The backfly would be used in reel-in phase to destroy the pull while the kite goes down fast, as shown on the videos. I got to do a backfly (see the video): it should be able to be modeled.
From the video the kite flies very slowly downwards. So this exact setup may not be terribly efficient in terms if time usage. You could fly the kite «properly» backwards if the kite was designed for this, but then it would also produce lift and tether tension.
Also you are «cheating» by saying the video explains everything. If you reel out by 1/3 windspeed upwards stroke and in by 1/3 windspeed downwards stroke (falling) the difference in force will not be so convincing. And even by this standard you are spending more than half your time unproductive (half the time plus losses)
Rather than trying to prove me wrong I suggest you rather listen to my arguments. If you dont believe me, do some calculations or tests and prove me wrong. Id be happy to hear that I missed a sound architecture. But, without any new design, I’m pretty confident that most of my remarks ate spot on on this one.
The Magenn up and down trajectory I have commented earlier, I dont thing it should have ever been taken seriously. In particular by the Magenn team who should have known better.
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I exposed my ideas by indicating then following about:
It is the opposite of what you made by stating quite wrongly about the factor of 9, then pursuing also wrongly by stating a vertical flight is not a crosswind flight, without even talking about the coefficients of drag that are obviously inapplicable in backfly configuration.
I had recently the same discussion about the vertical flight with an expert starting by stating the same as you did before admitting his mistake. It looks like a Pavlov reflex: what is not a figure-eight looks to not be crosswind, but this is wrong. A kite flying vertically pulls almost as strong that a kite flying by figure-eight, and by the same its speed is several times windspeed. As you don’t admit this evidence you could test it and you will see.
As I repeat I have manually tested the backfly, and it generates substantially no pull. And the video shows (between 8:30 and 8:42) that the backfly is fast. Even on my video we not see a substantial difference between backfly and upward flight concerning the speed. All these are observations based on tests and that it is possible to see on the videos.
Now about what is questionable, as I mentioned:
In other words I see a way to solve reel-in phase in vertical path by using backfly as the kite goes fast without pulling.
Now my questions are:I cannot describe how backfly works, what are aerodynamics forces and so on. A deeper analysis is suitable, but not on wrong basis which prevents all future discussion. The second question is if and how this maneuver can be computerized.
A simple test would be to go out in 9 m/s wind, fly up while walking 3 m/s into the lines, then backfly while walking the same distance backwards. Hopefully match the walking speed so that you and the kite arrive at the starting position at the same time.
Now look at the times used in each phase and then the distance walked and the feeling if the forces on the kite (eg you feel a factor 4:1 out vs in).
I think this test would be a better foundation for further discussion. The discussion would amount to how could this be optimized to produce whatever average power you could expect from a 3 m kite (0.5 kW?). Then we could argue if the technique could scale to useful 1 km2 size…
Nothing magic is happening aerodynmically during «backfly». The kite is probably just acting like a blunt body. The drag coefficient is probably not 2, but it is certainly greater than 0.05. I also see that the area of the kite itself is being reduced due to bridle compression.
The effect bringing the kite to ground is gravity. So in order to move faster to the ground the natural thing would be to add mass. This will anyways happen with larger scale kites by cubic law of mass scaling. If you plan on using many small kites, the effect will anyways hold true for the tether.
So even if you found a way to bring the kite contracted to almost zero area, you still need to find a way to fly the kite back to the ground. From experience I can say that a kite cloth not being in the normal inflated shape will not fly nicely to the ground in higher winds. It will be quite irregular.
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I count 6 seconds during upward flight, then 6 seconds during backfly on the video 8:30 to 8: 42.
Another false statement. Magenn’s flight is (was) stationary, unlike the another Magnus-Effect-Based I provided the link and which is expected to fly in yoyo mode by using vertical path. Please read it carefully, comprising the figure 1.18 I already mentioned.
Indeed commenting on your statements on this topic is not of great interest to me, as it masks the real questions that I may have on this subject and that I will leave open.
I did indeed provide some decent feedback on your initial posts, but it seems you failed in willingness to read them. Anyways I wish you luck in your progress on this strategy for AWE, and hope you prove me wrong
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It is already done on the specific points that I mentioned.
