energy kites. Perhaps something exists that is sun ray tolerant, strong and liggtweight, and does not disintegrate due to flutter (or does not flutter much). Lets hope at least that such a material will come to exist in near future otherwise…
In aeronautical science, weight-to-power is a key number. In Kites, weight-to-area is similarly critical, as power is a function of area. See Old Forum for latest WP Wing Loading article and relation to sustained power in turning. The rigid kite tends to use a lot of extra airspace to just to fly a pattern.
In fact, a soft kite is easily an order of magnitude or two larger than a rigid wing of equal mass, for superior theoretic power. But the aerospace engineer must account for mass and area to get this result.
Pierre is right to cite soft kite lifecycle data. He knows the cases vary according to well known factors. He has no equivalent rigid kite data to prove greater survival with time. No can he show that replacing cheap kites regularly is more expensive than a rigid kite over time. Capital cost of the rigid wing is far higher, that much is agreed.
Modern aircraft only last decades by human piloting, constant visual inspections and maintenance, 100hr and annual comprehensive inspections, and so on. Soft kites are not so complex to operate, maintain, and repair, and are more easily replaced.
“Nylon” and similar details are not key reasons for rigid kites. In fact, nylon’s elasticity can help a fast kite when it snubs up on its tether. We have good UV resistant fabrics whose UV protection can be renewed in a dip-bath.
Current rigid kite ventures are generating the missing reliability data to settle doubts. They need to reach >100khr without major mishaps for insurability under current aviation norms.
It is true in most configurations. SuperTurbine ™ and other rigid rotors can maximize their aispace.
The Super Turbine cannot scale well for many reasons. As turbine diameter grows, the angular velocity just gets worse, so inherent long driveshaft efficiency is poor. Again, no real data, just scaling laws to note. The ST driveshaft is not available for testing like power kites are because its predictably going to be very massive expensive and fragile. Operations are not worked out. Let testing prove the ST’s limits to Pierre and the world. Doug should be trying to scale up since 2009, since its his baby, but already its too hard, by Galilean square-cube mass. Anyone can buy a power kite, and they come in giant size too.
Lets be clear that our FAA designated airspace is the hottest airspace to tap into, but a bunch of STs are currently one of the least promising means to that space, that power kites have already reached. Given scaling law limits, no ST can come close to SkySails ship kites already proven in that sort of airspace. Testing to ST limits is needed for proof.
Lets keep Ship Kites in mind in the context of speculative scaling by non-power-kite contenders. Ship Kites have the highest power at highest TRL; the current wings to beat.
Soft Kites distribute stresses but rigid kites concentrate stresses, with a brittle-failure mode soft kites do not suffer from. The bigger a rigid kite, the graver the brittle risk, or at best, more parasitic structural mass. Without help, big rigid kites no longer lift-off in a breeze; big power kites do.
The highest theoretic power-to-mass in AWE depends on the strongest polymer unit-mass working closest to its load-limit. Load-distributing power kites are better at this. Current power kites are closely adapted to “most-probable” winds. Rigid kites offer less choices than power kite quivers. Try and wear a quiver out, but one crash, and a rigid kite is done.
Power Kites have the R&D advantage by large sophisticated user subcultures spanning decades and maybe a billion or so in past military R&D. “Power drones” must catch up and pass power kites to be “the answer”. Power kite state-of-the-art does not sit still, but improves daily. Rigid kites will never catch up at large scale, if cited scaling laws apply as predicted.
KiteLabs and kPower have long designed and flown rigid-soft combos, and even recommend them at small length-scale (<2m), but rigid structure starts to suffer >3m in length-scale, in normal wind conditions. Power kites rely on rigidities of air pressure, and sometimes spread anchors, at no serious weight-cost aloft.
Large soft kites, pilot-lifters, can hold up fragile rigid airframes from crashing, but even this soft solution does not make rigid kites essential to AWE. It may duly prove best to just let giant power kites work groundgens directly, without compounded complications. KIS, as engineers are taught.
I think right now the first step is to demonstrate a cost efficient safe way to produce airborne power. Making it run for 20+ years is only important if we can do that first. It is probably a difficult challenge to do this at scale (in terms og number of installed units), but I have no doubts it is technically possible to make a rigid wing last.
For soft kites, 20 years seems very optimistic. It seems that replacing the skin now and then is the most probable way forward. Though not a very big deal, kites as of today are manual labour intensive and also quite costly relative to current lifetime operating hours.
I would say the risk of not getting cost sufficiently low is bigger for soft kites, though there is a risk for both approaches.
It seems @kitefreak is overlooking the fact that composites offer both compressive and tensile strength. Carbon fiber has very good tensile strength and need not neccecerily be designed to concentrate stresses to a single point. Kitemill in the early days built a C kite in carbon.
The reason current rigid kites look like they do is because they are current best practise, closely related to aircraft designs. But wings may be made to utilize tensile strength also by adding bridling and arc shapes. We have surely not yet seen a final optimal design for a rigid wing kite (as we have probably not seen the final design for a soft kite)
Perhaps a better way to analyze the situation is if the kite should have three free axes (roll, pitch, yaw) and one attachment point, or one (fixed roll and pitch relative to kite lines and free or almost free yaw), or none (kite fixed in a network/mesh) or somewhere inbetween any of these.
Anyways discussing soft vs rigid is simpliying the matter to the nonsensical
It may seem “nonsensical” that standard power kites are definitely distinguishable as high TRL engineered soft-goods. Its true that rigid composites have great tensile strength, but this is not the fatal vulnerability; brittleness is. Hit a composite wing with a hammer or forklift, and its badly damaged.
The tensile strength of rigid composite is less than the same fibers in softer form, due to the resin content and need for greater thickness. More mass for less strength again. Yes Carbon fiber itself is great stuff, but very expensive. No one has proven it will be cheaper by LCOE to make large kites of it, rigid or not.
Its perhaps most nonsensical not to fly off standard power kites against the custom composite kites, except that its venture logic not to, rather than optimal experimental design. For now, kite sport data has soft power kites winning every race and market. Rigid kite prospects at larger scale are even worse.
Airliners are rigid in spite of crash risk.
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Sport data may be all we have but it is not completely relevant as kitesurfing or powerkiting as a sport is nothing like utility scale electricity production. Its like saying a combustion engine car should have wooden wheels because wooden wheels have ben proven through years of use on horse carriages (I know, lousy example, but I think the point is valid)