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?
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!
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.
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
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.
The single-skin lower mass favors a lower cut-in wind speed. So a significant advantage of a SS kite is a drastic decreasing of required takeoff and landing operations.
Lower Cut-in means less frequent landing and recovery operations
But the recent article
Vertical Takeoff and Landing of Flexible Wing Kite Power Systems
suggests lowering the cut-in of LEI, ram air and other kites.
Whilst also highlighting the complexity of handling this drone assist method requires.
I’ve done very rudimentary drone assist experimentation. Makes total sense. I’m all for the idea!
But an increased time in the air hastens UV-damage of kite and line without generating significant power during that time. To make a decision on whether the advantage outweighs the disadvantage you would have to look at windspeed data I would think.
What is the lifetime of uv-stabilized kite fabric out in the sun? It’s measured in the low digit thousands? You’d be lucky to get half a year of continuous flying out of that.
I’d be happy to be proven wrong.
Sailboat sails seem to last 2.5 years. I’d think most of the low hangig fruit has been picked here, and that a kite could not easily last longer.
If we have an automated deployment machinery for the kite(s) we might also assume that only half the time is windy enough for production, furter extending that. In northern and southern parts of the world, the windiest periods coincide with less amount of sunlight (winter), another factor to extend lifetime in practice.
For me it seems simpler to keep it flying year round than implementing such deployment machinery. Both are very hard though.
Cut-in wind speed concern should be considered for the whole flight, not only during takeoff (not landing I put by mistake) and landing operations.
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Indeed the lifetime of a flexible kite is low as AWE operation is most of the time. Appropriate and probably thicker fabrics with a better UV resistance should be studied. Rigid wings have a huge advantage in regard to the lifetime, excepted in case of crash.
Some posts about single skin kites:
OZONE KITES EXPLORE V2 Teaser / Jks-kitesurf
At 0:36 from the beginning, the structure of the wing is shown, with its “ram air wingtips” which I think improves profile and performance with little extra fabric just for the leading edge and the cords, most of the wing remaining single skin.
A similar structure is presented on:
La comparaison by Flysurfer: Peak 5 VS Peak 4 | Flysurf.com
These single skin kites seem interesting for AWE use because the profile remains efficient while the wing keeps its lightness.
Specifications
Size
4m
6m
8m
10m
12m
Weight
.65
.81
.91
1.06
1.21
Bar Size(cm)
45
45
50
50
55
Line Length (m)
20
20
22
25
25
Number of Cells
25
25
25
25
25
Projected Area (m²)
2.89
4.36
5.86
7.30
8.76
Flat Area (m²)
4.00
6.00
8.02
10.00
12.00
Projected Aspect Ratio
2.68
2.83
2.96
2.96
2.96
Flat Aspect Ratio
4.21
4.44
4.64
4.64
4.64
Root Chord (mm)
1180
1408
1593
1779
1948
Flat Span (mm)
4146
5210
6157
6875
7530
1.21 kg and 12 m² of flat area for the 12 m. The aspect ratio is rather high. I would like to know the lift to drag ratio. What not using giant wings like this for AWES?
See also single skin paragliders like
SPECIFICATIONS
SIZES 16 18 Number of panels 39 39 Projected area (m²) 13.9 15.6 Flat Area (m²) 16 18 Projected Span (m) 8.0 8.5 Flat Span (m) 9.5 10.1 Projected Aspect Ratio 4.6 4.6 Flat Aspect Ratio 5.6 5.6 Root Chord (m) 2.0 2.1 Glider Weight* (kg) 1.3 1.4 In-flight Weight Range (kg) 55-90 67-105
The aspect and area/mass ratios are still higher. Perhaps these would make excellent scalable high lift to drag ratio AWES, with reinforced fabric?
See also
An excerpt:
7. How does a mono surface work? A little flight mechanics…
A wing is made up of an intrados (the “underside”) and an extrados (the “top”).
On single-skin wings the upper surface stops just after the leading edge.
In terms of flight mechanics: the air flow linked to the movement of the wing generates lift: by depression on the upper surface (approximately 70% of the lift) and by overpressure on the lower surface (approximately 30% of the lift). So even without a lower surface most of the lift is generated: it flies!
The behaviour of this type of wing is a little different from a classic paraglider. This means that learning to fly with it has it specificities. This is an excellent first step into the world of free flight with paragliders, they are easier to inflate and take-off than standard wings thanks to their very light wing weight, between 1kg to 2kg (a “classic” mountain wing would weigh between 2kg to 4kg).
Can we deduce that the lift is reduced for a single skin paraglider?