Low radius loop

I think an implementation of @gianni 's RotoKite could fly a pattern similar, similar use of space (maybe even slightly better) to the one you show here @PierreB

Where you depict a single kite pulling low radius loops for Yo-Yo phased extraction… Multiple Kites could easily be coordinated and set to perform that manoeuvre.
A Kite Turbine rotor has similar proportions to the the wing and loop you depict.

Assuming your Low Radius loop kites are bound together rather Like Daisy Kite Turbines, but just set on a single line to spin and pull like a phased pumping kite. (This is what Moritz Diehl assumed I was doing first time he saw a Daisy Kite Turbine… So it must be good. )

This has already been done
Our old kite turbines have been adapted in the past to fly on a single line

It would not take a lot of actuation to coordinate phased operation of multiple sets of these turbines .

The whole system may even benefit from a multi-line or coaxial pulled line where counter-rotary sets avoid imparting twist to the pulled line.

My low radius loop kite does the same than a purely rotary kite, by using less material, only one wing, not inner parts connecting blades or kites which can be problematic for scaling, so roughly the same material used for eight-figure crosswind kite, perhaps a simplified automation management, but not as simple as for a rotary kite. Moreover it is possible to change the diameter of the loops according to the circumstances.

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The low radius loops (about 3.6 m diameter) of the 0.7 m² and 1.20 m span kite were realized in passive mode, by pulling on one of the two lines and keeping the position. The traction was strong (about 40 N with 4 m/s wind speed) while the loops gradually (about 1 m/s) descended.

These last days I tried to keep the altitude of the same kite by active manual steering, pulling (to rotate) and pushing (to ascend) the same handle, or alternating with both handles, which is the same. As a result rotation was a little broken at each loop, and the traction was far lesser, at least with a low wind, less with a strong wind.

I don’t still know the behavior of a large kite in the same comparative conditions (passive loops while descending, controlled loops while keeping or increasing altitude).

Today I experimented loops with another kite, being about 1.3 m², so twice the area of ​​the original kite, and a flat span of 1.8 m instead of 1.2 m.

The behavior was very different: the loop diameter generating a strong traction had to be proportionately much larger, 5 to 6 m at least. And, with large or low radius loops, the kite descended much faster, despite a slightly more favorable surface-to-weight ratio.

I don’t know yet whether to deduce that we would lose any real advantage (same power by flying in low radius loop, passively descending, hence a better power / space ratio and an easier control?) with a largely scaled wing.

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Still tests today with the same wing of 1.3 m², more in-depth because I no longer had the sun in my eyes. I added the fact of walking in the direction of the wind to represent the reel-out yo-yo phase, without noticing any particular difference except by a decrease in the expected traction.

The preceding observations are confirmed while being qualified by the following observations.

The kite seemed to descend more slowly with small loops generating little traction. But I cannot give a valid reason, having observed different things in the descents of the initial kite according to the tests, and sometimes no descent (because of a thermal lift?).

The initial kite was a Peter Lynn Vibe of 0.6 or 0.7 m² and 120 cm wingspan flat. Loops of 3 m in diameter should correspond to loops of 4.5 m in diameter for the kite of 1.3 m² and 180 cm wingspan flat which is an Ozone 1.5. As the Ozone lines are only 18 m instead of the 25 m for the Vibe, the larger kite (Ozone) was also closer. So there may be some error in my assessment: after all 4.5 m is not that far from the 5 or 6 m that I indicated previously.

And even if the loop is 5 or 6 m in diameter it is still narrower than a small figure of eight.

What I like about the loop, apart from saving space (without being huge), is the regularity of traction. In fact, the kite remains equidistant from the point of traction in the raised center of the flight window, unlike what happens with a figure of eight.

All this remains to be roughed up and tested with kites of different sizes and aerodynamic characteristics, and different winds.

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Today I experimented an old 2-line kite of 3.75 m² and 4 m span, wind 5 to 10 m / s. Operation was correct despite wear, still pulling strong at the zenith.

