Flettner Rotor Sails Again?

Very interesting. This reminds me Co-Flow Jets.

I would also like to mention that a multi foil wing could give us lift coefficient of >3 without using any fans, so that seems like an improvement over a few of these.

Brings back this paper to mind, promising C_L of > 5 and still glide ratio > 10 [PDF] Drag power kite with very high lift coefficient | Semantic Scholar

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Indeed, High lift coefficient and biplane kite mentions this @floba ’ publication but the link seems to not work now.

The pdf is also available on https://www.researchgate.net/publication/320742362_Drag_power_kite_with_very_high_lift_coefficient .

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Thanks Pierre: Downloaded the PDF wondering “what is a drag power kite?”, then saw the references to Loyd and realized it was all about a Makani-type setup. Looked like maybe computer study? Sorry I stopped reading after skimming a bit. I’d still say that “drag” terminology could use an update or a “re-take”. Any wind energy person reading it would be thrown off. It also makes me wonder, considering the high power predicted, compared with the actual results attained by Makani, how accurate such computer simulations are… :slight_smile:

Should be «hovering» kite power plant :stuck_out_tongue_winking_eye:

Page 291 (or page 2 for ResearchGate publication):

Because the power increases cubically with CL and decreases only quadratically with CD

Hi Doug: What interests me in this paper is the development of very high lift coefficients.

As already discussed the term “drag” in M.Loyd’s context has not the same meaning than in current wind energy. In order to align AWES with regular wind turbines @tallakt uses “hovering” for 16/27 (Betz limit) stationary swept area devices, and “bounding” for 4/27 (1/4 Betz limit) downwind swept area (yo-yo) devices.

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This is s fallacy and oversimplification. C_L and C_D are highly coupled. Its just a matter of finding an optimum for a given wing configuration. If you simplify to just looking at C_L and then leave the rest to best effort, you are probably not arriving at the best configuration imho.

For these ship applications high C_L does seem useful as larger wing areas impose other limitations (eg moments, mass, weight, cost, height and handling). Also drag does not seem highly important.

Also the amount of energy invested to maintain the wing is hugely important for ships.

OK there seems something fishy about this.
For example they like to place discs at the end to counteract “leakage” of the pressure differential. This reminds me of electric cars taking extra steps to eliminate drag - steps most cars wouldn’t need to bother with. We could place discs at the ends of jetliner wings, or wind turbine blades, helicopter rotors, etc., etc., etc. But why don;t we? Because we’re not desperate? The high coefficients of lift sound very impressive. And they say it “can’t stall”. Except there is a certain speed where rotation matches the translational speed where all lift goes away? I haven’t read the whole paper yet, but it is a good read. Have not gotten to the wind turbine section yet. But I think I remember seeing one before, maybe with the discs at the ends of the tubular blades? This sounds so compelling, but there seems something amiss: if it is so great, why aren’t people using it? I mean it has been around since the 1800’s. Is there too much drag? There has got to be a catch. Very interesting though. And if you know anything about airplane wings, “circulation” is a major factor, similar to a rotating (circulating) magnus/flettner wing. But using an airfoil, we can get self-powered circulation without spinning anything… Interesting subject but, so far, looking for the holes in it. Hard to believe it would not be used more by now if it really was so compelling overall.

An interesting publication including measured values (see Fig. 4, 7, 13) :

https://www.sciencedirect.com/science/article/pii/S0167610518307396

For the case with endplates (size = 2D), and for 1<k<3, it is shown that both CL and CD are larger than the results reported by (Reid, 1924), (Thom, 1934), (Badalamenti, 2010) and (Zhang and Bensow, 2011).

The pdf is available on request on:
https://www.researchgate.net/publication/331733434_Experiments_on_a_Flettner_rotor_at_critical_and_supercritical_Reynolds_numbers

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I have read a little more of that typewritten paper from the 1980’s, and came across a brief mention of a known drag issue with flettner wing airplanes. It was quickly glossed over as one of those “everyone knows this” issues, as though it scarcely merits being mentioned. In other words, while flettner/magnus rotating cylinders DO achieve very high coefficients of lift, they also entail high drag, to the point that the high drag is why people are not using them…
The reason I suspected this high drag is the increased frontal profile over a standard airfoil, and that the downwind (leeward) (rear) “edge” of the flettner/magnus rotor is not even an edge at all, but is as rounded as the leading edge.