I could agree, but many prototypes already exist and produce electricity. On the other hand, none of them details a credible plan towards a technically and economically viable AWES, at least for now.
No you did not prove, only stated disagreement.
You would need to make a wing that could fly forwards and backwards for this to work. Not one that turns into a potato sack for backwards flight.
The magnus kite has this property by changing the direction of rotation, or perhaps they use gravity.
For a rigid wing, it is not easy to get pitch stability going both ways.
If your kites are bridled, note that the angle of attack is very different along the trajectory, so also I believe you need a mechanism to adjust the bridle shape dynamically.
And because the kite will stop at its most downwind point, it will cause a difficult flight condition where kite and tether mass will directly slow the kite down. Because high efficiency kites fly very fast, if such a wing is made to stop, it must be energized to a high speed by other means than wind in order to start flying again.
Your crosswind flight pattern is indeed a correct one from fig 1.18 in the paper. I was probably too quick to state your poor description of your idea. Still, all my arguments are still valid and will provide engineering difficulties should you want to continue with this.
It was not taken seriously by me - I said it was whacked from the beginning.
And yet so many supposedly legitimate sources such as I seem to remember even NASA used the Magenn images in their promotion of AWE.
I feel that many of the ideas tried are not so promising, while many promising ideas are apparently not even contemplated.
Wouldn’t the kite-reeling setup pull a stalled kite down faster than a backwards-flying kite would descend on its own?
There is not disagreement as such, it is only 100% wrong. It is impossible as I explained:
You stated also (not for the first time), concerning a vertical trajectory:
A vertical trajectory is crosswind.
I don’t see how an analysis can be valid with such a poor basis on two mains points.
About drag coefficients please see my reply. I would add a Cl = 0 is not practically achievable as a little lift is required to stabilize the device (as you want) without undergoing a possible front stall which is different from the back stall, since it affects the leading edge instead of the trailing edge. As a result a significant traction is produced during reel-in phase, as shown on the curve on the figure 23.13 I link again.
I wrote something similar before but apparently you did not read it:
Indeed I have also already integrated that some problems can occur by using backfly although it can have more potential.
Why not? It is what I want: destroying pull then go down fast in a controlled way if possible.
The magnus kite slows down the rotation.
I don’t know. A vertical trajectory is not possible with a constant length line. Here the vertical trajectory is possible thanks to reel-out action. As a result the progressively additional length of the line may make the change of angle of attack unnecessary, especially as the apparent wind and the real wind are in the same direction throughout the trajectory. Finally it is a point to clarify.
On my tests (I put again the video below) the kite started again quickly after backflying quickly and without touching the ground, if we consider that soft kites have a limited L/D ratio of about 4. I think to other possibilities I will perhaps explain later.
I think no, the kite “backflies” as fast as upwards flies, so likely faster than the kite-reeling setup could. The “low speed” is only due to the use of a soft kite which is slower than a rigid kite. With 5 m/s wind speed, the kite speed could be about 15 m/s, and 30 m/s with 10 m/s wind speed.
Tallak it is true I am not explaining all the possibilities that come to me. There are so many highly-credentialed “geniuses” promoting AWE. If they are so darn smart, they don’t need me to think for them, now do they? As another example, it has been long-documented that I thought of the basic configuration as a teenager in the 1970’s, so cleverly named “laddermill” by Ockels after he had a PhD and won a trip to Earth orbit. No matter how celebrated he became after the laddermill idea was publicized, and no matter how it was discussed, modified, improved, rendered, etc., nobody ever built one. To my knowledge, nobody even tried. No “students” or “interns” or anyone ever built even the most rudimentary attempt. Instead of building laddermills, they all built excuses why it would be too hard, would not work well, etc., and went back to kite-reeling, while retaining the empty name “laddermill”. You’d think one group, somewhere, could have at least tried one. I mean, you could make one with bicycle parts, some fishing line, bamboo cocktail skewers, and plastic bags. Personally I was pretty excited when the idea was first popularized, and was hoping to see it in action. Wah-wah-wahhh… no sorry, nobody can seem to do much of anything, besides flying a kite and pulling the string. Whoopee-doo.
Well at least all those kids got to tell their mothers “Look at our group-selfie on the internet Mom! We’re flying kites! Wheeeeee…”
I may at some point start publicizing all the possibilities I see, as they come to me, but like you, I might say I’m “not allowed” to just spew them out ad hoc as though there is no value to them.
