Single skin AWE

Single skin kites might be the ultimate AWE kite in terms of lift/mass ratio. Are single skin kites suitable for AWE? Id really like to hear any thoughts on this.

http://www.peterlynnhimself.com/newsletter/2017/April.html

I agree. Indeed a 1.1 kg paraglider can be a good basis for an AWES. Even by using a thicker fabrics for a higher lifetime the AWES can be still light.

https://www.flyozone.com/paragliders/en/news/headlines/22455?source=rssfeed

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Dave Santos just posted on the topic:

“SS” means “single-skin”, of course, but “TS” is more obscure; its Peter Lynn’s “Twin-Skin” (parafoil) usage to avoid confusing DS “double-skin” initials with DS flight. We have long noted that SS wings are theoretically superior by power-to-mass and power-to-cost, also found that to be true in empirical practice, and we have followed the ongoing revolution in SS wing design in recent years. The NASA Power Wing (NPW) remains the standard workhorse, Dave Culp’s OL Shipkite is a classic radical simplification, and new SS paragliders and powerkites are the avant garde. Dave Culp long ago predicted that SS kites would eventually dominate large power kites, which I agree is likely. This message adds Luff Rate to comparative SS-TS theory.

Besides lower-mass-by-area and higher-power-to-mass of an SS over a TS, the most distinguishing characteristic between the two kites is the Luff Rate. An SS kite will luff faster and recover faster. SS AoA tolerances are a bit lower, not as much as one might fear. On the TS side, valved ram-air intakes add a bit of mass and further reduce luff. At a given velocity, Luff Rate is determined by the thinness (depth-of-section) of the wing, the thinner wing having an inherently faster luff. Kite Control does not react in time as easily to a fast SS luff. However, for equivalent-mass SS and TS wings, the larger SS kite’s luff rate is moderated by its larger size, and flown in weaker wind, and this slower response is operationally significant. A smaller TS in faster wind can be twitchier than an equivalent power SS in slower wind. An SS kite is more prone to flogging (harmonic luffing) damage, but excess flogging is a duly avoidable state.

This is a start at understanding Luff Rate as an SS v TS design and operational factor. Its an open task to formalize luff mechanics in terms of Strouhal numbers, Re, quantum-of-action, etc.

Adding here some damping-factors on Luff Rate and Luff Amplitude-

  • SS Luff is damped by fore-and-aft multi-bridling and camber of the wing. Multi-bridling inhibits bulk oscillation and high camber restricts luff to the LE area.

  • TS Luff is damped by parafoil-cell (airbeam) buckling, which can be interpreted as a slow-phonon effect, under sonic relativity.

Luff Rate (Frequency) and Amplitude reflect a close relation to harmonic flapping (and flogging) dynamics.

The single-skin lower mass overhead favors a lower cut-in wind speed. So a significant advantage of SS is a drastic decreasing of landing and recovery operations.

@tallakt replied.

Due to their extreme advantage in lift to mass ratio, single skin kites will always be very interesting for AWE and any other use if kites exploring the limits of what they can do.

I just purchased a few of these kites and I will see what they are capable of for myself. It seems such kites are still in early development phase, and that the advance of the technology is difficult and slow.

I can say from testing the Flysurfer Peak (single skin four line kitesurfing kite popular on snow) this winter, that the low wind range was exceptional (low weight probable cause) and it also had very good lift-to-size ratio. What I didnt like much was some flapping at the edges which is likely to cause some premature wear, but hey, Flysurfer probably has some data on this matter based on actual damage reports, as the kite has been in the market for some years already.

Ozone released a similar kite I believe this year.

Now, there is one thing missing from your analysis that was part or the «ram air» vs LEI kite design discussion back in the days. A ram air kite will contain a lot of air between the top and bottom layer. For a 12m2 kite we could estimate this to be 4m3 worth of air, weighing approx 5 kg. This mass will severly affect turning speed as the air travels with the kite. A single skin kite or the more traditional LEI (leading edge inflatable kite) does not have trapped air in the same sense.