Loops were about 3 to 4 wingspans (12 - 16 m diameter). At the end of two loops the kite was already largely lowered. So I think I can say that the passive descent while turning is not a good solution because the kite goes down too fast. A swash plate in @someAWE_cb style for control could perhaps be adapted, or something simpler and a swivel.

I continued the tests until I had to let go of a handle because of the pull too high for me (and for one of the two bridle knots that went off) during a loop, and comparable to that which resulted of a long trajectory as achieved in a figure eight.

Page 12, from the table 2:

Flexible wing Lift-to-drag ratio 6
Rigid wing Lift-to-drag ratio 20

On this table, the Max flight circle radius is about 5 wingspans for both rigid and flexible wings. I think things are perhaps different. Also take into account the tether drag.

https://www.skybrary.aero/index.php/Radius_of_Turn :

Radius of turn is dependent on both airspeed and bank angle. The radius of turn at any given bank angle is directly proportional to the square of the airspeed. Doubling the airspeed results in a radius of turn that is four times greater while tripling the airspeed would result in a radius that is nine times greater. Conversely, if the aircraft speed remains constant, increasing the bank angle will decrease the turn radius.

As a first approximation the turning radius of a rigid wing will be higher not directly because of its higher mass, but due to its higher speed resulting from higher L/D ratio. The same when comparing flexible wings with different L/D ratios. So a good efficiency could result from an accurate balancing between a high enough L/D ratio, but not too high in order to keep a tight turning radius and gain some space.

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Kite turbines kinda break the rules here
The bank angle of the blade on a kite turbine (outer tip down to ground station) would promote the opposite turn - out of the loop. The tethering between blades keeps the loop tight (tensile and size)

Yesterday I experimented the same old 2-line kite of 3.75 m² and 4 m span, wind speed being 5 m/s. As the wind was constant during some minutes and not to high (for me), not too low, the experiments were more precise, although not measured. 4 wingspan loops were required for an optimal traction, more or less being equivalent of what I obtained by using large figure-eight paths.

Picture page - Kitemill (pictures 13 and 15): loops are used.

I think that loops provide more regular power because the wing is always at the same distant of the middle of the flight window. But I don’t think we would save much space by using loops. Indeed the spacing between the units is necessary even if the loops are tight. And by using figure-eight paths, the second loop of one unit could match the first loop of the next unit, even more with large paths.

Just sensational work.

I’d like to fall back on this older thread which quite relevant, I have reviewed some studies graciously provided by this group.

In @Florian Bauer’s PhD dissertation from TU Munich, he mentioned that the ideal tether inclination and length, along with the most efficient altitude for kite systems, may not be as large or as high as previously thought. It seems that we could benefit from focusing less on issues like tether weight and drag, and instead, concentrate on designing more efficient, low-altitude kite systems.

One aspect that caught my attention in the dissertation was the comparison between loops and figure 8’s. @Florian Bauer suggested that the Makani systems, which use circular paths, require a complex hardware setup to prevent tether twisting. He also mentioned that figure 8’s might be a preferable option, considering their relative simplicity and the avoidance of potential patent issues.

However, one area I felt could have been explored further was a more in-depth analysis of the pros and cons of loops vs figure 8’s.

Looking at the discussions in the AWES Forum, particularly on @PierreB’s “Low Radius Loop” thread, it was interesting to see how the community is experimenting and learning together. @tallakt made a noteworthy point regarding line twisting and suggested that alternating loop directions could be a potential solution.

The results from @PierreB’s experiments with low radius loops were quite thought-provoking. The experiments showed that these loops could potentially allow for larger kites and produce smoother forces. Yet, the point that tight loops might not necessarily save as much space as initially thought does raise some interesting questions.

Also, @kitefreak’s positive experience with low radius loops further solidifies the case for this approach.

