Lift and drag in AWE field vs in current wind energy

Engineering Math & Physics
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The title of this topic is problematic because normally the AWE field should be a part of all wind power. However, the respective meanings of “lift” and “drag” seem to be reversed, as demonstrated by @dougselsam in the post I link below:

I link also @tallakt’ reply:

I think that two logics clash: the aeronautical approach developed by the scientific literature in the field of the AWE since Loyd’s publication, and the traditional approach of current wind energy.

And traditional wind turbines don’t fly, and airborne wind turbines seem at the very least limited in power generation.

So it may therefore be a deeper problem than a simple question of semantics.

I suppose that when the analyzes in the AWE domain will correspond to those of current wind energy, something can happen in the development of AWE, particularly in the management of space.

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Pierre: In regular wind energy, a lift-based turbine still has drag. The blades have a lift-to-drag ratio, and the bearings have some drag, but the main “drag” is electromagnetic drag of producing 3-phase (usually) electricity. The machine is still called a lift machine, because it uses aerodynamic lift to spin the generator. Kite-planes driving onboard, small, high-speed turbines, do have drag on the main wing, caused by drag on the small high speed turbines, ultimately produced, as with the regular turbine, by electromagnetic drag of the generators, just with an extra step of blades driving blades through the air (propeller as gearbox).
Just because it has an extra layer of blades driving blades, instead of just blades, does not suddenly, magically, convert a lift machine into a drag machine. You have to realize, wind energy already HAS terminology. Just because some newbie is the next idiot to come in with some whacky new turbine design does not make it OK to redefine all the well-established terminology, as though his idea is so radical that the terminology just somehow doesn’t apply. No, it usually applies very well. So it behooves the purveyor of a supposed improvement to an art, to know the most basic terminology of that art… To not understand such basic terms is reduce one’s credibility. It makes a wannabe innovator seem ignorant. If they arbitrarily want to use the terminology already applicable to their contribution backwards, or sideways, it reveals the next typical whacko wannabe wind energy supposed-improvement claimant who “just won’t listen” to anything or anyone and who thinks the rules don’t apply to him. There is no logic in pretending a makani-type craft is a “drag-based wind energy system”. It is already a lift-based system, all of which use lift to fight the drag. Now the kite-reeling systems do use lift too. So I’m not going to say the kite-reelers can’t call their apparatus a lift-based turbine, but it is interesting that it throws away the high-speed of the blade, really using the lift to establish a thrust or “push”, utilized the way a drag machine utilizes the thrust or “push” from the wind, pushing the ultimate working element, a cable, downwind at a slower speed than the wind, rather than faster than the wind the way a lift-based machine normally operates. In the field of wind energy, even farm water-pumping windmills are termed “drag-based” machines by I;d say about half of the people qualified to comment on it, due to the slow rotation and high solidity, where the blades are more just “pushed” in a circle rather than “flying” as blades of a regular wind turbine are understood to do. For anyone who has ever worked with wind turbines, as the rotor begins to spin up, there is a point where you can sense it metamorphose from simply being “pushed” in a circle, while mostly stalled, to actually flying. It;s like drag…drag…drag…(taking excruciatingly long to catch some real lift), then ZOOM! as it finally develops real aerodynamic lift and quickly accelerates with a huge burst of energy.
That moment is when, for a brief period, the rotor can exceed the Betz coefficient because it has not yet slowed the airflow! Yes you CAN beat Betz, but not continuously, just for a brief burst til Betz gets control of the situation and slows the wind going thru the rotor. I know, I know, nobody knows what the hell I’m even talking about, and I guess that;s my point. Most wind energy wannabes “don’t know what they don’t know”…
Sure, call your obviously-lift-based machine a “drag” machine, and call your machine that uses lift, but operates like a drag machine and really just uses the lift to virtually create the increased blade area of a typical drag-based machine, really using lift to create a drag turbine configuration, and call it yoru lift-based option over the lift-based one you just called a drag-based machine, and mark yourself as the know-nothing-new-guy. Go ahead - whoever wants to not learn the most basic terminology in wind energy, it probably means you really don;t understand the basics, which might explain the inability to power a single house after how many years now…?

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Excepted that the turbines onboard slow down the wing by adding drag. This issue is not really studied in current wind energy because current wind turbines do not have secondary rotors.

Why not name a (crosswind) “drag-based” wing (with secondary turbines onboard) as a “lift-drag-based” wing, and a (crosswind) “lift-based” wing (yoyo system) as a “drag-lift-based” wing (the first qualifier being predominant)?

