For a Makani-type device: 2-stage lift device
The wing is the "primary working surface"
Each propeller blade is a "secondary working surface"
or
The wing is the "first-stage working surface"
Each propeller blade is a "second-stage working surface"
For kite-reeling: lift-to-drag device
For the second one (kite-reeling), I can imagine further possibilities:
It could also use “2-stage” as in "2-stage lift-to-drag device"
"lift-powered drag device"?
The point is, in wind energy, when the working surface moves downwind slower than the wind speed, with a non-power-producing return cycle that uses power to return that surface to its original upwind position, that meets all the requirements to be called “a drag device”.
But it is using “lift” to “drag” that tether downwind.
Note that the Makani kite did not “drag” its propellers (rotors) through the air so much as push them through the air.
Some may say the name “drag device” for Makani is because the power-producing turbines create drag on the primary working surface.
However, by that logic, using the currently-popularized AWE nomenclature,
a regular horizontal-axis turbine (classic “lift device”) would be called “a drag device”, since the generator places a “drag” on the blade rotation.
Something to think about. Maybe a starting place to get in line with current existing nomenclature.
Well now that I see the term “lift-to-drag” device, I can see that it might be confused with “lift-to-drag ratio”.
The term “lift” in aerodynamics, has long-parted with being restricted to the literal meaning of “upward force”. An airplane banking at 90 degrees is still using what is called “lift”. A wing on the rear of a race car is specifically generating a strong downforce, but it is still called “lift”. The blades of a jet turbine or wind turbine generate forces in all directions as they spin, but again, the phenomenon is called “lift”.
I can see why the kite-reelers like the term “lift device” since it is so literal, but the problem is the well-established terminology long ago veered away from that usage. The wing itself is a lift device, but the tether is functioning as a classic “drag device”. When depowered and being reeled in using power, even the kite is acting as a drag device. Altogether, kite-reeling is using lift to create a virtual surface of a classic drag device. So I only throw out these ideas as a starting place for how to categorize kite-reeling within established, traditional, well-known wind energy terminology.
I’m a little more confident in the two-stage lift device for Makani-type wing-driving-rotors (usually skygen) device.
That is MUCH more descriptive that merely calling it a “drag device”, which a normal wind person would envision as some flying or elevated Savonius turbine.
A Makani-type airplane HAS drag, but USES lift to generate power. The drag is a consequence or aftereffect of generating that power, but “drag” is not used to generate power, per se. Any aircraft naturally has drag, but I don’t see naming an aircraft around such a common and peripheral aftereffect.
I realy like two stage lift as a term for what makani is doing. Or even just «two stage» awe. But note it doesnt have to be two stage, if you put rotary shaft AWE in the same box. Maybe just call it «traditional mode awe» to say you have a blade flying crosswind that is being slowed down.
For Yoyo maybe «downwind motion lift» would be ok for me. Calling it drag because you are dragging the tether (or did I get it wrong?), is too easy to confuse with aerodynamic drag… I think the describing thing would be something related to moving downwind, hence my suggestion
So, after slight thinking, I may suggest
slowing down AWE
downwind motion AWE
… at least as an input to the discussion.
If its not too late to change, I really like the idea og getting rid of the names «lift» and «drag» mode awe. They are indeed very confusing
Some more or less connected discussion occurred on
I suggested two main categories:
Stationary
Downwind
These terms refer to the swept area.
Stationary AWES would concern static or crosswind fly-gen, but also rotary devices like SuperTurbine™ or Daisy or Kiwee or Rotating Reel, and also carousels, which could be subcategories. Betz’s limit of 16/27 applies (more or less, rather less) to all these devices.
Downwind AWES are aligned or crosswind yo-yo (kite-reeling) devices. Betz’s limit drops to 4/27.
So, to avoid confusion, Makani-type devices could be called “fly-gen” as is already done, and kite-reeling devices could be called “yo-yo” or even “kite-reeling”.
Pierre, I think you are confusing separate, but related topics.
“Flygen” and “groundgen” refer to where the generator is located. Nothing more.
“lift” and “drag” devices have been well-defined for as long as I am aware, and I started with wind energy way back in the 1970’s.
If AWE people ever want to be taken seriously, they need to stop calling a treble clef a bass clef, and vice versa.
To call the Makani machine a “drag device” is exactly as wrong as my mom guessing that “drag racing” was so-called “maybe because the driver has to drag his feet to stop”.
Obviously the last thing any drag racer would want is to have to drag his feet to stop after exceeding 200 MPH in a quarter-mile race. I can forgive my Mom, and once our family had been to an actual drag race I’m sure she would have never said such a thing.