This exploration into flight path mechanics is proving to be quite a learning journey. The discussions around tether weight, drag, optimal flight paths, and the impact of patents on the design choices are all significant considerations that will help guide the design and optimization of future AWES projects

Here is an example of how kite surfers are already paving the way, Triple loops.

We can see the measured traction (at 0:26), and also wind speed at the beginning of this now old video (2012) which is linked to some other comments including Low radius loop - #4 by PierreB.

There is no power for an AWES if the lines are static such as this, The arrangment as recoreded would be perfect for downwind sailing. Exceptional actually. For an AWES however power from the kite is associated with the tension on the lines being let out. So the question here is; what is the optimum shape/flight path as the lines extend under tension?

Are we going for a long cycle where we let say 100m of line out then recover the kite? or is it better to have a kite that oscillates between 50-60 meters and recoups/resets after every loop or so? Perhaps there can be a scheme that takes advantage of gusts and subsequent lulls?

My tests were aimed at measuring the force I indicated, not directly the power in reeling mode, which can however be easily deduced by multiplying the force (N, so here about 40 N) by the reel-out speed (1/3 wind speed) and then by 4/9 because of a lower apparent wind. Result with 4 m/s wind speed: 23.7 W. (L/D)² was about 7. We then come back to the classic formula, here without counting cosine losses: kite area (about 0.6 m²) x 1.2 (air density) x 2/27 x wind speed³ x (L/D)².

I’ve been diving deep into your tests on kite force and the way you’ve approached power in the reeling mode. Your results, especially the 23.7 W power output at 4 m/s wind speed and the L/D squared value around 7, got me thinking.

I’m playing with an idea, kind of like how a clock’s escapement mechanism works. Imagine a system that adjusts its resistance when the kite’s tension goes beyond a set threshold. So, if we hit a certain torque, we’d let the line out at 0.2 meters every second.

Let:

T = tension
T_threshold = our set threshold

Rate of line release (R):

R = 0, if T < T_threshold
R = 0.2 meters/second, if T >= T_threshold

When the tension (T) increases, motors would need compensate to keep the line payout consistent. This isn’t about spinning faster, but adding resistance.

Breaking it down:

Motor voltage is determined from a raw value and scaled.
Motor current is calculated as: Motor voltage divided by Motor’s internal resistance.
Line tension is then derived from this motor current.
The motor’s effort adjusts based on the difference between the current tension and our set threshold. It’s a feedback loop that keeps the line payout consistent, no matter the pull on the line.

Just like how cruise control in a car maintains a consistent speed regardless of uphill or downhill terrain, the motor system ensures the kite line is pulled out at a steady rate. Even if the kite’s pull varies, similar to a car facing different slopes, the system adjusts itself to keep the line’s release consistent, ensuring the kite ascends at the rate.

When orbiting, the difference between all the control lines remains relatively static, in the video below you see that during a loop only slight inputs need to be made to the tether line line while the other line orbits around

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I measured only the force. You can see on the video (0:26) that the force peaks at 50 N at the top and then gradually decreases as the kite descends, certainly because the wind (being measured at 4 m/s at my height) was stronger at the top, which more than compensated for the cosine loss.

The power is only deduced by calculation, and has not been tested as such.

Measuring force was a prudent move. To emulate drawing energy from the system, one could just walk downwind with the kite. I’ve observed that when looping the kite, it’s smoother if there isn’t a twist build up in the line. The mathematical rationale behind this is still something I’m trying to decipher.

I don’t understand how the tether tension/power is only a function of the kite area and the wind velocity. What if the kite rotates faster and sweeps more area? Doesn’t the power increase?

(L/D)² is in the formula. This is the ratio of lift to drag squared. The higher (L/D)², the more power the kite generates.

To control the loop with a single tether, we can implement a pod like on

We add some weight with the pod, but also save weight by using a single tether (as for SkySails system).

That said OKE Precision Winch "Reel and Rotate" Technology maybe could remove the requirement of a control pod by using cyclic lengthening and shortening of the orbital tether.