Indeed as I mentioned the paper indicates that "AWE drag power systems can harvest up to 16/27 of the power available in the wind " and "AWE lift power systems can harvest up to 4/27 of the power available in the wind ", “AWE lift power systems” being yoyo systems. The used terminology is exactly the opposite in the field of current wind energy, although these values ​​correspond well to the respective limits of lift devices (for example three- bladed HAWT) and drag devices (for example Savonius-type).

I think the AWE logic is a bit different because AWES fly and are tethered, while current wind turbines don’t fly. AWE works (or doesn’t work) in near 3 D while current wind turbines work in near 2 D.

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Pierre: Thank YOU for making my point better than I was able to. What you said: "AWE drag power systems can harvest up to 16/27 of the power available in the wind " and "AWE lift power systems can harvest up to 4/27 of the power available in the wind ", exactly nails the case shut. Your reliably-well-targeted skills at analysis have made the case better than I would have thought to do. Your insight is the bow on the ribbon of my argument, the icing on the cake I was trying to bake! You have unequivocally confirmed exactly what I was trying to point out. The numbers themselves confirm what I was trying to explain.
Now I will ask you a couple of questions:

  1. Does a regular HAWT or Darrieus VAWT blade have anything trying to slow it down (load), or is it flying free, with no load?
  2. Since this load is slowing the blade down (thank God or the machine will exceed Mach 1 and explode!) should we then call a regular HAWT or Darrieus VAWT a “drag device”?
  3. Does a Makani secondary blade have anything trying to slow it down (load), or is it flying free, with no load (even more likely supersonic)?
  4. What load is slowing the Makani secondary blade?
  5. So then, what load is ultimately slowing down the Makani primary blade?
  6. What is the difference between the load slowing down a regular HAWT or Darrieus VAWT, and a secondary Makani blade?
  7. What is the load on a regular wind turbine blade?
  8. What is the ultimate load on a Makani secondary wind turbine blade, which also provides the load on a primary Makani blade?
    In each case, the load slowing the blade is the electricity being generated.
    Whether the blade is supporting itself in the air is a separate topic.
    It does not affect whether the machine is a lift device or a drag device.
    You have to understand something about wind energy: There are ALWAYS newbies to the field, thinking they have a new approach, who often must slowly come to the realization that much of what they assumed was wrong, as they slowly get up to speed on the well-developed art of wind energy. Sometimes these people cannot be helped - uncurable crackpots, other times they can learn and get up-to-speed on the basics.
    Now I have long-maintained, “since the air is invisible, wind energy is a magnet for crackpots, and airborne wind energy is a neodymium supermagnet”.
    Of all the crackpot concepts I’ve ever seen, however, I’ve never seen anyone trying to reverse what is a lift-based device versus a drag-based device. That is where the “supermagnet for crackpots” comes in. “This time it’s different” is what the newbie often believes. When you add being supported by the air against gravity, that is just enough of a diversion to encourage this sort of “this time it’s different” thinking. But no, the basics are still intact. Just as adding an extra layer of connecting rods does not stop a piston engine from being a piston engine, adding an extra layer of rotors does not stop a lift-based wind energy device from being lift-based. As the promoters describe their device, it is supposed to represent “just the tip” of a wind turbine blade: The tip of a lift-based wind turbine blade. Lift-based. Basic. Not changed because someone wrote a paper getting it backwards in 1980.
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Thanks Doug. I am not a specialist in wind energy. I only try to deduce the divergences and convergences in the evaluation methods of current wind turbines and AWES. For now it is difficult to me to reply to your questions. However I think we should distinguish the load from the generator and the aerodynamic drag. Peter Jamieson clearly explains the secondary rotor concept in his book page 128. Otherwise it has to do with Induction factor. The secondary rotors lead to an additional aerodynamic drag in the whole system compared to a wind turbine. But for a flygen AWES (and also for a wind turbine as Peter Jamieson indicates) it can be a solution as the device scales.

By changing the pitch? By changing the trajectory? By changing the orientation of the secondary rotors?

I think the small generator inboard (per each turbine) produces the load. But it is not enough to prevent overspeed.