I sense the flavor of high drag just looking at one. I saw a passage mentioning a prototype wind turbine offered a low-cost, high-efficiency configuration that could survive high winds, and thought "Easy to cite all these combined supposed “advantages”, but I’m not so sure I believe it actually offers low cost, high efficiency, or high-wind survivability. If it does offer high wind survivability, that could be the deciding factor. (Still, you need basic efficiency.) Most wind energy wannabe developers and even actual developers ignore the high-wind survivability requirement, until it ruins their business. They develop a turbine that tests out well in normal winds, but are surprised what happens when a storm or sustained high-wind event occurs.

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“Wingsails has generally more efficiency and wider working range”

Flettner Rotor vs Wingsails[ENG]20180315 (naos-design.com)

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Hi Doug: concerning Magnus-based effect Flettner rotors, the very high drag coefficient is well known.

As represented on the figure 4, the lift coefficient goes from 2 to 9 for a spin ratio of 1 to 4.5 times wind speed.

As represented on the figure 7, the drag coefficient goes from 1 to 3 for the same spin ratio of 1 to 4.5 times wind speed.

So the glide number would be 2.5 or 3 at the best.

For an aircraft, this is obviously not a solution.

For a boat, it will be difficult to benefit from both the lift and drag forces, except perhaps by going in broad reaching.

For a yo-yo (reeling, bounding) AWES, the drag would add beneficial force (as for any wing, due to the low elevation angle) during power reel-out phase.

And also an AWE use as a lifter kite and perhaps as separator is a possibility.

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Thanks John, (or is it Dave?) good reference.
This seems to verify my skepticism over the flettner rotor concept.
Some may say the extra drag is not a problem for elevating a wind energy device, however that would lower the tether angle.

The reference cited above, if true, (measured or “modeled”?) would place the flettner rotor in a similar position as a darrieus wind turbine: An “improvement” that is worse. (By the way, how many Darrieus airplane propellers do you see? Could there be a reason?)

So why do trained people who “should know better” keep glomming onto the Flettner-rotor-as-sail concept?

Well, I believe it is psychological, and I have come to see that much of science and engineering are just as subject to psychological manipulation as any other field, maybe more, although the people in these fields do not suspect they are so vulnerable to Bulls***, but they are.

For example the food industry, Proctor & Gamble in 1911, found they could replace lard with hydrogenated vegetable oils (Crisco), so they funded the American Heart Association and pointed them toward promoting the idea that ever-increasing heart attacks were caused by animal fats & butter, and the hydrogenated vegetable oils were healthier.

Turns out none of that was true, but they had even heart surgeons parroting that line, making more money than ever, since heart disease kept getting worse. The food conglomerates removed animal fats and replaced them with partially-hydrogenated seed-oils, sugar, and high-fructose corn syrup. That made everyone fat, and heart attacks increased. And we now know that the seed-oils went rancid and cause cancer.

Today many former heart surgeons have abandoned surgery in favor of helping people adopt (real, not fake) heart-healthy diets around more animal fats, more vegetables (NOT “vegetable oils” which are really half-rancid seed-oils) and eliminating sugar.

SO, there is an example where “official science” (which “everyone” agreed with) was 100% wrong, and nearly everyone believed it, and due to inertia, many still do!

As far as explaining the urge toward flettner rotors, I’d like to point out, or maybe add, an aspect of what I call “The Professor Crackpot Syndrome”:
“Interesting” and “unexpected” - effects that seem to defy the laws of nature are of course “interesting”. And if something like a spinning cylinder can be shown to generate high amounts of “lift”, then that seems “unexpected”. So when we combine “interesting” AND “unexpected”, those two combined characteristics carry a lot of emotional weight, and despite all our logic, we humans tend to make decisions based on emotions.

So if something is seen as very interesting and unexpected, people are willing to stop asking any further questions (such as “Yeah, but is it BETTER?”), and just accept that such a “new” principle MUST (?) be worth implementing, and we’ll worry about the actual numbers later.