I thing the severity if this effect scales quite quickly with kite size. They will trap air with wingspan to the power of three given otherwise equal dinensions. Add to this the general observation that bigger kites turn slower.

The conclusion of these observations is perhaps that single skin is the only viable option (from these designs) to be able to actually turn with reasonable speed when scalled to the 100 m2 size.

Just thinking out loud here.

Tallak Tveide
SW Engineer Kitemill

Interesting to consider single skin kites here in [Subsystems / Components] [Crosswind kites]

Hope my thoughts can help … Usual incompetency waiver …

I normally consider a crosswind kite to be the main power-converting blade of an AWES.

To maximise power output, a crosswind kite wants to fly permanently close to its maximum performance with apparent wind very forward.

Given a long straight sweep AWES (Such as an ocean crossing AWES, snow kiting, kite surfing, paragliding…) A single skin kite (or even a network of single skin kites) seems a fine choice. Especially so if cost of material is a main design driver. Ocean clean-up might be a good application.

A single skin kite may be of various materials and rigidity (generally soft) and varied performance. (So jealous of the Flysurfer Peak testing.)

Single skin kites have incredibly tight turning. But - Max line tension comes from larger radius turns. Single skin kites can be stacked. This could cumulatively generate huge line tension for a yo-yo.

However - Any large AWES needs control to be safe. Control is mechanically difficult without large networks of bridling and tethering. Power to weight of single skin kites is impressively high at the proven scales, but their scaling is problematic without control.

So far, I have tested single skin kites, (at any noteworthy scale), only as lift kites or personal traction kites. I tested groups of single skin kites flying together in stable networked lifting (not rotary) configurations. Lift kites are not normally crosswind; they normally just hold position and tension. (I currently use a KAP foil for simplicity) There is no reason not to have a crosswind lifting single skin kite if you can design the controller…

Tensile rotors (such as Daisy kites) benefit from being centripetally expanded. As such, the tighter form control of rigid wing profiles will greatly increase the power available from tensile rotors. I have modelled huge arrays of soft kites working together as network turbines… Without controllers and drone assistance, I assume the deployment and recovery would be tricky. A couple tricks left there no doubt.

Designing for maximal power crosswind performance (a very forward apparent wind, thin profile) leaves a single skin kite with very little capacity to handle change of direction and with a high chance of flutter and wear.

Fine for small systems and travelling sets – unlikely for utility energy.

AWES blade progression has been similar to the blade progression seen in yacht racing and early flight. Test loose and safe, progress winning formulas with tight and powerful.

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Clarification: I moved the thread to [Subsystems / Components] [Crosswind kites] as it was uncategorized and I interpreted the original question that way. Maybe not the right place. Should it be in [Lifters / Pilot kites] or do we need another category?

I am worried that too many categories will make navigation hard, the Yahoo forum only had one category. But I dont mind whatever you decide @Luke.

@Rodread youre welcome to join me in testing kites at the Hardangervidda Plateau this winter

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I guess for tensile rotors some mass may be beneficial to keep the wings apart. This probably rules out single skin kites for this purpose

Disclaimer: I don’t know anything.

I think the question is backwards. Instead of going, I have this [solution], what [problems] could it be applied to? You should go, I have this [problem], what [solutions] could work? If you have a good problem description, if you know what characteristics you are striving for in the solution, you can slowly start considering materials and designs.

And what is a single skin kite? Can it have spars or inflatable sections?

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This topic spans both categories. We definitely need the two categories as they are very different subjects.

I see my preferences for choice of technology for AWE changing so fast, so keeping a good insight into any possible tool is valuable in itself.

In relation to rotors benefitting from tensile design.
https://www.nasa.gov/sites/default/files/files/Moore_EternalFlight.pdf
The CSR (Centrifugally Stiffened Rotor-wing) as proposed by Mark Moore NASA Langley
Suggests applicability to AWES
and of course, was discussed previously on yahoo, but the links are so tangled…
@tallakt I had 4 days snow kiting in Haugastol for my 40th birthday. Heaven!