I am not sure, because 4/27 can be a huge value for an AWES. Makani wing could reach the theoretical 16/27 (slightly less because of the secondary rotors) but it should be far below 4/27 because it harnesses a tiny part of the annulus and nothing inside. In the other hand if reaching Betz limit (16/27) becomes possible within a huge area, that could make a difference. AWES undergo their own weight in addition to efficiency requirement. The space use is another major requirement (imho). However Loyd’s analysis takes into account of the wing area, not the swept area, and states an equivalent efficiency for both drag and lift devices, which does not contradict the 16/27 and 4/27 mentioned above.

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Pierre: None of the minor details you cite changes whether a wind energy device is categorized as a lift device or a drag device.
The terminology of “lift” versus “drag” does NOT describe the “load”, it decribes the working surfaces.
EVERY wind energy device has a purpose, which is to drive a load.
ALL loads impart what could be called “drag” on the working surfaces.
The nature of the load does not determine whether the device is a lift-based device or a drag-based device.
The nature of the working surfaces describes whether the machine is lift-based or drag-based.
If the working surfaces travel downwind, slower than the speed of the wind, that is the ultimate drag-based machine.
If the working surfaces travel across the wind, at a higher speed than the wind, that is the ultimate lift-based system.
Every wind energy system has a load.
In all cases, the load imparts “drag” to the working surfaces.
We often see wind energy newbies asking “Why not use a hydraulic pump as a load at the top of the tower?”, with the idea that the generator could be moved to the tower base, as though a hydraulic pump is lighter than a generator and a hose is less of a hassle than an electric cable.
Such a device is still lift-based. The substitution of a different yet equivalent load does not change that. The working surfaces are still operating exactly the same, even though the vanes or piston of the hydraulic pump act as “drag” on the working surfaces. It doesn’t matter exactly what provides the load, or drag. ALL wind energy devices pull against the “drag” of a load. That is why we build them!

Let’s take a water-pumping wind turbine as an example:
The “load” is the water pump. The water pump slows the blades. But some people have tried to fit generators with a gearbox to water-pumping wind turbines. Does that substitution change whether the machine is lift-based or drag-based? NO, that question has to do with the nature of the working surfaces, NOT the nature of the load.
A farm water-pumping windmill sits on the fence as to being categorized as a lift device versus a drag device, due to its high solidity and slow rotation. As a possible drag-based machine, it is less efficient than a high-speed rotor, by about half. Substituting a generator/gearbox for the pump does not change that. Changing the load does not change whether the device is considered lift-based or drag-based.

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Here is the reply of course!

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Pierre:
An old-fashioned square-rigger sailboat, traveling directly downwind, cannot travel faster than the wind. It is less efficient for two reasons: 1) By traveling with the wind, it reduces the relative windspeed, and 2) it does not collect as much wind as a sailboat traveling across the wind.

A sailboat traveling across the wind can go faster than the wind, is more efficient, because 1) its motion raises the relative windspeed, and 2) the motion adds to the area of wind collected.

The first example is in drag mode, the second example is using lift mode. If you add a propeller-type turbine to the sailboat, to extract electricity, that does NOT change the fact that the crosswind sailboat is operating in lift mode and the downwind-square-rigger sailboat is operating in drag mode. Make sense yet?

Same with a Makani machine - a crosswind sail pulling a propeller along. Lift-based.

If a person wants to say the Makani wind energy system is a drag-based machine due to the load providing “drag” against the working surfaces, then all lift-based turbines are drag-based machines.

If the thrust force on Makani’s spinning secondary rotor is said to categorize the Makani device as “drag-based”, then the thrust force on a kite-reeling rotor or kite flying a crosswind pattern is also “drag”, which would make kite-reeling devices drag-based devices too. They do operate on the well-established drag principle of the working surfaces being “dragged” downwind.

Now the fact that the working surfaces achieve this downwind thrust force by rotating blades or a kite flying a pattern is just another way to harness a higher amount of thrust force from a larger area. A larger, stationary (non-crosswind-flying) kite of the same area, could achieve the same thing, and it would be called a drag machine.

So, to my way of thinking, it would be the kite-reeling concept that has aspects of both a lift-based machine and a drag-based machine, NOT the Makani-type devices. Still, overall, I would say the kite-reeling machine operates in a similar fashion to a drag-based or savonius-type device, even though it uses lift to achieve that, it does not take advantage of the high speed of the working surfaces to directly drive a generator at high-speed, but instead throws away that high-speed motion, instead using the low speed of the resulting thrust force to spin a generator using a gearbox. And the working surfaces have an upwind phase where they use power rather than generating power, characteristic of a drag-based, Savonius-type machine. As you point out, the theoretical efficiency is that of a drag-based machine such as a Savonius.
:slight_smile:

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I think that initially the secondary rotor area was calculated to add half of the drag of the wing alone in order to optimize the complete kite. But that does not justify the qualification of drag device as you indicate.

