The pull of the Megafly would give us approx 300 kW output power in an AWE application. I also am looking at the extremely high number of bridle lines that would cause a lot of drag. Supplied glide ratio 3.75:1 is very low for AWE in my opinion, though the aspect ratio seems a lot higher than Skysails’ kite
The Megafly does though indicate a 300 kW unit could be made at a kite only mass of around 400 kg, which is a medium number. The mass of the KCU remains the only unknown. The lift to mass ratio would be
I am assuming the KCU would have a mass of 400 kg. This is double that of the one specified, though I imagine gliding flight is much easier than flying a pattern for AWE
Are you thinking of something otherwise than just a bigger Skysails kite? My logic is a kite could scale to any size unless mass is an issue… I did not calculate that though
Disregarding mass scaling factors, some things that come to mind:
I think it makes little sense to make a giant kite that then either makes a low radius loop, so the inner part of the kite moves slowly, or a larger radius loop, but due to a limited top speed of the kite, it only makes one revolution over some long period of time. So from that perspective, large kites make no sense if space use is at all important and you are only considering a single kite and are not making use of swarm logic or a MAWES.
I suspect that you don’t have to increase the chord by all that much before boundary layer separation becomes an issue, even with the wing having the perfect shape and angle of attack, you have perfect wind without any shear, and so on, all of which you won’t have, and will likely become bigger problems the higher the chord.
If your kite is making a large loop, you also need a large vertical (and horizontal) distance of uniform wind blowing in the same direction. On land, you’re likely to run out, assuming you want to fly above the earth’s boundary layer and want to limit the ground area radius your kite can overfly.
You’ll perhaps want to vary the shape/camber of the wing for different (wind) speeds and different modes of flight, which I think would become more difficult the longer the chord.
Longer time and higher cost to launch and land, and to construct, inspect, repair, and deploy the kite, for example.
There are lots of variables that have to be just right, and I think with most, the probabilities are just a little or a lot worse the larger the chord is, so the outcomes when you multiply the probabilities, say capacity factor or efficiency, are much worse.
In addition to my previous questions:
Other questions might be: what is the size of circle or figure eight you want to make given the (local) weather and wind characteristics, among other considerations; at what range of altitudes do you want to fly; and how long should a single loop take.
You use wing span as the scaling factor, then you scale width and length with that same factor, resulting in the square. I would have sooner chosen wing area so it is a bit more explicit that width and length of the kite can be independently varied and the area=power relationship is clearer. An assumption is that an increase in chord has no effect on efficiency. As I say above, I think that’s an interesting question to explore, and I would probably go for an increase in aspect ratio first.
This is also what I approximately get with 1/3 wind speed (of 14 m/s), assuming a L/D ratio of 4 (glide ratio of 4:1), which is reasonable by taking account of tether drag, so (L/D)² of 16.
But 1/3 wind speed is the reel-out speed of the tether. So the apparent wind speed therefore becomes 2/3 wind speed. And I get approximately 13.4 tons, and 10 tons by assuming a squared coefficient of cosine with an elevation angle of 30 degrees (cosine = 0.8660).
By the same, concerning the calculation here I would get approximately 66 tons, and 49.5 tons by assuming a squared coefficient of cosine.
We would have to add the mass of the tether. For the 900 m² wing, by extrapolating towards a breaking load of 100 tons to be safe enough, we get a weight/m of more than 0.6 kg. So a 1000 m tether would weight about 600 kg.
Concerning the power during reel-out phase, with air density =1.2 and cubed coefficient cosine = 0.65: 1.2 x 2/27 x 900 x 14³ x 3.75² x 0.65 = about 2 MW. The average power during the cycle, taking account of time, reel-in consumption and other losses) would be about 500-600 kW. This could match the SkySails power curve (average power during the cycle) at 14 m/s, assuming a possible global L/D ratio of 4.
I am thinking here that the mass of a kite and the maximum load it can carry is correlated. So a 900 m2 kite carrying 10 ton would not have the same mass as a 900 m2 kite carrying 60 ton.
Maybe we should put all these kites in a table or a plot, to make more sense of it.
When estimating power earlier in this thread, I am just multiplying max tension in the tether by 3 m/s approximately. What conditions required to achieve such pull I have not considered.
