Bryan Roberts' Gyromill (Sky WindPower)

There should be a topic about this on here I think.

http://www.skywindpower.com/ww/images/feg_test2.jpg

The Original Flying Electric Generator


Sky Windpower - Power Generation Flight - Angle 2

Sky Windpower - Power Generation Flight - Angle 1

First Completely Autonomous Quadrotor FEG Flight

Skywind Power - Animation of Full Scale FEG Flight


The Stability of a Tethered Gyromill - D.C. Rye (1981).pdf (503.4 KB)

Optimum Twist for Windmilling Operation of a Tethered Rotorcraft.pdf (3.3 MB)

The Flying Electric Generator: evaluating the claims of a largely ignored proposal for generating electricity from high-altitude winds ||| web.archive link

Airborne Wind Energy (2018)
Chapter 23
Quad-Rotorcraft to Harness High-Altitude Wind Energy
Bryan W. Roberts


https://web.archive.org/web/20191030083420/http://news.bbc.co.uk/2/hi/science/nature/1248068.stm

https://web.archive.org/web/20191001010208/http://skywindpower.com/ww/home.htm

I think Bryan Roberts had a good grasp of where his basic idea could possibly go. There are a few points though, that concerned me:

  1. The term “gyromill” had already been in use as a well-accepted term in wind energy for many years, referring to a modified vertical-axis wind turbine, using straight blades. I believe it usually referred to a scheme where the angle of the blades was adjusted in real time as each blade reached a different point in its circular travel, with respect to the wind direction. This adjustable-pitch gyromill concept was a typical “rescue” attempt of the already-“superior” concept of a vertical-axis wind turbine operating on the Darrieus principle. See, it starts with the stated “advantage” of the vertical-axis turbine concept: “It doesn’t need to aim!”. But then… well… some wise-guy decided they could work better if they DID aim…
    Nevermind the previous stated advantage of “not needing to aim”, the “rescue” plan was to make them aim nonetheless, often using a tail that could be used as a physical directional reference for a set of mechanical linkages to change the pitch of the blades as they transited their circular path. In other words, the blades would adjust their pitch to make the most power at any given point in their travel, in response to the overall wind direction. They didn’t seem to realize that this step eliminated the main “advantage” of vertical-axis machines (not needing to aim). I mean, if you now need a tail to aim the machine into the wind, the sensible thing to do would be to just aim a regular propeller which uses a fraction of the material to sweep the same area, with no on-the-fly adjustment needed. So the pitch-adjustable gyromill never caught on - just a more complicated way to use more material to sweep a given area less efficiently, with less reliability, at higher cost. (Hello Professor Crackpot!). I will say though, I never really agreed with the name “gyromill” for these straight-bladed vertical-axis machines: I think the name is better suited to the gyrocopter. As I’ve said many times, a wind turbine WANTS to fly, all we have to do is let it.
  2. I never liked the single-blade-with-counterweight concept:
    a) It seems destined to weigh more than a regular two-bladed rotor, since the counterweight must weigh more than the blade to be located so close to the hub to avoid excess drag.
    b) It may be statically-balanced, but can never be aerodynamically-balanced.
    c) If you need a counterweight for a blade anyway, what’s the problem with having a second blade?
    d) I saw this counterweight as a typical “Professor Crackpot” step of “let’s ruin a perfectly-good invention by incorporating an unnecessary SECOND invention to show our unlimited GENIUS!”
    e) Why not just build single-bladed gyrocopters, helicopters, airplane propellers, and wind turbines, if that is a better way? Is it possible that single-bladed propellers never caught on because they don’t work so well? Is it just possible that maybe the developers of rotors of all types had some idea what they were doing?
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It is a naturally good concept as it is a tilted flying wind turbine. The main issue is the length of the tether compared to a unit. A possible solution would be gathering the units, perhaps in flight, in such a way that the number of tethers would be globally reduced while collision risks between units would be prevented.

