A (train of) parasail(s) as AWES ?

Engineering System Design
24

An analysis of the figures and references given on pages 18 and 19 of the “Parasailing Safety” document did not allow me to corroborate the enormous Cd of 2,727 mentioned here. I deduced values ​​less than half.

Some investigations in the meantime, about not steerable square rescue parachutes and steerable square and Rogallo rescue parachutes.

This not steerable square rescue parachute seems to have a far higher drag coefficient Cd (perhaps about 3.5, guessing projected area as for other parachutes ?) than a classic parachute (perhaps a Cd of 2). But their extrapolation would not lead to kites because they have no lift.

About steerable square and Rogallo rescues parachutes: as they are steerable, they have a (very small) glide ratio. However this low glide ratio of 1:1.5, so 0.666, common to both (for the Rogallo, a likely similar glide ratio is achieved with the pre-brake device, see also the video, with a possibility to achieve 1.2, “finesse 1.2” in French by steering), can be good for AWE once extrapolated into a kite, resulting in an angle of elevation of 33 degrees and 40 minutes, avoiding too much losses by cubed cosine.

Some complements: Airborne Wind Energy System based on steerable square rescue parachute, and above all Airborne Wind Energy System based on steerable Rogallo rescue parachute (DOI : 10.13140/RG.2.2.32994.13762). For both, and with a value of 0.666 for the glide ratio, from no complete data (no projected area given) I deduced, more or less wrongly, a thrust coefficient Ct about 3.425, a drag coefficient Cd about 2.86, and a lift coefficient Cl about 1.9.

The Rogallo rescue seems more suitable for an AWE use: Rogallo parawing kites were used.

If we take a look on the specifications page 6, we can see a low sink rate of 3.93 m/s, loaded with 115 kg, with a horizontal speed of 2.62 m/s for a glide ratio of 1/1.5 (0.666), elevation angle of 33°40’), calculated Ct, Cd, and Cl similar to the values above; then a still lower sink rate of 3 m/s, loaded with 90 kg, horizontal speed of 2 m/s, calculated Ct of 4.56, Cd of 3.8, and Cl of 2.53.

These values ​​seem very high, and I could have made a mistake somewhere, not having the value of the glide ratio for a given sink rate, nor the projected surface area, which I deduced arbitrarily.

Another point from the last example: it seems that as the load increases, the coefficients decrease. We can perhaps deduce that the force of the wind, much greater than the indicated loads, would reduce these coefficients even further.

However, if the high coefficients are real for sufficiently large units, tether-aligned devices can perhaps be an alternative (maybe simpler and consuming less space) to flexible crosswind kites, knowing that there is already a working parachute system.

25

I think a C_d of 3.5 would not happen. But sometimes the coefficients are scaled by other baselines than wing area…

26

This seems likely, but then how do you explain such low sink rates with such small surfaces, knowing that the part of lift is minor for a glide ratio of 0.66 (1:1.5), and zero in its absence?

No coefficient is mentioned on the documents and pages linked to my comment.
I tried to deduce them from the sink rates, the given loads, and the projected areas which I deduced arbitrarily. That said large Cd about 3 were tested.

But I think you are right. I had noted in particular:

About the “enormous Cd of 2,727 mentioned here”:

There may be a relationship between the coefficients and the load-to-area ratio, Reynolds number and scale.

I found the thrust and drag coefficients very high, as well as the lift coefficients for steerable rescues.

I would like to know more about this topic.

27

I wondered how, alongside particularly high drag coefficients Cd, such high Cl could also exist, knowing that these are flexible wings.

The reason seems simple: these Rogallo rescue parachutes only stall at an extremely low speed and could have a very high angle of attack (being able to fall almost vertically as a fully drag-based parachute) that paragliders would be far from reaching, paragliders having also a far higher glide ratio. Figure 3 on the document below shows an angle of attack less than 10 degrees.

Experimental Study of Paraglider Aerodynamics
Authors: Sarah Becker and Paul Bruce
DOI: 10.13140/RG.2.2.33674.16321

Longitudinal Flight Mechanics of Paraglider Systems, see the figures 22 and 23 showing Cl and Cd decreasing as the speed increases.
The author is Rene Falquier:

28

Maneuvers on the ground, like a kite.