Now let us see the weight of the system for the 1 ton load 102m² kite (the second kite of the list) named FireFly: 73.5 kg. It would be in 0.7 kg/m² range, so a little more than the 900 m² wing which has a weight of about 575 kg including the canopy plus the airborne guidance unit, so being in 0.6-0.7 kg/m² range.
Beside the weight issue, other already evoked problems can occur when scaling. Peter Lynn Himself :
Scaling Effects: As a kite is made larger, there are changes in three relationships that effect the way it responds to angular and lateral displacement, and that therefore effect stability.
-For rigid, framed, and even soft kites (for which fabric thickness must be increased for very large sizes- even when they have super ripstop reinforcement), area (and therefore lift and drag forces) increase with the square of dimension while structural weights increase at a bit less than the cube of dimension. Big kites are therefore heavier and/or distort more and aren’t able to fly in strong winds.
-And, for large kites, the relationship between their rotational inertia (flywheel effect), weight, corrective moment (the length of the upward-seeking pendulum), and aerodynamic forces, cause them to respond more slowly to being knocked askew (by turbulence or wind direction changes for example). As far as I can figure so far, response time is inversely proportional to dimension squared. Large kites therefore respond much more slowly than small kites to lateral and angular displacements- even proportionally. This suggests that large kites should be more inclined to superstability and less inclined to volatile instability than small kites- but from experience, I’m not sure that this is true.
But what I am sure about, is that this is true for ram air inflated kites- because of disproportionate increases in the mass of air trapped in the cells. A soft kite’s area (and weight if the same fabric is used) increases with the square of dimension while the volume of air it contains (at 1.23kg/cu.m) increases with the cube. This causes larger ram air inflated kites to tend towards superstability.
Commentary: When the area of a ram air kite is increased by x 2, the mass of air it contains increases not by a factor of 2 but by 2.83. A negative consequence of this relationship is that every successful ram air inflated single line kite will fly optimally in only one size- make it bigger and it will be inclined to superstability, smaller, towards volatile instability. To counter this effect, the Guinness record size kites we have made use thru cords rather than cells so that most of the internal air mass does not have to rotate with the kite’s body.
Particularly:
As far as I can figure so far, response time is inversely proportional to dimension squared. Large kites therefore respond much more slowly than small kites to lateral and angular displacements- even proportionally.
Should we expect the crosswind (eight-figure or loop) figure area to expand faster than the kite dimensions in scaling process? If yes, that would lead to an increase of the non-used wind energy within the crosswind figure or/and the impossibility of completely performing the figure in extreme scaling cases, so practically a brake of scaling.
According to a different or even opposite interpretation of the previous one, scaling the kite area would simply lead to scaling the crosswind figure, both scaling by the square, assuming the weight scales also by the square, which is not quite the case.
Besides that I think the author meant “response time is [inversely] proportional to dimension squared”, “inversely” going against.
This could go with the above, with the surface area of the kite increasing by the “dimension squared”, as for the “response time”.
Still I am not able to decipher without deeper investigation if turning rate scales inversely to wingspan or if it is otherwise.
In any case, I would not trust numbers derived from a 6 m2, 9 m2 and 12 m2 to apply for a 1000 m2 kite. I would bet Skysails maybe know though, what happens at a few hundred m2 size.
But assuming turn rate scales inversely to wingspan, flight pattern simply scales with wingspan, and turning speed is not a scaling issue (that is I agree with your assessment I think)
Let us though assume scaling of a huge kite with air trapped inside. The volume of air scales with x^3. For the 200 m2 kite the volume may be 100 m3 and already account for 120 kg mass. The moment of inertia would scale x^2 but the moment created by changing drag at either wingtip would change x^3 (distance to wingtip times wing area creating drag). So this would indicate turning speed would speed up with scale. Which is probably wrong.
If we otherwise assume turning rate is dependent on difference in glide ratio left and right wingtip, turning speed would indeed scale inversely to wingspan. And air trapped in the kite would further slow down turning because the air mass scales x^3 and the forces involved scale x^2. But, because pattern size will increase, time spent will also increase by a factor x meaning that also this difference is offset (like we discussed earlier).
Anyways, an analysis based on wingtip glide ratio points to no scaling issue for turning speed of huge kites.
I wonder if Filippo’s first question from his PhD holds true here too?
For a given wingspan he found there was an optimal mass; As mass is the factor which most affects turning radius. Radius too tight = induction issues, Too large = gravitational potential energy exchange issues.