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Can anyone remember details of the kite with the trailing edge net of multiple flexibly in-plane rotors by an English guy. The net seemed to have quite a weight and drag issue but had about 12 props on it. Sure there was a youtube video of it…
Anyways…
If the Bryan Roberts design went well and lifted well you could surely drop the blade area to tether length ratio with creative application of nets for rotor groupings… like using fractal arches with multi points for hosting rotors on arches, net domes, stacked net domes etc…
Trickier to launch of course. Needs sequencing and good ground organisation.

The weird thing to me, which should be quite familiar by now, is what I see as a pseudo-intellectual paper full of equations meant to analyze the concept in minute detail, seemingly never getting to the point of showing if it is even a workable or good idea in the first place. If it;s a good idea, why not just build one at even just an RC scale and show us how good it is rather than dragging everyone through page after page of mathematical equations that would seem to have not provided any useful answers? I mean, where is SkyWIndPower today? I don;t recall seeing anything working well despite such a promising concept.
As far as calling any use of multiple kites or rotors on “nets” as “fractal”, isn’t this just taking us back to calling the same sort of claim that any occasion of multiple units is “quantum”? I mean when I think of “fractal” I think of the Mandelbrot set where you can zoom in at a million times magnification and find the same shapes and topology, at a much smaller scale, as you see at a larger scale. I don;t think a series of rotors on a trailing edge meets this same definition of “fractal”. Just as I don’t see the incidental use of tying a bunch of harbor-freight blue tarps together in such an incomplete way by only joining the corners and not attaching the whole seams, leaving the multiple tarps only partially interconnected to make a leaky arch that gives up its possible advantage of a high aspect ratio, as “quantum”, “fractal” or any other such misapplied terms trying to make cub-scout-level projects sound like they are straight out of CERN or beyond. I say if you want to add multiple units, just do it without trying to sound like the next Einstein or like you-know-who trying to sound einsteinesque. I’m about to go get a flat tire fixed. But gee the vehicle has four (4) tires, that all roll together! It must be quantum, or fractal, or something out of another dimension, right? 2 + 2 = a time-warp wormhole into another dimension, yet the promoter can’t dress himself or walk and chew gum at the same time. How many sticks of gum in a quantum pack of fractal gum anyway? Maybe that’s the next big question in physics. Or maybe it will be how to use quantum fractal principles to try to tie your shoes! Stay tuned folks, as we journey on together into the next unknown dimension in ultra-advanced physics, starting with first-grade addition of integers, and ending at the same place, all the while pretending we left the known universe and came back through a wormhole.

It flies at a wind speed of at least 10 m/s. Even my small gyrokite requires at least 5 m/s wind speed and it has no generator and produces nothing.

For harnessing winds of at least 10 m/s on average, you have to climb very high, enough so that the weight and the drag of the electrified tether prevents it from flying. And more the rotors scale up, more weight penalty occurs.

As a result a demo of a small device flying at low altitude is likely not achievable. Perhaps a solution would be a large and light frame with numerous rotors (far more than 4) for only one tether and the appropriate bridle. An alternative would be the crosswind flight, the L/D ratio of 3 or 4 allowing both to sweep more and to lower the cut-in wind speed.

Hi Doug
If I’ve used the word fractal to describe kite tethering, or kite networks or kite lattice… it should only be fractal over a few steps at most… To a size which makes sense.
e.g surf kite bridles are 1 step fractal where a main bridle birfurcates ( I guess trifurcates is a word…) once. The 2 thinner bridle-ettes look just like the bigger one when you’re a bit closer.
Some fractal line patterns allow you to fill voids and cover surfaces really well with few main feed lines, spreading out to smaller more numerous unit lines.
Trunk line, branch line, twig, leaf / kite

I would like to compare the FEG system with the system proposed by Bolonkin.


