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.
Iāve done very rudimentary drone assist experimentation. Makes total sense. Iām 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? Itās measured in the low digit thousands? Youād be lucky to get half a year of continuous flying out of that.
Iād be happy to be proven wrong.
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Sailboat sails seem to last 2.5 years. Iād 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.
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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.
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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 āram air wingtipsā 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 (mĀ²)
2.89
4.36
5.86
7.30
8.76
Flat Area (mĀ²)
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 mĀ² 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 (mĀ²) 13.9 15.6 Flat Area (mĀ²) 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 mechanicsā¦
A wing is made up of an intrados (the āundersideā) and an extrados (the ātopā).
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 āclassicā mountain wing would weigh between 2kg to 4kg).
Can we deduce that the lift is reduced for a single skin paraglider?
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Nasa ParaWing (NPW) have been mentioned on several topics including the present topic, including kites based on Rogallo rescue parachutes for tether-aligned AWES.
NPW are also used as power kites, including kites up to 150 mĀ² for Windsled.
Originally their lift to drag ratio is about 2.5 to 3, without the tether(s).
https://www.2e5.com/kite/nasa/reports/
Page 7:
The maximum lift-drag ratio of the single-keel wing shown is about 2.5 while that of the twin-keel wing is about 3.
See also the curves of lift and drag coefficients, and lift to drag ratios from the figure 16 on
NPW are cheap and have an advantage: they pull very hard (perhaps due to their inflated parachute-like shape), but in a narrow flight window (due also to their inflated parachute-like shape). I will explain why it is an advantage.
The flying window is a bit smaller, partly because the NPWās need a certain angle of attack to stay inflated properly but also because they weigh so little that there is no momentum in the kite that helps to expand its flying window when in flight.
The smaller flight window is presented here as a disadvantage. But I think that for AWE use it is an advantage because the trajectory of a kite crossing the flying window greatly exceeds the useful swept area. Moreover in yo-yo mode it is preferable to make U-turns at the limit of the flight window, during short reel-in depower phases. So a narrower flight window induces a natural flight limit, perhaps allowing several unities to be closer each other. Therefore a NPW wastes less space, and we can increase the density in a kite farm.
I like this line of thought @PierreB
Makes me want to go and check back over my soft kite data.
But Iām sure the rigid blades made a lot more power because of their speeds.
Also
I had a single skin kite (flysurfer Peak5) flying waaay past straight up over my head today
Just come back in from leaping/running/kiting up and along the ridge of hills ā¦ The wind blowing onto the hill gave huge lift. Was getting flung around a fair bit. Fun 