Other than that the secondary rotor area increased a lot compared to the wing area, taking account of induction factor, VTOL requirement, tether drag…

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No more than an inventor of a new musical instrument, who happened to like sad music, would be justified in naming a minor scale as a major scale, and a major scale as a minor scale, due to his preference for what were really minor scales. Say the instrument was invented in 1980. Someone could come along later and say “Well, for THIS instrument, the major scale we’ve been using for 40 years has been the scale you say is a minor scale, so I see no contradiction”… What is it? Nonsense. Just as a musical scale remains major or minor, no matter the instrument, or mood of its promoter, new wind energy concepts fall into the existing categories of lift devices vs. drag devices, based on the operation of their main active working surfaces or blades, not based on how a load is applied.
I’ve been castigated in the past for saying I had never read Loyd and saw no reason to. Now I know why. Not sure why people are so impressed with excessive detail regarding formulae, angles, etc., since the gist of Loyd is what any kid flying a kite could see:
“A powered kite-reel could extract energy while reeling out, more if the kite flies a pattern, or a wind turbine could simply be placed on a kite flying a pattern.”
There are Loyd’s two main concepts, taking only a single sentence.
Whether he understands what is a lift device versus a drag device in wind energy scarcely matters. Aside from his own misinterpretation of that question, there was never any confusion on the issue. Personally I experienced the realization that a powered reel would make reeling a kite in easier, and that such could generate power when reeling out, as a kid way back in the 1960’s. It did not seem like an original thought to me, even way back then.

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Yes if we consider the swept area as for current wind turbines as it is discussed above. Within a given swept area the limit of really drag-based reeling yoyo kite is 4/27 (as mentioned above), while it is 16/27 (Betz limit, as mentioned above) for really lift-based-turbine onboard or some rotary kite systems such like SuperTurbine ™ or Daisy. But Loyd’s formula considers the kite area, not the swept area, finding an equivalent result for drag (in fact lift) and lift (in fact drag) based kites. And as developers consider that a crosswind kite harnesses a tiny part of the swept area, the numbers 4/27 and 16/27 have no practical effect for the time being. The problem is also that for crosswind kites the swept area is not easy to define accurately. Indeed a kite can swept a huge swept area while the real harnessed power is tiny due to its possibly low power harvesting factor. Imho a practical measure would be by using the Power to space use ratio, defining the ratio of the wind power within the frontal airspace / space and land use area.

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I would not be so quick to adopt drag mode in AWE as @dougselsam says is normal from conventional wind power. This is because in conventional wind, reeling out doesnt make much sense. That would mean eg. placing a HAWT tower on a rail cart and then letting the cart travel downwind.

I would not compare drag mode AWE to a savonius windmill because the blades are actually travelling crosswind for drag mode AWE.

Lift and drag mode AWE stems from describing two distinct modes of generating power. Like you already described in a separate thread about a backstalling, vertical motion kite, sometimes even the lift and drag mode happen at the same time.

Lift and drag mode are important concepts for AWE. For traditional wind, its more meh…

Lets just stick with what wording was already decided since the 80s, then rather discuss ways to make it happen

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Do you mean lift (yoyo) mode AWE (which is considered as drag mode in this topic by @dougselsam and me)?

Indeed in crosswind yoyo (and also flygen) mode the kite travels crosswind unlike the blades of a Savonius turbine. But in crosswind yoyo mode the whole swept area goes downwind as for the Savonius rotor whose the power blade is pushed by the wind. It is the reason why the limit by swept area is 4/27 as indicated by the publication, as for a Savonius-like turbine.

That said AWE works in 3 D, which complicates the qualification of movements and definitions. And for what we know this does not necessarily lead to a superiority of the flygen or rotary modes compared to the yoyo mode, many other factors coming into play. And again an AWE system which harnesses 4/27 from a huge frontal airspace can be successful.

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This is not related to the merits of any method. I’m just saying yoyo is not drag mode like a savonious. If you say this, you are adding meaning to the older «drag» term that is probably not obvious to those already using the term. Also, you are using the opposite term relative to everyone else in AWE except @dougselsam

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If we consider the swept area as a reference, the AWE theory has indirectly inverted the qualifiers of lift (example HAWT) and drag (example Savonius type) used in current wind energy.

