As you likely know, I’ll finish by arguing in favour of a hollow-axis, multi-stage, kite-autogyro. e.g. A stacked daisy kite network, which is a mechanical drag mode kite turbine. (Can anyone come up with a short & sweet term for it?)
It looks at kite plane inductance factor (as per Rachel Leuthold)
Drag mode has more power potential than lift mode… (he he rotary … wha? nope)
Best to have an induction factor close to zero or basically no kite in the sky for best efficiency
So skinny wings in wide-open space but working crosswind.
Unless your ground station is moving and high up…Crosswind is an unfortunate term for AWES analysis to base itself on.
For a normal kite, “Crosswind” nearly happens, only rarely. And it nearly happens only in the sweet spot right downwind low down in the wind window. (where the air is normally slow)
Lets say you can operate from a raised platform or bridge. You sweep across the wind window directly at the level you stand … then for a moment right downwind of you (in your wash) for a fraction of a second …magic… I flew a kite crosswind.
OK, bit over-dramatic @Rodread … but the point is,
kites have to cope with wide envelope flight regimes
kites are flown from the ground up
Big wide open space, low solidity, is best for optimal theoretical efficiency, yep, but can your big wide loop fly anywhere near crosswind? Nope.
I’ll finish by arguing in favour of a hollow-axis, multi-stage, kite-autogyro. e.g. A stacked daisy kite network, which is a mechanical drag mode kite turbine. (Can anyone come up with a short & sweet term for it?)
Solidity and inductance can be set on multiple light wing network kite turbine systems.
2 Questions @tallakt is this too simple an explanation of why you prefer small loop systems?
and as per the start
What is the most crosswind kite power system?
I investigated Low radius loop - #7 by PierreB to show that it is possible to harness as wind energy as for a large crosswind figure, because a more powerful central zone is harnessed, leading to a more regular traction.
If there are several connected wings the low radius loop becomes a rotary kite.
I’m sorry about the nonsense. I was away and did not find the time to write a proper reply.
The paper (which i only skimmed yet) mentions drag mode and lift mode which is fine. The Shakira™ rig is obviously a drag mode rig.
To categorize a rig in terms of “crosswindness” I would say the important factor is only the angle between the kite/blade motion and the wind. This should be close to 90 degrees. Lesser angles are still crosswind, but if they are drastically different from 90 degrees, one would pull out the “cosine loss” term.
A a personal note I would like to add that a kite moving from zenith and upwards is not really a crosswind kite in my opinion, as reeling out will eventually cause the kite to fall into the power zone. For me crosswind only makes sense as a term when the kite is flying deep into the wind window with a speed that is greater than the wind speed. This is when you get the “multiplication of power” that is the foundation for efficient AWE imho.
I’m becoming increasingly doubtful as to the usefulness of this forum and its format.
Got a weird e-mail from Roddy through this forum asking me where he had said something about drag / lift. I obligingly clicked on the “go to topic” button offered and found myself plunkced down into some “new topic” which consisted of ONLY a single message - a repeat of Roddy asking where he had said anything about lift/drag. I had to open a new “new forum” window so I could first find the original topic, then slowly go back and forth cut-pasting Roddy’s words regarding lift/drag then post the message. Now I do not even see where my message went - where is the “new topic” to be found? Why did the reply to my message come pre-paxcked into its own topic? Why did Roddy even ask me where he had mentioned lift/drag when it appeared three times (3x) in his post? God forbid there could just be one topic: AWE. This whole “forum” thing is reaching its limits of either non-usefulness or unuseability. How many “topics” are we up to by now? A few hundred? A few hundred splintered discussions of how many angels can dance on the head of a pin? Anything and everything except anyone powering a single facility of any size whatsoever using airborne wind energy…
Here waas my message - where it is now, who knows?
OK this is bizarre: I responded to a message in a topic, and the reply is in its own new topic rather than in the original topic so I hve to open multiple windows to find your quotes:
Roddy said: “As you likely know, I’ll finish by arguing in favour of a hollow-axis, multi-stage, kite-autogyro. e.g. A stacked daisy kite network, which is a mechanical drag mode kite turbine. (Can anyone come up with a short & sweet term for it?)”
Then, in the same post, Roddy said:“ Drag mode has more power potential than lift mode … (he he rotary … wha? nope)”
Then after that, still in the same post, Roddy repeated:“I’ll finish by arguing in favour of a hollow-axis, multi-stage, kite-autogyro. e.g. A stacked daisy kite network, which is a mechanical drag mode kite turbine. (Can anyone come up with a short & sweet term for it?)”
4.**Sorry but I just have no idea what you are talking about regarding lift versus drag modes, so I was asking you to explain.