His system is essentially the same as Kitewinder but on a much larger scale and with an LTA balloon instead of a lifter kite. The key difference is that the turbine is oriented to directly face the wind. This means that Bolonkin‘s system will produce at least twice the power due to the elimination of cosine cubed losses. I have always believed that turbines should be used only for power generation and lifter kites only for lifting. Having kites and turbines perform dual functions (lift and power), will make both of them perform suboptimally. Bolonkin‘s system is much lighter due to elimination of generators and the conductive cable. The large lifter kite which is required, can be launched at much lower wind speeds.
I don’t believe that crosswind action is suitable for these high altitude systems. The high velocity winds are already at the limit of the turbines capability. Crosswind action will only increase the chance of tether or turbine failure.

I am not sure this is a good rule of thumb. The downwind pull of a rotating windmill is quite large. So you need a really large lifter to maintain altitude if the windmills are directed lower than the elevation angle of the tether.

If this lifter is lighter-than-air its ok as force is directed upwards. But I think lighter than air is infeasible for this purpose due to the large force required. My guess is a steel/composite tower will be more feasible for this use.

If the lifter is flying it will not generate a lot of power upwards at zenith, but it will generate more force progressively deeper into the wind window.

My guess is the largeness of the lifter compared to the size of the windmills makes this infeasible or at the very least uneconomical.

So, it seems to me the better option is to keep the windmills oriented slightly higher than elevation angle, or aligned with a small lifter to compensate for gravity.

The only useful addition to this rule of thumb is to have the tether curved so that the highest kites in higher winds provide lift, while the lower altitude windmills provide more downwind pull and less lift. The reason to do this would be because it is easier to transfer energy a shorter distance (eg if the tether was conductive)


The graph page 45 “Lift And Drag Coefficients Were Obtained Through 90 Degrees In Gliding Tests With The PCA-2” provides curves of Cl and Cd / Hub Plane Angle of Attack (deg).

  1. Angle of attack of 45 degrees: lift coefficient = drag coefficient = about 0.8. These values lead to an elevation angle of 45 degrees, and also to a power value of 0.35 due to cosine cubed loss.

  2. Angle of attack of 90 degrees: lift coefficient = 0 and drag coefficient = 1.2. This is also a similarity case for a ground-based wind turbine.

It would take about 2.85 times the area of ​​the rotor tilted at 45 degrees (in 1) to obtain the power of the rotor not tilted (at 90 degrees in 2).

Now as a first approximation let us try to determine the area of the lifter kite that is required to lift the not tilted wind turbine (like Kiwee) in 2 (drag coefficient of 1.2, no lift) at an elevation angle of 45 degrees, assuming a lift coefficient of 1.2, a drag coefficient of 0.5, so a lift-to-drag ratio of 2.4.

The kite area should be a little more than 1.7 times the rotor sweeping area. So for a rotor sweeping 10 000 m² the lifter kite area would be about 17 000 m².

To stay reasonable for a side we have three tilted rotors sweeping 9.5 m² each (Sky WindPower or SkyMill gyroplane systems). On the other side we have a HAWT rotor sweeping 10 m², the lifter kite area being about 17 m² (@Kitewinder or lifted HAWT systems).

So, what is better or less expensive, about three rotors with an angle of attack of 45 degrees, or only one HAWT rotor lifted by a kite of 1.5 time the rotor sweeping area?The answer is not so obvious, not to mention other AWES which are not specifically related to the current topic.

@PierreB I think your use of the swept area may be misleading because you dont know easily how must pull per swept area is generated.

If I were to make a first order approximation, say we have a static lifter at 90 degrees (not possible of course, but simple in thought) and a tethered lifted windmill pulling straigth downwind. The elevation angle is 45 degree so the lift of the lifter must be the same of the windmill (lets consider a single rotating kite)

If the power producing kite is operating at a glide number of 5, it will fly 5 times windspeed. So the force is F = 25 \frac{1}{2} \rho w^2 C_L S_p. For the lifter the same equation is F = \frac{1}{2} \rho w^2 C_L S_l.