When you combined that aspect of “The Professor Crackpot Syndrome” with what I playfully call “Global Warming Derangement Syndrome”, where we tend to accept any possible bad idea, if it could possibly be rationalized, or at least funded, on the basis of “saving the planet”, you can understand why people are using flettner rotors on ships instead of sails, since sails are so “old-fashioned” and seem to be “long-disproven”, being seldom used in commercial craft.

So, while the flettner.magnus spinning tubes date from before aviation got started and the shaped airfoils used on windmills for 1000 years were as yet unrecognized for flight uses (even though birds’ wings use airfoils), they spinning tubes are somehow seen as “new” or “cutting edge”, when in actuality they predate airplanes and are a 170-year-old, mostly-failed idea.

Not to say there is no use for them, but they are in fact old news - VERY old news, not some new “breakthrough”.

The reference serves mostly as a commercial brochure as they are not presenting the inputs to their comparison rather just what they conclude, which, no surprise, looks good for those who wrote the brochure.

Relevant input in this case would be lift coeff, drag coeff and power expended to run the sail. Ideally as some kind of polar plot

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https://hal.archives-ouvertes.fr/hal-01759173/document : Fig. 1.24 represents a vertical trajectory in yo-yo reeling (bounding) mode, where the elevation angle (which is also represented in Fig. 1.17) increases during ascent, reaching about 45-50 degrees at the top (rather high value), and only about 10 degrees at the bottom (rather very low value). This vertical trajectory would allow to maximize the potential, putting the bottom of the trajectory far to the winch.

Ding-a-ling-a-ling - “Er, um Hello? May I pleathe thpeak with Profethor Crackpot? - uh Hi, uh yeth, Profethpr Crackpot, I’m reading your typewritten paper from the 1980’s on Flettner devitheth, and I’m a bit perplexthed at what I thee in Fig. 15, on page 43 of your typewritten document (or page 51 of the PDF file):
You thow two windmillth, one being twithe the diameter of the other (4 timeth thwept area), claiming the Magnuth devithe at 1/4 the thwepth area nonetheleth produtheth the thame amount of power ath the 4 timeth larger regular wind turbine. Can you pleathe comment on how thith relateth to the Betthz coeffithient? Athuming the regular turbine ith operating at the Betthz coeffithient, are you thaying the Magnuth turbine operateth at 4 timeth the betthz coefficient? Thith would mean the Magnuth rotor captureth more than twithe the power actually contained in the wind itthelf - how ith that pothible? Thank You.” :slight_smile:

https://hal.archives-ouvertes.fr/hal-01759173/document Fig. 1.24 page 25. Explains of the authors below, page 24:

In Fig. 1.24, one can see the vertical trajectory of the MW scale system. We also
present a comparison with an equivalent conventional wind turbine. Even though
the Magnus effect-based system is less efficient to capture mechanical energy from
wind, it produces the same amount of power as an 80 m diameter wind turbine
(around 1.4 MW for 10 m/s wind speed) since it works on a larger area. In other
words, an 80 m diameter wind turbine works on 5000 m² with a power coefficient
cp = 0.45 where the Magnus effect-based system works on 13940 m² with a power
coefficient cp = 0.157.

This is very clear and understandable.

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Hi Pierre: I was referring to this typewritten document from the 1980’s, on Flettner devices, Fig. 15, on “page 43” of the typewritten document (or page 51 of the PDF file): https://apps.dtic.mil/sti/pdfs/ADA165902.pdf
The document you are referring to seems compelling. I would think if AWE is that easy then we can all stop working on it now, since they have such a perfect solution. I think we’re done here/ Let them come forth with it and we can all relax. Global warming is now solved. :slight_smile:

Hi Doug, now I see what you are referring to. This is not the document I quoted, at least not recently.
As you point, Figure 15 page 43 leads to a near impossibility.

Yes that document I was reading was a few days back or maybe even a couple of weeks in the thread. Makes good reading until you see impossible statements like that and wonder how factual the rest of it is.
Funny how you can identify when technical “information” about wind energy comes from people who don’t know the facts about wind energy.