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Here is an alternative to backfly that could work well for a fixed wing.
Rather than flying backwards, do a quick 180 degree pitch maneuver to fly upside down on the way downwards. In the simplest case the wing profiles are symmetric and thus fly as well upside down.
The idea works better if the kite is constructed from many smaller subkites, reducing the time needed to change direction.
Another variation is having a fixed wing with a symmetric wing profile, then let the fuselage move (motorized) such that the wing is attached at either side (front and back) when moving up or down. The tail becomes a canard and vice versa, but you get stability due to movement of the main wing.
I think this a valid route for your idea, my initial concerns still remain
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Thanks for the indications Tallak.
I thought about the 180 degree pitch maneuver for horizontal path (which has some similarity with vertical path, aiming also to fill the space) to replace classic turns by rolling which take too much place and time. The problem is that you should be able to pass the tether and the bridle (if there is a bridle) through the wing during the U-turns. For this a slot should be installed in the wing to allow this passage.
Concerning the variation the motorized fuselage move looks to be a complex solution.
And also, above all if rigid wings are used, the kites should be stopped at the end of the downward flight before crashing at full speed and tension.
I had some (different from your variation) idea of a reversible wing comprising both a tail and a canard and flying back and forth, but the efficiency of a reversible (as for your variant _ for what I understand _ but unlike the symmetric profile for 180 degree pitch maneuver you indicate in first. Some clarification would be welcome, and also from me) wing profile could be reduced a lot, the stability was not assured, and the possible crash could be by the tail (or the canard) which would be likely and easily broken.
Dave Santos indicated also some flexible wings derived from single skin Nasa NPW and Dave Culp’ sails could fly back and forth.
Below there is a video showing stacked horizontal paths.
Hi @dougselsam, I know you think reeling (yoyo) mode is a nonsense in regard to wind energy, being as if a wind turbine retreated to produce, then advanced without producing to complete the cycle. I discussed about backfly with several experts (of which @tallakt here but not only): all said that it is something like a worse thing in aeronautic. The worst in aeronautics may well go with the worst in wind power. What do you think?
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Hi Pierre and thanks for asking. I think your backfly idea might work considering the kite may be dragged down using power supplied from the groundstation anyway, which may result in a stable descent, however I personally would not be pursuing kite-reeling because, as you say, the concept is in retreat as it produces power, resulting in using power to recover position while losing time which could have been used to produce power. To my way of thinking, an important aspect of any wind energy system is to “stand its ground” and not allow the working surfaces to travel downwind. The concept is “crosswind”, which as I say, is so elementary to wind energy for the last 2000 years, it should not need to be said. That is not to say these observations can’t be overcome by overwhelming economic viability in spite of the inefficient aspects. Every energy production scheme has tradeoffs, so the proof will be determined by how it all works out, but kite-reeling, as popular as it is, may not turn out to be an economical way to harness wind energy. To me, wind energy should be simple and reliable. There are too many things to go wrong in wind energy, and the idea is to minimize what can go wrong, not add to it. Thanks for asking my opinion, which is subject to change when evidence to the contrary emerges.
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Today I tested again my 4 m² 4 line kite, with a higher wind speed (for the previous test wind speed was 1-4 m/s), from 4 to about 6-8 m/s. During ascending phase I was deported and lifted forward, the force being enormous. During backfly phase the kite descended quickly but only a few yards, then slowly descended until I put some traction back on. So it descended by several steps like on the video, but more slowly because of its area compared to the smaller kite area on said video. Indeed a larger kite takes more time to be inflated after being depowered. In the end descending backfly phase took far more time than ascending phase.
My experiments confirm this. So I am going to leave this idea behind while trying to explore a reversible kite that can do U-turns without turning, in order to make it larger.
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Today I experimented the same 4 m² 4 line kite, including a full backfly. The explain of the full loss of pull is simple: the kite losses thrust including lift and falls like a rag under gravity; the tethers are no longer under tension. I didn’t manage to get it up, as I didn’t have enough time when the kite was almost down.
The manual control on the videos 2 (not mine) and 3 (mine) was achieved by alternating backflies and stages with tethers under tension during the descent.