And once anyone knows anything about aerodynamics or wind energy they would never call a Makani airplane a “drag device”. Drag means “pull”. The Makani device PUSHED the rotors through the air. But even that is beside the point. The point is the Makani used a lifting device (wing) to drive a second stage consisting of another lift device (wind turbine rotor), so it is a two-stage lift device. To call it a drag device is exactly the type of thing that makes wind energy people nauseated at what they (rightfully?) see as the stupidity of almost any wannabe wind energy “inventor”, and that certainly applies to AWE as I found out talking to a fellow wind turbine manufacturer who got it straight from a former NREL engineer. Really, I’m trying to help reduce the ignorance in this field, but at every turn in the road, it seems increasingly hopeless. I mean, there is a saying sometimes seen on a T-shirt or bumper-sticker: “You can’t cure stupid”. That saying is pregnant with meaning. Four words that explain a lot about the world. I’m trying to seriously come up with helpful nomenclature for lift versus drag so AWE people do not come across as completely ignorant. You are mixing this up with where the generator is located. You can’t categorize devices that have never been seen before using associations made by what has been done so far, and even so I don’t think what you’re coming up with hits the mark at all, but nice try.
Thank you Tallak. And two-stage AWE would be a broader category, that could include a Savonius turbine pushed through the air by a wing, for example, but specifically, with regard to the use of the term “lift” as currently used in common aerodynamic parlance, the Makani airplane was a lift device driving a second lift device, hence the two-stage lift device term that occurred to me. And I think it is important to note the first lift device is harnessing wind energy that ends up as electricity after going through the second lift device, which is not the same as just elevating a turbine using a kite, which might require a different name, such as “kite-elevated turbine” or “lift-elevated lift device” as opposed to a “lift-elevated drag device” which would refer to a savonius turbine elevated by a kite. Geez this is getting complicated! OK now, anyone got any ideas for a three-stage lift device?
I agree for the last sentence. As I mentioned in my previous comment, my view is the division of all AWES into two main categories: stationary (16/27 Betz’s limit) and downwind (4/27 limit) AWES, both concerning the swept area.
Why “two-stage”? There is only one swept area. The power is insured by the wing. Loyd’s formula works for crosswind fly-gen as well as crosswind yo-yo systems.
I’m currently considering / proposing the term "LIFT-AS-DRAG DEVICE" for kite-reeling,
since it does use lift,
but in the manner of what is traditionally called a drag device.
(traveling downwind at less than the windspeed)
In traditional wind energy parlance, a “lift” device takes advantage of a forward vector fraction of the total lift, but at much higher than wind speed, across the wind, to capture up to ~the Betz Coefficient of power.
One traditional cutoff from a “lift” device to a “drag” device is the working surfaces of a lift device travel faster than the wind. (drag devices’ working surfaces usually travel slower than the wind)
Another is the working surfaces of a drag machine usually travel downwind.
And one more is usually high solidity for drag devices.
Therefore a farm windmill is often referred to a drag machine, due to high rotor solidity and blade tip speed right around the wind speed, or a little slower.
The reeling kites DO actually travel faster than the wind, but their resulting aggregate movement as far as driving the generator (pulling the cable) is slower than the wind speed and in a downwind direction, therefore “Lift-as-drag device”.
In other words, you have lift, but the configuration does not allow that lift to be utilized to its full potential. Still, pretty impressive numbers seem to be occasionally generated.
Now let us take an eye on a classification of AWE concepts (click on “image”):
“Fixed GS” leads to reeling-kite (yo-yo). After we have “tether-aligned”, including Omnidea, Laddermill, Guangdong. Would be them “DRAG-AS-DRAG DEVICES”?
Ever notice how anything undesirable always needs a new name every few years?
Pull-kite - sure, but then people will still want to know how it fits into the lift/drag spectrum.
Tallak: In the case of kite-reeling, lift is indeed employed, but only to create a “virtual surface” which is then used to power a classic case of a drag-based wind energy device, where the working surfaces turn a generator by traveling downwind slower than the windspeed.
You might even call the path of the kite a “virtual drag surface”.
In the context of kite-reeling specifically, since there is a tether involved, “pull” and “drag” are synonymous, since you could say the tether is “pulled” or “dragged” off the drum, however, most “drag” devices actually push the working surfaces, rather than pull.
Kite-reeling utilizes lift, but employs that lift in the manner of a classic drag machine, with power produced by the working surfaces traveling downwind slower than the wind speed.
That was why I suggested “lift-augmented drag machine” (or device) etc.
Other classic drag machines could be said to “push” the working surfaces, bu whether push or pull is used, the power-producing movement is always downwind, always slower than the wind speed.
However you slice it, kite-reeling does not fully utilize the “lift” it enjoys.
In “lift” mode (Guangdong parachute, SkySails…) the kite uses both lift and drag, above all a tethered-aligned kite. It is why the terms of “pull kite” could perhaps be used, as the kite pulls to unwind the tether. While in “drag” mode (Makani), pull is not directly used.
Twisty Tethers