I like this concept then: An inflatable wing using the principle of Tensairity

Jup. That is just wild. I guess the invention of using inflated skeletons for kites belong to Bruno Legaignoux and his C kite

Some more info:

http://www.lesfoilz.com/phpBB3/viewtopic.php?f=4&t=1713

I don’t yet have certain knowledge of this, but will stick my neck out here and say that less than 1sq.m poses no issues, and that at the other end, I don’t expect anything terminal until around 300sq.m- and maybe much more.

and

High performance foils might achieve unloaded L/D’s of 5 or even 6, but under load no conventional traction kites do much better than 3.5 (this is from the extensive data collected for kite energy systems ) . The best SS kites might start out at just 3.5, but they only drop to 3 when fully powered , so aren’t as far off the pace as might be thought.

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Dave Santos posted:

Thanks for the input. Some further details-

  • SS power kites (NPW and Barish PG) are almost as old as parafoils (>50yrs). The NPW is still competitive by core metrics, as its design continues to be refined.
  • Turn rates between SS and TS are closer than one might think. SS kites do not have internal air mass, but the “ugly” bottom surface still has to drag/churn more entrained air mass that the smoother bottom of a TS parafoil.
  • Turn rate is more sensitive to AR (aspect ratio) and frontal C-shape. Larger SS wings of equivalent TS mass are slowed in turns by simply being bigger.
  • A bit of SS soft flutter is not very damaging. SS repair is fast, cheap, and easy (field-repairable). Its only hard flogging that damages quickly. A tensioned leech-line is the sailmaker’s solution to modest TE flutter.
  • SS wings generally operate in non-dimensional lower wind velocities (kite gets bigger in proportion to constant wind velocity) compared to equivalent-mass TS wings, with lower wing-loading (by unit-area), which means higher L/D than one might expect.
  • SS have the theoretic fastest payback by being the cheapest power kite (capital-cost-per-Watt), so they may economically beat any other wing type, even if they last less time (not yet proven).
  • Bridling is a complex design factor. More SS bridling, up to a point, gives higher L/D, but simpler bridling is better operationally (OL has just three bridle lines) and high CL “grunt” power most counts, when high load-velocity is not required (truck v racing car).

I’ve tested PL 4 line SS Skin kites are flyable in a huge range of winds, from very light to full on gales.
They have often needed repair after high wind days.
The backward flying mode in the PL single skin takes a lot of getting used to.
Operating wind range is a huge consideration for the applicability of your wind energy system.
A rev kite (effectively a tight single skin with a rod stiffened leading edge) while faster and more controllable can’t handle the extreme ends of wind range which a purely tensile single skin can.

Kite networks enable advantages of modular deployment (setting a kite appropriate to the wind) and mixed soft and hard kites set in the appropriate place on the network.
Single skin can have a place as a crosswind kite.
Then again, a recent EU review of AWES said my rotors aren’t crosswind devices. WT. do I know?

Your kites are downwind for sure :joy:

Dave Santos posted:

The modern Power Kite was always the standard WECS basis to compare against all other AWES schemes. We include SS, ship-kites, and pilot lifters in the standard kite group.

Many early AWE R&D players bet they could come up with something better than the power kite, but after years of trying, and hundreds of millions spent, most of them have clearly stalled or completely disappeared from the AWE race. Meanwhile, we have explored in far greater depth specific operational and design factors that predict power kite success, and standard power kites have only gotten better. Non-experts now have a mountain of supporting evidence to better guide growing investment in AWE R&D.

Where are we now? The standard power kite has slowly gained over fashionable fringe concepts. More than ever, the AWE race seems to be how best to harness power kite rigs. New rounds of investment are shifting accordingly. We may still see large investments in non-standard wing development, but the power kite has surged ahead of every other wing; not just in validated performance, but also in direct AWE R&D. The natural failure of fringe AWES concepts is a major form of progress.

Thank goodness for the power kite, the Cinderella Wing of AWE.

Though power kites are interesting, I don’t buy that they are the only viable solution. Rigid wings like Kitemill, Ampyx, Makani and others are building seems to me to have no problems wrt feasibility. You are also neglecting a kite that has not been designed yet, but might prove siperior to anything we know of today.

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