First test of 1/3 scale Rogallo chute

Siklóernyő mentőermyő - Vonblon Papillon rogallo (Paragliding reserve/rescue parachute)

Questions: is a Rogallo rescue stable enough for kite use, despite the low (about 30-35 degrees) elevation angle, but sufficient and suitable for AWE use to avoid excessive cosine losses? What happens to its Cd and Cl at different wind speeds?

29

Cd and Cl may be fairly constant but the effective kite area will probably change due to kite distortion.

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30

Hello @gordon_sp , you raise a good point. Additional suspension lines could then prove useful.
Moreover, but as a secondary factor, the lift coefficient can be changed (reduced) as air speed increases, as with paragliders, although the lift component is not predominant for Rogallo rescues.

31

This thread is about a train of parasails.

One thing that bothers me about single skin kites (like the Rogallo) is stacking them in a train.
The problem I find with it - Once there’s an upper kite, which has a tether line to a kite below…
Where do you attach that tether on the kite below?,
So that any misalignment or movement in the upper kite does not crush or diminish the kite below
e.g.
Either
A The upper kite tether connections crush the inflation of the lower kite skin. This points towards using only a single line connection or some sort of rigid spreader on an upper connection set
or
B Misaligned direction of the upper kite (absolutely to be expected) pulls the lower kite sideways changing the air flow over the lower kite, rendering it less performant than usual and potentially deflating it.

We have to have a better way of stacking single skin kites for this to work well.
Passing tethering on main lines through the skin without perforations and form compromises is the ideal
Where do you pass the tethering through the skin to avoid steering and crushing of the lower kite?

Pure arch forms and C shape surf kites can get around this by passing tethering up the outside edges.
Box kites often stack by having holes in the middle of the frame to pass tethering through

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32

Good observations @Rodread. I intended to investigate very high drag parachute kites based on parasails, Rogallo rescues…, for improving the power/area for tether-aligned AWES. Then I tried to see if trains of these kites was a possibility, being based on the Chinese parachute-based AWES.

For symmetrical parachute kites (toys flying like parasails) I had this problem: to solve it I installed a central line in each parachute except the upper parachute, and in the extension of the tether, in order to avoid deformations caused by traction on the upper stage.

In one way or another, it is necessary to block the longitudinal extension of the parachute: this is what the Chinese team did with more suitable means such as pods blocking and sliding alternately in order to achieve the reel-out and reel-in phases.

Instantané 4 (15-04-2024 13-25)
Instantané 4 (15-04-2024 13-25)1920×1080 54.2 KB

I think that for single line Rogallo kites, we should have a third group of suspension lines whose lines end at the central “keel” or “spine” over its entire length, then leave again joining the Rogallo kite above, and so on. This would perhaps allow:

  1. to create a train of Rogallo kites;
  2. to sketch a depower system for reel-in phase, in yo-yo mode, releasing the two other group of suspension lines.
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33

This is a plausible explanation.
Another explanation is that sometimes the manufacturer gives two sink rates for the same steerable (Rogallo or cruciform) parachute: one for the maximum load, the other for an average load. The sink rate with the maximum load is proportionally even higher than the sink rate with an average load: the explanation may be a slightly higher forward speed (and therefore a slightly higher glide ratio) leading to an additional sink rate from a certain value, this data not being precisely provided.

34

The issue of wake effect can be harmful for trains of parasails or other parachute kites or any static kites.

This issue is mentioned on https://repository.tudelft.nl/file/File_4816ada5-be86-4040-b6f7-da37711e3bf9?preview=1 (from the Book of Abstracts from AWEC2024).

I put again an excerpt of this one page PDF:

With only one high Cd (at low wind speed) giant unit, the power available in the wind in pumping mode could perhaps be higher than expected (4/27) if we don’t take account of the surface of the disturbed airflow around the parachute.

But the wake effect would lose a lot of energy density due to the need for spacing, if confirmed and depending on the degree.

I wonder if the wake loss from tether-aligned kites in trains would not be even worse than for cascades of crosswind kites like those of Kiteswarms.

