# Reel-in phase: how to further depower flexible and rigid kites?

Two papers nearly 10 years apart publish test curves, the first for flexible wings, the second also but on a larger scale (Figure 15), also publishing for a rigid wing (Figure 18):

Development and validation of a real time pumping kite model (Ruppert, M.B., 2012, Table 8 page 44)

Pre-print on:

Concerning flexible kites:

On the first paper: Table 8 page 44, dataset 5: Mutiny (25 m² kite, see page 8), 8 m/s wind speed, 5.19 kW average power, 14.1 kW average generation phase.

On the second paper: Figure 15 (average power 92 kW, with peaks of 300-400 kW.

My observation: on Table 8 page 44, dataset 5 the average cycle power / average generation phase ratio seems to be roughly the same as said ratio deduced from Figure 15.

For both papers, one can see that losses during reel-in phase are important due to its duration and power consumption. As a result, the average power is low (1/2 or less) compared to the power during reel-out phase.

In fact the kite is not fully depowered and generates a significant tension.

Perhaps a mean to shorten reel-in duration and to more fully depower the kite would be using the principle of 3 line kite, the 3rd line being a depower line (video below, 1:10):

Unfortunately the kite falls like a rag, being difficult to control for an automated system.

So maybe some means could be used to allow a control a minima, such like a central bar or inflatable beam, preventing the kite from closing completely, coming back like a flag (second sketch):

Concerning rigid kites:

https://www.ampyxpower.com/technology/

After this reel-out phase during which electricity is generated, the wing glides back towards the generator and the process is repeated.

I have a toy: a plane flies like a kite, then when you release the rope, it comes loose and flies like a glider. I think that for a system at scale the control is not easy. And from the figure 18 (second paper) I guess that the use as a glider was likely not be made, but this is only a guess. The duration of reel-in phase is relatively far shorten than for a flexible kite, but the energy consumption is far higher.

I reput the link (see from 3:07 : comparison between reeling 3 m² plane (2.5% of potential, huge difference between average power and maximum power per cycle) and fly-gen 4 m² (25% of potential, so 10 times more)):

So, as a temporary conclusion, some structural elements could be added to a flexible kite, accompanied by an adapted flight control, while for a rigid wing it is only an affair of (difficult) flight control.

It seems to me that the most efficient retraction method is the Kitegen system. The kite is converted to a ribbon and can be retracted at high speed. Do we have any data on this system?

Someone or a company would have to test several methods.

Today I made some experiments about the system on the sketch above, and on the photo and the video below. The kite warped as it descended, but not as much as it did without a central bar. So that’s a possibility.

For airborne wind energy systems using yo-yo mode with power reel-out phase alternating with depower reel-in phase which consumes a lot of power during long time of no production in the current art. This low quality video is like a sketch to investigate means like a central bar connected to a 3rd line, allowing high speed while the kite is not too ragged and remains controllable. Further tests and devices are required to improve stability which is naturally improved by the high speed.

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Cool @PierreB
Are you able to alternate between the two states yet?
E.g. re-engage the outer power lines after a period of flagging out?

Not yet. Maybe I can manage to chain depower and power phases manually, but without holding the camera, and without rewinding, risking tangling the lines.

On the video we can clearly see the kite spinning, which is not necessarily good when the power is regained.

In the real world (?) the 3rd line would be integrated into the (KitePower style) control pod as in the sketch above, for several reasons of which avoiding line tangling. Indeed with 3 lines connected to the ground station, it would be a mess.

The central bar aims to limit kite deformations and perhaps would facilitate automated control.

I think an automated system should act on the steering suspension lines in order to stabilize the kite during reel-in depower phase.

For the current 3 line kites, the 3rd line is connected to several suspension lines on the trailing edge. Perhaps this could lead to some variant without bar.

I think anything flapping is deemed to have a lot if drag and also a lot of wear.

The greatest cause of wear is physical wear from dragging around on the beach or sitting flapping in the breeze and general mistreatment.

A rigid kite will often quite easily allow a very low lift/drag setting by setting the angle-of-attack very low. For rigid wings we have very precise control of AoA so this is quite feasible. Also multi foil rigid wings allow the wing configuration to change which is another way to achieve close to zero drag.

For soft kites this is very difficult. A single skin kite needs the whole upper side to be underpressure to avoid flapping. This inevitably leads to a lot of lift which again messes up the retun path.

A soft kite with chambers (foil kites) needs the internal pressure to be larger than the impact of turbulence, and as you show, it may not even hold its shape with super low AoA even with a dedicated bridle.

All in all makes on think whether yoyo is worth it without a rigid kite. Maybe consider what rigid is. An inflated leading edge may be a good compromise to a 100% soft kite for this application

TRPT [rotary torsion] dont have these issues to deal with. Just saying.

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I tried a similar experiment with the semi-rigid kite below, simply walking against the wind while holding it by the 3rd row attached to the nose of the kite. The result was similar: it spins in all directions, having no stability.

The explanation is quite simple: with a neutral AoA, the apparent wind attacks the lower surface then the upper surface alternately.

Not only the AoA must be positive, but also it must be positive enough to prevent pitch oscillations leading to chaotic flight.

That said the reel-in conditions are quite different since after the reel-out power phase the kite returns to the starting point by a descending slope steeper than the normal glide angle in free flight, mitigating said oscillations, but increasing aerodynamic complexity (variable apparent wind vs real wind) and likely drag by higher AoA (by apparent wind) excepted if the kite flies in a dive.

As a result it looks like the big loss by reel-in phase cannot easily be avoided. This is what the curves show for soft or rigid wings over several years (links already posted, soft Table 8 page 44, dataset 5, rigid from 3:07, soft figure 15, rigid figure 18).

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If a flexible kite can behave like a flag during the reel-in phase, it will be good for a more or less significant reduction in the drag (which remains nevertheless quite important), although bad due to more wear. Using a rustic kite a parachute in aligned (no crosswind) flight would likely not generate positive energy because, even in “flag mode” during reel-in phase, the apparent wind speed (real wind speed + reel-in speed) cubed would lead to a more or less equivalent power consumption than the generation power resulting in a far lesser apparent wind (real wind speed - reel-out speed) cubed.

Flag (C values from Munson et al., 1998)
A=DL
C=0.07 if L/D=1
C=0.12 if L/D=2
C=0.15 if L/D=3

The drag coefficient decreases as L/D decreases, possibly due to a lower flapping amplitude.

I agree. But in the end the control to keep a minimal traction remains required, and the configuration becomes close to that of @Kitepower 's leading edge inflatable kite (not like a flag).

Today I experimented the same 2 line kite with a 3rd line connected to the trailing edge (photo and video below): the kite remains probably uncontrollable while drag is a little mitigated (as for a flag), but not so much.

So I understand better why diminishing time and traction during reel-in phase is so difficult, if it is really possible.

Using the exact same principles described.
It a oscillating version of the thing you describe.
Just another visual to hon in on what your after.
It a fixed system that moves freely. Found it in one of my many searches. Read the post above reminded me of it.

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Airborne wind energy systems using yo-yo reeling method operate by cycle comprising the power reel-out phase and the recovery reel-in phase. The latter consumes a lot of power and lasts a long time. Different means are tested in order to compensate for these losses of average power: the current means by reducing the surface of the kite by deformation seems promising. These experiments are preliminary, and the variation in the size of the kite is barely visible but is there. Some depower configurations like in accordion style are designed to increase size variations.

This video and the reeling parachute system is described on Zhonglu High Altitude Wind Power Technology - 中路高空风力发电技术 and Slow Chat - #71 by Windy_Skies.

An example of depower (for reel-in phase in yo-yo mode) is shown just after the start of the video, then along it. I wonder what force the inverted and flapping parachute exerts: maybe its drag is like the drag of a flag, so not quite neglecting.

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Pages 112-113: Ingo Mewes, AirWing, a self-regulating control system for kites:

AirWing is part of the ground station. It essentials consists of 6 pulleys, 4 of which are fixed and 2 are movable with a central spring element.

It is a mechanical structure aiming to assure power, depower, power and steered, without current, without pod.

Translation:

Armand Torre is the man who designs our kites. We owe him the invention of the Bird mode on our SeaKite kites. An ingenious invention that allows to reduce the power of the kite during the launch and recovery phases.

Other videos:

The Birdkite is a kite specially designed for towing boats, it is launched in single line thanks to its capacity to keep the auto-zenith and then to fly it in 4 lines.

My comment: it is a great invention which also would allow to solve (automated) takeoff and landing for flexible kites, power (reel-out) phase in crosswind flight by being fully deployed, depower (reel-in) phase by being reduced, and also overspeed protection.

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