It shows that for a glide number of 5, and everything else being equal, the lifter must be five times larger in area compared to the power producing kite.

You can make the deal somewhat sweeter by using a lower elevation angle than 45 degrees. But I think my original sketch of this problem stands firm. Probably not feasible, with practical and economical contraints taken into account.

@tallakt, I cross-checked data. The graph on page 45 is essential for that. It shows that the glide number is 1 when the angle of attack of the plane rotor is 45 degrees, leading to an angle of elevation of also 45 degrees as I mentioned. And both Sky WindPower and Kiwee are stationary devices as only the blades work crosswind. So I tried to compare stationary devices.

Certainly these are old data but I have not found equivalent recent data, except by the glide number of a more recent rotor which is a little more efficient.

Anyway, a glide number of 5 is completely impossible with an angle of attack of 45 degrees.

On the graph a glide number close to 5 is approached with an angle of attack of a few degrees, at most 10 degrees according to this document and other documents on gyroplanes that I have consulted. You can see on the graph that the coefficients are low.

So I think my approximation stands, although I did not take account of the weight of the HAWT (Kiwee) or gyroplane (Sky WindPower). And 1.7 m² kite area at 45 degrees elevation angle would correspond to 2 times more kite area at more than 60 degrees which is the angle used by Kiwee with a lifter kite of 4 m² for a turbine sweeping 1 m², corroborating my approximation, except that the drag coefficient of the HAWT rotor of 1 would probably be more appropriate than 1.2 indicated on the graph for a gyroplane rotor.

Now things are very different if you use the gyroplane as crosswind device as I also suggested:

I think you suggested also a crosswind flight by:

Indeed even with low coefficients of lift (Cl something like 0.2) for a still smaller coefficient of drag and for an angle of attack of 10 degrees, it would be interesting. The graph shows that an angle of attack of 15 degrees allows a good compromise with a Cl of 0.6 and a Cd of 0.2, so a glide number of 3. I evoked Crosswind gyroplane? But for now and for the comparison I made on my previous message, we were on stationary devices.

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I am not sure to understand. Should the lifter not be 25 times larger since the lifted HAWT should sweep 25 times more area? Unless the gyroplane of a glide number of 5 would have a power efficiency of 0.2 in regard to cosine cubed loss, when it should be 0.65 with an elevation angle of 30 degrees? I suppose the "power producing kite " is a HAWT that is lifted by the lifter?

In all cases a gyroplane system working crosswind could be far more powerful than a stationary kite-HAWT system of equal area concerning the stationary HAWT vs the crosswind gyroplane.

Answering my own question of
Can anyone remember details of the kite with the trailing edge net of multiple flexibly in-plane rotors by an English guy. The net seemed to have quite a weight and drag issue but had about 12 props on it. Sure there was a youtube video of it…

It was our very own @philip
https://www.youtube.com/channel/UCBSHjlgLP5hKrsWiUJOk6eg

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That guy is quite productive

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Gosh - did someone come across my ancient propaganda video?! (Made before I became (semi-)convinced that we’re the victims of a cosmic conspiracy: helium, helium everywhere but precious little of it on earth, and carbon nanotubules/graphene sheets impossible? to produce to indefinite lengths/extents - at least at a reasonable monetary or energy cost.) At the time I was thinking that the materials issue would be solved: the 100-fold or whatever specific strength gain in covalently bonded carbon over the best fibres we have available would make it feasible to lift rotor and generator arrays with very large kites or kytoons.
I still potter about but not very productively, and haven’t had anything useful to contribute of late. My hunch is that, in the absence of some significant breakthrough in materials it will be very difficult for any AWES to rival the conventional HAWT for large-scale electricity production. The deep water question is perhaps more open: obviously the mooring costs rise steeply while the safety concerns diminish, and there might be scope for something Darrieus-like (omnidirectional and not suffering from the cosine effect) but with the guy-ropes replaced by a lifter.

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