Indeed parachute kites block a significant wind area, perhaps more than crosswind kites which work like blades of wind turbines at equivalent wind power.

And it does not seem that the wake effect penalizes the SuperTurbine ™ rotors…

35

A solid wall must block more wind to the solid wall directly behind it than any other configuration where the wall is smaller, moving and going over a larger swept area, and not directly in front of the wall behind it. I’d guess the wake from a wind turbine is larger but more diffuse than from a wall. I’d also guess that a single large wake dissipates slower than multiple smaller wakes.

You could look into wake dissipation and shape between different kinds of objects, kites, wind turbines, trees, buildings…

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36

Parasails and fast blades have very different wake signatures.
The more static nature of parasail flight than blade flight and their smoother “tip-less” design suggests to me that vorticial flow will be less tightly intense and distinct in the wake field of a parasail (I’m often wrong of course)
Any wake field disturbance progressively diminishes and merges with background wind flow with distance downwind from the initial disturbance but I am no expert in the exact dynamic.

Parasails also typically have quite a lot of vent holes - they’re certainly not a flying brick wall

If it’s OK I’d like to insert a response from Perplexity.Ai here - on a question concerning
The relationship between wake, vorticity, and turbulence in the flow field behind a wind turbine blade
(Loads of lovely pictures … all the best science has lovely pictures )
https://www.perplexity.ai/search/relations-ship-between-factors-HsEzsHUgQN2lxZzIDbir4Q

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37

I must say I was surprised to learn, years ago, of the amount of mutual interference in a kite train. :slight_smile:

38

Is it not the same for the rotors of SuperTurbine ™?

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Hi Pierre: The data shows otherwise. The source of the information that a kite train really requires a lot more spacing between kites than one would expect came from Santos on the old forum. I took it at face value since his stated expertise was kite-flying, which I never doubted. I think he may have even cited a more authoritative kite source. But I really can’t speak to that alleged phenomenon beyond that limited supposed information from years ago, of which I have no specific experience nor real verification. You’d think after 16 years of “kite experts” trying to make electricity, basic stuff like that would be well-established by now, wouldn’t you? Anyway, one reason I was surprised to hear it was the good results we were getting from ST.

40

Hi Doug, It is entirely possible that the behavior of SuperTurbine ™ is quite different from that of a train of kites.

41

IIRC that was based on a YouTube video where someone flew a train of powerkites at a spacing between kites of no more than a few meters or so. That would obviously be too close.

42

This message referred to the 25 Flexifoil Stackers.

43

Oh Yeah, thanks for reminding me - that was the source I was referring to.
I noticed the wind was not very strong during this video, so that might be part of the problem…
Toward the end, the guy mentioned the low wind speeds before explaining how normally he waterskis behind just 10 of these 6-foot Flexifoil kites.
He did not mention the spacing of the kites when waterskiing.

I’ve watched plenty of people flying stacks of kites years ago when I lived near the beach.
Never really gave too much consideration to the spacing, although “it seems obvious that” at some point, the kites could be spaced too closely for optimal performance.

The reason I placed “it seems obvious that” in little finger-quotes, is that for me to even utter those words hits a nerve. I can’t tell you how many times I’ve heard those exact words, and other similar phrases from the “Professor Crackpots” of the world, when they share their deepest thoughts on wind energy. Like “It seems obvious to me that expanding the speed range of wind turbines is a breakthrough” in response to a recent article I shared about turbines claiming continued output during category-5 hurricanes, or “It’s obvious that there is a lot of energy seeping through that rotor with only 3 slender blades” from some know-it-all with a PhD - (EXCEPT - dingggg - you guessed it - his PhD was not in wind energy.)

In fact, I’ve heard it so many times that I automatically add a “Professor Crackpot” lisp:
“Well it theemth obiouth to me that…”

It gets really funny when the subject of vertical-axis turbines comes up.
They always repeat the same uninformed narrative:

It’th rethponthive to windth from any direction, tho it ith thuperior in urban environmenth!"

Uh-huh - sure thing, Profesor Crackpot! :slight_smile: