Hi DaveS,

The Zhang paper does not cover the dynamics of a Flipwing or the Fluttermill. It is limited to flapping devices with a fixed leading edge, which they do not have. The Flipwing does not work like a tacking sailboat sail. That is a very loose analogy at best. Your comments suggest that your knowledge of the Flipwing is rudimentary. But why? You are an engineer, aren’t you? Where are your measurements? Where are your vector diagrams? Where is your math? Where are your published papers?

I’m sorry to hear that you have no technical data on the Flipwing because people who might wish to make use of it will need to reproduce all of your experimenting in order to do so.

Why not just write a paper about it and include what you know, so as to save other experimenters and entrepreneurs the time and energy you have already put into that project? Even if you don’t understand the physics details, you could at least report on its behavior under different circumstances. That would give experimenters a head start about what to look for. Perhaps you have additional videos that could be gathered together?

Your comments confirm that you do not have a reliable launch and retrieval system for the Flipwing. Sorry to hear that. But maybe one can be devised that uses a short tower, a spring loaded tether, and a pumping lever. The pilot kite would require a separate tether that could be fully retracted. And the Flipwing could be retracted into a space behind a wind shield on the tower.

If it is true that most classic kites will eventually relaunch by themselves (which I seriously doubt), that is irrelevant to making them reliable and practical. “Eventually” must be replaced by “immediately” when the normal launching wind speed becomes available.

You apparently have not yet devised an overspeed control system for the Flipwing. Overspeed control is mandatory for WECS. If you assume that one is not necessary, please explain why not. Self-launching from the ground is unreliable and not practical.

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You did not specify anything I said -- about the Fluttermill blades staying in phase -- that you did not understand. So I will assume that you are not interested in understanding it in detail. That’s fine with me.

PeterS

Hi DaveS,

If you have no concern, that’s fine with me. Perhaps I misunderstood you.

PeterS

Hi DaveS,

You can find your statement as easily as I can, so if you believe you did actually have a concern, then please find it yourself. I’m assuming that I misunderstood you, that you did not have a concern,  and that the matter is settled. Your lack of recall of any such concern supports that assumption. If you ever remember such a concern, you may also recall that there is a solution to it.

In the future, I will do as you suggest. It is a good policy.

PeterS

Hi DaveS,

Thanks for the information.

You did not address my specific objection as to why the Zhang paper is not an adequate explanation of the Fluttermill or the Flipwing. Yes, there are some similarities between different forms of flutter. That’s obvious. But what is important are the specific differences, the details. If and when you figure out the details of how a Flipwing works, I would love to see them. Even if you don’t know precisely why it does what it does, the first step is to analyze exactly what it does. From that information, hypotheses can be tested to determine why it works as it does. And once you know why it does what it does, you will have tools to optimize it and to estimate its potential.

I have already explained how two Fluttermill blades force each other to stay 90 degrees out of phase due to the oscillations of a see-saw connecting beam. You have yet to state what you do not understand about that. Please do so. Perhaps it would help you if I drew you a picture of the four positions that are involved for the blades during one cycle. Please see the attached sketch.

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Your reply to my concern -- that kites cannot reliably self-launch if the wind shifts 180 degrees while they are on the ground -- was not responsive to what I said. Please show me any evidence you have of a kite relaunching from the ground when the kite tether is fully extended directly upwind, and please show why that technique can be counted on to operate independently and efficiently (meaning an immediate launch when the wind is available) for many months at a time. I say it can’t be done without first moving the kite to a downwind position, either manually or mechanically. I would love to have you prove me wrong. Shorelines are a place where winds often reverse 180 degrees from day to night, and shorelines (or off-shore) are in important place to install energy kites.

Why would you want me to show you a better way to launch kites if there is no problem in the first place? You are affirming that there is a problem while denying that there is a problem. Please make up your mind.

Automatic launch and retrieval is a very difficult problem to solve for small-scale energy kites because most solutions would make the kites too expensive to compete with small-scale wind turbines and windmills. It is a critical problem that deserves much more attention because it is impeding further progress on small-scale energy kites. I hope that you will agree.

PeterS

Hi DaveS,

I agree that Flipwing blades should be semi-flexible using fabric and battens, rather than flat blades with hinges, such as blades made using flat Coroplast. But it’s good to hear that you-all tried Coroplast.

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The Bird Windmill is probably quite limited with respect to how large it can be, so it is probably suitable only for small-scale. Arrays can be used to increase the total power. If the blades are made of fabric, the frame holding the fabric must be rigid during normal operation. The blade needs to be over a minimum weight per square meter in order to be able to fly upwind, so that is another limitation. So there is no need to make the blade ultra-light.

While the Bird Windmill can be suspended from a kite, that is not its strength. It’s strength is that it can be dirt cheap when the blade is suspended between poles. For a small-scale energy kite to be successful, it would need to produce energy for less cost than the Bird Windmill (or an ultra-light Sharp Cycloturbine used in a similar manner, and operating more efficiently). That may be quite difficult to do for a small-scale kite. The Bird Windmill supported by poles may be thought of as a short-stroke pumping kite. It’s maximum practical power rating might be as low as one kilowatt per blade.

The Sharp Cycloturbine can be scaled up, probably to large-scale. It can be made quite light, but not nearly as light as a fabric kite of course. But in terms of the total mass of the system, it might prove to be a lot lighter than most energy kites, and cost a lot less for the same power rating. To show you what I mean, see the attached sketch of an unbalanced Sharp VAWT for short stroke pumping. The parts are all suspended on cords. There are no rigid connection (except the blade parts). So in that sense, it is maybe a kite. It would be much more powerful than a Bird Windmill (TSR = 2) due to its considerably higher tip speed ratio (TSR = 3 to 4). Like a Bird Windmill, it would be dirt cheap, but it would be a bit more complicated. So it’s a useful standard for comparisons with short stroke pumping kites. They are not likely to beat it on simplicity, versatility, cost, or system weight, but they might eventually beat it based on their capacity factor. I hope so, because I favor whatever lowers the cost of wind energy.

PeterS

Hi DaveS,

On Coroplast Flipwings: I understood you to say that 2D structures, such as Coroplast Flipwing blades, scale poorly, so fabric Flipwings are preferred. “but of course stiff 2D structure scales poorly,…” But now you are promoting their use anyway. Please clarify.

You seem to have misunderstood what I said about the pole mounted Sharp VAWT in the recent sketch. I said nothing about flying it beyond pole towers. Please quote me.

You did not respond to my point about how small-scale energy kites are not likely to be competitive with small scale wind turbines, and I showed you an example of a very cheap, pole-supported, Sharp VAWT to illustrate my point. It can do what short-pull pumping kites can do for a small-fraction of their system cost because it does not require a complex launching and retrieval system. So the broader question is at what size energy kites can begin to compete with wind turbines, and under what circumstances.

That point may be at around 100 kW, because there is some evidence that computer-controlled cycloturbine VAWT need to be roughly that large to justify the cost and complexity of computerization. 100 kW is a typical dividing line between small-scale and medium-scale wind turbines.

PeterS