Is there an issue with Flygen?

Airborne wind energy systems using flygens (onboard generators) do have several drawbacks, despite their innovative design.

  1. The Square Law: From a purely physics standpoint, power generation is proportional to the cube of the wind speed. A flygen system that uses onboard generators to create drag and convert kinetic energy into electrical energy is less able to exploit increases in wind speed, because the additional drag slows the kite down. For instance, if the wind speed doubles, the potential power increase by a factor of eight is difficult to fully realize because the generators add drag that prevents the kite from accelerating as much.

  2. Weight: Onboard generators, along with a conductive tether and other necessary equipment, add significant weight to the kite. This reduces maneuverability, can decrease flight time, and increases the energy required to launch and keep the kite airborne.

  3. Cost: Flygen systems are typically more expensive to construct, operate, and maintain. The generators themselves, the conductive cable for the tether, and the advanced materials (such as carbon fiber) for the kite structure, all add to the costs. These systems also require more complex control systems to manage the power generation and flight, further increasing costs.

  4. Safety: With the increased weight and complexity, there is also increased risk of failure, which can lead to accidents. If a flygen system crashes, it could cause significant damage due to the weight and speed of the system. This risk is especially high in populated areas or sensitive environments.

  5. Energy Transfer: The energy has to be transferred from the kite to the ground station via a conductive cable. The energy transmission over long distances may lead to power losses, reducing the overall efficiency of the system.

  6. Maintenance: Maintenance of these systems is more complicated than for simpler designs like tension-based systems. The onboard generators are exposed to harsh conditions and require regular checks and servicing.

  7. Limitations on Site Selection: The extra weight of the system may also limit where these systems can be deployed. They require stronger and more consistent wind conditions to be economically viable, which limits their site selection.

In contrast, a ground-gen system that relies on tension on the line to generate power can potentially extract more energy from an increase in wind speed, because it doesn’t have the drag from the onboard generators slowing down the kite. These systems are simpler, lighter, and often more cost-effective, especially in terms of installation and maintenance. Their safety profile is also better because they do not require heavy equipment to be kept airborne. Therefore, for many applications, ground-gen systems can provide a better balance of power generation, cost-effectiveness, and safety.

Certainly, it’s crucial to emphasize that the points raised above are not an attack on flygen technology, but rather a critical analysis of some of the challenges that this innovative approach faces.

There is immense potential in flygen technology, and the exploration of different methodologies and designs is what drives advancement in any field. Each approach has its unique strengths and weaknesses, and by acknowledging these, we can better focus our efforts on improvements and innovations.

I wholeheartedly welcome feedback, thoughts, and additional perspectives on this topic.


Welcome, ChristianH;
Your observation is one that I also have considered and I still wonder that the flygen route has been able to attract more investments since the days of Google’s Makani.
Perhaps because AWE being at the intersection of the aviation and the Wind power industries has attracted more funders from the aviation sector than from the Wind Energy field has therefore drawn typical aeronautic minds.
Do you know of JAL and Dave Santos?
DaveS, an aeronautic roboticist with much kite knowledge and Kiting skills; had long concluded along the lines of your observation and insists that the flygen approach will not scale to serve the global energy needs to combat Climate Change.
The ‘Advanced Kite Networks’ by JAL proposes sports style power kites and ground generators hybridized for maximum up-time at all scales.

I will add my opinions on this:

1 The square law; I dont agree to any physical fundamental problem. I do think though that noise is a very serious concern because the blades are spinning at very high speeds. Early wind projects maybe often don’t focus on noise, relative to just producing power. But noise is a showstopper. BUT, let me add AWE has potential noise benefits due to the distance kite to ground. This benefit will not come to flygen I believe though, due to tether length not scaling with wingspan.

2 Weight; I fully agree this is a problem, to the extent of being a probable showstopper by itself

3 Cost; Yes complex gear is costly. Though maybe the overall cost is not a problem. Only making a design and calculating cost could get close to answering this

4 Safety; Yep. I think complexity and many moving parts are making safety harder to address. Flygen is a poorer starting point safety wise. Though this may or may not be an issues depending on other factors (eg offshore safety may not be very demanding)

5 Energy transfer; Definite problem. The mass of the tether is the major one in my opinion. The length of the tether will not scale with wingspan due to cubic scaling of mass, or perhaps even worse once scaling of metal
in wire is included in the calculation. But the drag of the tether due to the tether «belly» while looping would be killer unless you add fairings to the tether. You just dont want to go there.

6 Maintenance; Yup. Big problem. Though AWE in general has a benefit that the kite can be landed for easier maintenance, and even the kite can be replaced for maintenance off site.

7 Site selection; Im not sure about this one though. You would have to expect they could make this work with similar performance compared to alternatives. I dont think there is a market for a windmill that requires very high wind to start production, at any site

Also would like to mention some things you did not mention;

The aerodynamics of the interaction main wing and generating blades is serious and complex. It is a poor starting point for an optimized design.

The process of making a combined propeller/generator blade is extremely complex and difficult to achieve. The alternative of keeping these separate would presumably have a huge mass penalty. Though I do see some possible avenues to abate this.

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I wish only to point out that this question hearkens back to the earliest days of the recent decade+ long hype cycle over AWE: Flygen? Groundgen? … :slight_smile:
Back to square-one?

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Flygen just means an AWES with an airborne generator. It doesn’t need to mean a system like Makani’s, which is a drag-mode AWES. A flygen system that had the conductive tether and the generators stationary, with perhaps (partially) tensile rotors for example, would have different characteristics.

  1. I do wonder about the fatigue in a conductive tether. The voltage drop in an optimized electrical cable is probably better than the discontinues reeling in and out that most tension based systems have.

What are the benefits of drag mode AWES? An “innovative design” isn’t a benefit and with only mentioning drawbacks your analysis is incomplete.

Maybe for a small system you don’t need much of a ground system, the kite and cable are still relatively light, so relatively portable and safe, and with the crosswind flight you still get respectable power? Could you design a drag mode AWES that could fit in a suitcase and produce a continuous 5kW in good winds?

Indeed, when I refer to ‘flygens’, I’m discussing airborne wind energy systems (AWES) that feature onboard generators, irrespective of their design specifics, and employ a conductive tether. The strategic essence across all these systems is constant, unless I’m overlooking some significant variation.

The fundamental issue with kites using propellers to extract power from the wind is that it inherently imposes a limit on scalability. Two critical factors come into play here:

  1. The weight of the conductive tether, which is a necessary component of the system but adds significant heft, and
  2. The deceleration effect of the turbine on the wing, which inhibits the system from reaching optimal speeds.

Considering the cube law from physics, power generation is directly proportional to the cube of the wind speed. When onboard generators introduce drag, it impedes a flygen system’s ability to fully leverage increases in wind speed. If the wind speed were to double, in theory, the potential power could spike by a factor of eight. But the added drag from the generators restricts the kite’s acceleration, thereby causing the system to miss out on harnessing the full potential energy increase.

In light of these factors, one could argue that, from a physics perspective, flygen systems confront inherent challenges right from the conceptual stages. However, this doesn’t discount the potential for innovative solutions to mitigate these issues in the future.

I used to pepper these forums with the phrase “idiots, idiots, idiots”.
Of course this was not well-received in many circles, but I thought I’d take a moment to explain here.
First, I don’t really think too many involved are actually idiots. The terminology is just shorthand in wind energy for saying they haven’t done much background investigation and are pursuing a wrong idea.

Idiots 1) In wiind energy, we typically call EVERYONE who thinks they have a new, improved, basic kind of wind energy device as “idiots” because they don’t appreciate that so many alternatives have already been tried and failed, whereas the simple propeller-style turbine is ALREADY reduced to its bare essence - already simplified and optimized, possibly to the maximum extent possible. The wannabe improvers have no clue of any of this and just think some offbeat method that occurs to their untrained mind will suddenly eclipse thousands of years of refinement. Also, these people typically have no concept of a real, industrial-class wind resource, how necessary and how powerful and destructive it is, so they make little cardboard models on backyard stands to barely spin when some lucky bit of breeze is able to reach down to ground level among buildings and think it is relevant.
So idiots 1 = not appreciating the magnitude of the challenge, including the brutality of the wind, the existing level of progress (perfection?), and the difficulty (unliklihood?) (hopelessness?) of the challenge to improve on it.

Idiots 2) Once “idiots 1” is in place, the next step is to think the way around their bad choice of a target concept will be to see how much money they can raise, how many employees, interns, professors, technicians they can hire, what facilities and buildings they can rent, as though all of that misdirection of other peoples’ financial resources can solve the basic problem that they don’t know what they’re doing in the first place.

Idiots 3) This is where no matter how simple wind energy already is, the “team” will suffer from an inability to grasp any existing knowledge from wind energy. Examples are coming in with statements of “beating the Betz coefficient”, misusing or misapplying existing terminology, and just generally ignoring the basic principles of wind energy, often going back to not appreciating aspects well-understood thousands of years ago. I’m trying to stick with three (3) “idiots”, so I will fit this one in here: No personnel experienced in real wind energy being directly involved, should be a big red flag, but this obvious vacuum of proven results and lack of familiarity with the art of wind energy often goes unnoticed and unappreciated. Think of any cutting-edge technology that doesn’t include anyone from the existing industry…

The misuse of the term “drag” to describe a Makani device, would be an example of number 3, where the terms “lift” and “drag” regarding the principle of operation were already in common use for wind energy as the most fundamental category, somewhat opposite in meaning to that wished to be applied by AWE people. Kind of like what they say about a game of poker - “if you don’t know who the mark is, it is you.”. The perpetual parade of new people insisting they have the big breakthrough are easy to identify, and stick out like a sore thumb. Why, just the other day, I had a friend with a new house, with some solar already installed, explaining to me how he is looking into wind energy - mentioning vertical-axis machines for sale on Ebay. What can you say? The ads are just lies and the frail plastic machines would be unable to make almost any power at all, and would be destroyed in even a decent wind

Not that I don’t have my own favorite collection of whacky wind energy ideas, some better than others, but I also have enough of one foot in real wind energy to know what I’m getting into, and to somewhat know what I’m doing.

Also, the experience (school of hard knocks) of building and running real wind energy devices makes me less likely to fall for other dubious schemes smacking of “green”, and hopefully clean, whether it is a windmill or another vaporware air-taxi.

So anyway, it’s not that I really think anyone is an idiot, just relaying some (hopefully-amusing) sarcasm from the world of experienced wind energy people.

Yes. So for anyone using «flygen» existing knowledge would be to somehow understand this simple thing I wrote a few years ago

Btw; i can mention the «flygen» design I would consider a «hovering» design, explained simplified by the constant tether length during production. Any reeling system, like we do in Kitemill would normally be a «bounding» system.

I came up with this terminology because flygen/groundgen or lift vs drag mode don’t accurately capture the concept we would like to describe.

What I see is people still stuck at ground-zero, still pondering the original basic questions of whether to have a generator on board or not, whether to fly in a circle or a figure-8, etc. Still talking about kites pulling ships with the occasional demo that is then discontinued. In some ways it seemed that we were closer to success before anyone had even tried anything than we are now. At some point it doesn’t matter how many papers or talks are presented. By now there should be some unambiguous actual positive results. :slight_smile:

I do remember an early, seemingly promising AWE “player” was Joby, now involved with the electric air taxi concept, with over 1000 employees(!!!???)

It will be interesting to see how that field of endeavor pans out. I see similarities in the amount of hype (a lot) versus products in daily use (none?), after how many years of occasionally-working demos(?)

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Joby learned how to survive in these «markets» it seems :stuck_out_tongue:

Well, for now, using up investors’ money I presume, maybe some of Joe Ben’s own cash too, all based on the supposed promise of using drone technology to carry people… to get them to the airport…
I’m just thinking, over 1000 employees?

Contrast this with the airplane - many working models from various manufacturers almost immediately, use cases quickly found, with the planes working first, then people thinking of uses for them. Here we have the use already decided (getting airline passengers to the airport), and super-expensive development of a craft that seems unnecessarily complicated, clumsey, and improbable as an aircraft.

It doesn’t look very graceful, to me anyway. And, while I’ve always been “a big fan” (get it? big fan?) of tilt-rotor technology, it DOES seem that tilt-rotor efforts (The Osprey) have had very limited success, with a lot of problems, and a never-ending series of catastrophic crashes. Wasn’t there another one just the other day?

Accidents and incidents involving the V-22 Osprey - Wikipedia

(1) Triple-Fatal Crash of V-22 Osprey - YouTube

I believe Kitemill’s solution is very elegant. If my understanding is correct, the props are exclusively utilized during the launch and landing phases.

Kitemill employs a robust wing design that, despite its higher cost, enables it to withstand significantly stronger winds. I just fear crashing.

The process involves unwinding a spool, maximizing the apparent wind for the foil.

I would like to discuss the topics I recently came across while watching a TU Delft graduate presentation. They were discussing the concept of utilizing flygen, metal kites shaped like box kites with rotors in the ocean… Paid no mention to tether weight… didnt seem practical at all…

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Here is an excellent analysis of the airborne generation concept by Florian Bauer:
Multidisciplinary Optimization of Drag Power Kites, PhD Dissertation, TU Munich, 2021.


Hi @rschmehl , indeed @floba and KiteKraft made a fine work:

An interesting feature:

Multi Element Airfoil

This airfoil configuration generates a very high lift and thus enables a very high power density.

A direct link to his publication (Multidisciplinary Optimization of Drag Power Kites):

Some discussion on High lift coefficient and biplane kite


Hi Doug, does this mean that the small wind turbines forming the flygens do not have a high life expectancy, given their low mass, their high rpm, and the high apparent wind speed?

Hi Pierre: Well, here’s how it works: Any generator can burn out its windings under a load greater than its capabilities. It can happen fast, as in a couple minutes with some new configuration, or you might have to wait for some sustained high winds, and your turbine that worked fine all season suddenly is burned out from running for a couple of days sideways, trying to protect itself from a relentless wind.
In my experience, you won’t know where you stand for some time, with regard to generator overheating. The more steps taken to avoid this overheating, the better your chances of survival. There are many ways to achieve overspeed protection, but none is a magic solution. Some combination of techniques may be warranted. It is current that burns out alternators, not voltage. In general, the more copper you use, the better. Thicker wire carries more current. And higher voltages help keep current low.
Anyway, yes, I would say a crosswind-flying flygen kite would obviously place a huge load on a generator. The same could be said for EVTOL motors in general. One more potential failure mode for them. You can get a lot of power from a small generator for brief periods, sufficient for a quick demo. But long term operation is another story. This may be one more reason why so many projects seem promising, only to mysteriously never enter regular operation. :slight_smile:

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I should clarify, a generator enclosure of a given size can only hold so much copper, so a higher voltage will require more turns of wire, meaning the wire will have to be thinner to fit in the enclosure, so working at any voltage has about the same overheating issue, using the same enclosure. But, the tether can be thinner at higher voltages, and that will save weight. how do I know all this? Years of building alternators of all voltages from low to high, and burning out a lot of them while adjusting and fine-tuning overspeed protection measures. Typicallly, every time you think you’ve got it solved, you get two days of sustained high winds and another smoked generator/ That’s just how the game goes. That’s why most people never get a good turbine model developed, and most turbines on the market will break down pretty soon, because they are not fully torture-tested.
Well, and it is common, basic knowledge, for anyone who knows that sort of stuff.

Hi Doug, what do you think about (high rpm) ram air turbines (RAT) for flygen AWES?

Hi Pierre: I was actually contacted by the leading maker of RATs about the possibility of using SuperTurbines as RATs. Anyway, that’s beside the point. What do I think of crosswind flygen generators? To me, the answer seems obvious, in that they are challenged with a combination of conflicting factors:

  1. They want to be lightweight so as to be more easily kept airborne
  2. They need to handle much higher amounts of power for the same size propeller (rotor), which tends to make them much, much heavier, not just from multiples of the amount of copper, but everything must be beefier, more aluminum, more steel, beefier bearings, possibly a cooling system.

I’ll pass along some info here: I watched some videos about electric cars. When you see a Tesla advertised as being able to go zero-to-60mph in 3 seconds, what they DON’T tell you is, they can only do it once or twice, before the motor’s temperature sensors tell the computer it cannot keep putting that much power into the overheated motor. This is why Teslas can’t compete with fuel cars in longer races. The liquid-cooled motors STILL get too hot, very quickly. Then the sensors and computers limit performance. The instantaneous performance sounds impressive, but it can only be implemented for short periods.

With most small wind turbines, there is no liquid cooling, no computer to shut it down when it is making too much power, and in fact no WAY to shut it down. So a crosswind ram-air flygen system is a definite challenge for any generator. This may be one more reason you never see one in long-term operation. Details, details… :slight_smile:

For a crosswind flygen using SuperTurbine ™, the kite chord is too narrow to accommodate a large installation. Perhaps an additional bar would allow to implement more rotors.

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