Miles L. Loyd used “drag power system” expression because of the drag induced by the turbine aloft slowing down the wing. After that the two expressions are now used in scientific literature.
A “drag power system” designs a flygen but could also apply to rotary systems like Kiwee, Daisy, Sky Windpower’s autogyro-helicopter, SuperTurbine ™ and other rotating devices, even HAWT. It is a reason why I would prefer to use “torque power systems” terms instead of “drag…”.
Indeed AWE is perhaps the one of the two fields with numerous patents, categories, publications, conferences, before any practical existence, the other field having been the space conquest, but with a more uncertain outcome.
I actually find «drag mode» and «lift mode» to be a nice way to describe either.
Are you generating energy in the direction of the wing lift, or in the direction of the wing drag? If the tether length is constant, you are probably using drag mode…
Note about the forum: I think the new forum is quite nice. There might be room for improvement (I dont know what), but I dont think that would solve any real issues. The yahoo group had serious issues with usability, and now it seems also serious issues with the owners who decided to shut it down. The value of a forum is mostly in the discussions, not in storage of «useful nuggets». As long as I can see what messages are new, I’m happy. That being said, the search function here actually works well, as opposed to the yahoo group.
For Daisy the tension of the ropes assures the torque transfer along the stack of rotors.
For SuperTurbine™ the torque transfer is assured by a rigid structure, mainly a central shaft.
It is quite different.
OK I understand why the term “drag” is used: a wing “drags” a rotor through the air, right? Is that what you mean?
Well I think this is backwards.
By this logic, a sailplane (glider) operates on drag, since the wing provides lift which then could be said to “drag” the glider through the air. However the glider wing is operating on lift. Drag is minimal - like 1/50th of the amount of lift. And the rotors of wind turbines could be said to “drag” the generator around in a circle, but that is a different use of the term “drag” than aerodynamic drag. What you’re doing is seeing that aerodynamic lift is used to fight drag, overcome drag, and that a load acts to provide drag which the lift must overcome, but every part of even a Makani machine generates lift, not drag. The rotor slows the airplane through lift, not drag. Sure, that lift provides an impeding force that slows the airplane, but aerodynamically that slowing force is from lift, applied backwards to the usual direction, but lift nonetheless. The nomenclature of wind energy was well-developed when Loyd introduced his contribution. I’m still happy to have never read the Loyd paper. In wind energy, a drag-based machine uses actual aerodynamic drag directly, to counter the physical retarding force of the generator.
If you have a yo-yo kite-reeling configuration, it generates lift but uses it in the same way as a drag machine: pulling directly downwind. I think this use of the term “drag-based” to indicate a positive rotational force perpendicular to the wind direction is wrong. Ultimately it is the GENERATOR that is providing the type of “drag” erroneously applied to lift-based machine in this discussion, but the generator ALWAYS serves in this manner to resist rotation, for ANY wind turbine. According to the Loyd notion under discussion, all wind turbines would be called “drag” machines, since the working surfaces are slowed (dragged) by the generator, or these lifting surfaces could be said to be “dragging” the generator in a circle. It is sometimes after a while to try to have wind energy discussions with people from outside the field of wind energy. Sometimes seems like a deja vu of the first day at school, discussing the ABC’s. Well it took me quite a while to get all this under my belt too, so don’t give up.
Airbone wind energy scientific circles consider efficiency in regard to the wing, while wind energy field considers efficiency in regard to the swept area of the whole rotor.
I «read» the paper today. It is a bit heavy reading. Anyways: I dont think we should use every convention from windmills same as we should not use every convention from aeroplanes.
I dont believe the induction factor a is the most useful approach for AWE. Sure, for a windmill, the swept area is kind of fixed, and finding the best induction factor could make a big difference in overall performance.
For AWE, we must assume that the benefits of less materials and higher altitudes will counteract the issue of very low induction factors.
So I dont agree in the shift from power to induction factor. At least from a yoyo single kite point of view.
I am also not sure if assuming crosswind/downwind is a good idea. Looking eg at the recent Makani videos, elevation angle and loop radius are quite huge. Not taking this into account might make the results less relevant, I don’t know.
I’m just talking about basic wind energy terminology. “Drag-based” refers to machines where the wind simply pushes (drags?) the working surfaces, such as a Savonius. Or a 3-cup anemometer. The usual convention is if the working surfaces have a tip-speed-ratio of 1 or less, it is a drag-based machine. Therefore some call a farm water-pumping windmill a drag machine. Even though it is crosswind.
I was not aware of this convention. In that case «drag» based AWE would be when the glide number of the kite and tether is less than one? I dont think this is a useful distinction for AWE because such a system would not be very effective in terms of power to weight (my general hunch).
(In my previous post my reference to conventions was meant for the discussion about induction factor in the paper)