In AWE field the effective swept area of a crosswind yoyo kite could be replaced with a full not crosswind surface (like a large parachute) going downwind mainly by drag with a similar result (15% or 4/27) to said crosswind yoyo kite, and in a similar way as the Savonius-type turbine blade making power by being pushed by the wind, unlike a crosswind flygen or rotary kite. Similarly, the reel-in recovery phase can be compared to the other blade of said Savonius-type turbine going against the wind. As a difference a Savonius-type turbine has a high solidity while a crosswind yoyo system can have a low solidity thanks to the possible high glide ratio, but that does not affect (imho) the downwind swept area of both to make power.

It may be useful to question this reversal. It is not for nothing that recent publications increasingly deal with the swept area and the Betz limit unlike Loyd’ seminal publication.

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From the Loyd paper:

When a cross wind kite pulls a load downwind, as described above, it is essentially the lift of the kite that acts on the tether to produce power. That mode of operation may be called lift power production. Power can also be produced by loading the kite with additional drag. Air turbines on the kite result in drag power.

That looks like a quick off-hand way to introduce a new variable to his analysis. It’s confusing if you don’t keep the context in mind at all times, so I don’t like it. Also every AWES would then almost be drag power as every system to extract the energy introduces extra (aerodynamic) drag on the wing, either via extra turbines on the wing or tether drag.

And it gets extra confusing when wind turbines are also classified as drag or lift based, using a different criterion.

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The classification of current wind turbines into lift and drag devices stands because wind turbines predate AWES and are massively marketed unlike AWES.

So AWES have to adapt to current wind turbine classification, not the reverse.

So what is named as drag AWES (crosswind flygen kite, working according to a stationary swept area) should be named as lift AWES (flygen and also rotary kites) in order to correspond to lift-based (stationary swept area) HAWT, both having a (Betz) limit of 16/27, both being perpendicular to the wind flow, with the cosine correction for the kite.

By the same what is named as lift-based AWES (all yoyo systems, and working according to a swept area going downwind) should be named as drag AWES in order to correspond to drag-based Savonius-type turbines, both having a limit of 4/27, both having a swept area (the power blade for Savonius-type) going downwind, pushed by the wind.

The drag power expressed by M. Loyd is a secondary feature _ for secondary rotors _with respect to its lift-based stationary swept area feature. Certainly the slowdown of the wing by the turbine is equivalent to the loss of power during the reel-out phase, this in terms of efficiency by kite area, but not by swept area (16/27 vs 4/27). The concept of swept area should prevail over the concept of efficiency by kite area. The qualification of lift-based crosswind kite is possible but within the swept area which is stationary for flygen and some rotary kites, or going downwind for yoyo systems. See the sketch below:

Classification per swept area
Classification per swept area1005×271 10.4 KB

It is not for nothing that recent publications increasingly deal with the swept area and the Betz limit unlike Loyd’ seminal publication. This paper corroborates all that I repeat about the lift (both lift-based HAWT and flygen) and drag (both Savonius-type and yoyo) systems, with the respective values ​​of 16/27 and 4/27, and what the publication of M. Loyd does not mention.

So the terminology in Loyd’s paper and following publications should be modified in order to adapt to the existing terminology in current wind energy.

In a practical point of view, the evaluation by swept area is a very important step to determine the power/space use ratio which is a key for the viability of AWES. Yoyo systems are not advantaged for this concern, and moreover their drag-based swept area also results in slow reel-out motion which requires significant gear reduction, not to mention their intermittent production.

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One can see Loyd’s reasoning, however, he is attempting to characterize on the basis of the load applied to a primary working surface, whereas the industry and academia have already long-characterized on the basis of the mode of operation of the primary working surfaces themselves, not the load applied. If we were to apply the Loyd definition to regular horizontal-axis wind turbines, they would all be characterized as “drag” devices, since power is “produced by a loading the blade with additional drag” in the form of a generator slowing the blade. Without slowing the blade, you have a runaway situation that often destroys turbines, due to the no-nonsense high effectiveness of the configuration. But it is not the loading that determines the existing naming convention. The best I can say for the reel-out methods is some of them do use lift, however they use lift to create a similar drag.effect to existing drag devices, with similar mode of operation. I like Pierre’s chart - makes sense. At least we understand how these various machines work, regardless of what labels we place on them. Still, there is a place for consistency in terminology.

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