Low radius may also apply in figure-eight. Some arrangement for a kite-farm where kites fly in figure-eight can allow a better maximization of the space use. I may describe how in a further topic.
On the video at 1’10" from the beginning we can remark the fast turn of 25 stacked kites What kite plans do you like?.
Increasing the power by keeping a low radius turn could be interesting for AWES.
2 stacked kites would be a beginning of a possible High lift coefficient and biplane kite in a flexible kite version.
What this stack of flexifoils shows is how weak the kites are when crowded together, the high pressure wanted under a kite cancelled by the low pressure over a kite below. That’s why one skinny guy can fly them all, and they fly very slowly compared to a single flexi, which helps the pilot manage. An old heuristic is to space kites in a power stack at least 3 Wing Spans apart, and there is still considerable loss of power. A properly staggered multiwing has a different (trailing) geometry than a (vertical) stack, and even then may not beat one good kite matched to conditions. That this stack may do a low-radius loop does not offset the other losses.
Thanks for some interesting input on the distance between kites heuristic. Would it be safe to extend the heuristic to a looping kite, such that wind must travel 3x the wingspan og the kite between every loop? In that case this could be a rule of thumb to design minimum flying radius for a given wing and windspeed…
3 x wingspan is also roughly the diameter of the loop during my experiment on Low radius loop and
If the loop diameter is lower, so only 2 x wingspan, there is more loss due to the no speed of the chord toward the center, in a similar way as a classical wind rotor.
If the loop diameter is higher, from 4 x wingspan and more, the benefit of the higher swept area is annulated due to both the higher variation of power by the variations of the cosine, and a lesser power far from the central zone of the flight window.
No, in principle an Archimedes Screw is effective (at low Re), and the stacking heuristic is mostly independent of wind velocity factor you introduce (that ranges toward high Re).
There is an optimal proportion to drogue apex holes, as an annular sweep analogue, if one is seeking Betz disc efficiency. Wingtip is traded for hub area.
Reminds me of the early ship propeller development. They moved away from an archimedes screw very quickly.
Yes, ship screws operate at high Re, so long screw is not preferred. The odd fact is a long screw turbine in an open flow can develop more power along its axial-depth than the Betz disc assumption allows, by entraining flow all along itself.
Other close real cases are high solidity rotors like irrigation wind turbines and stealth submarine propellers, where they have not “moved away” from the form.
Note: I will now desist from posting in Rod’s favor, as he seems to think I posted improperly on the reeling topic, but I don’t see the error.
A low radius loop allows an implementation of a far larger kite for the same tether length. And as the first experiments show an equivalent power that obtained by large crosswind figures as envisaged usually, the potential of power density increases.
Let us summarize data.
I measured again the wing area for 0.6 m² (not 0.7 m²). The diameter of the loop was about 3 times the kite span (1.2 m).
The traction was 40 N with 4 m/s wind speed. In yoyo mode that leads to a power of 23.7 W. This value is deduced from 40 (N) X 1.33 (reel-out speed) X 4/9 (loss of traction due to loss of apparent wind).
Now, assuming the tether length is 1 km, a similar kite of 120 m span would lead to a power of 3.7 MW with a wind speed of 10 m/s. As it would work by yoyo mode, the power would be between 2 and 3 MW by taking account of the time and the consumed energy during reel-in phase. And a kite of 240 m span would multiply the power by 4.
Now is it possible to build a soft or semi-rigid kite of 240 m span? See
https://www.copybook.com/companies/airborne-systems/articles/worlds-largest-autonomously-guided-ramair-parachute (60 m span) and https://airborne-sys.com/wp-content/uploads/2016/08/ASG-DragonFly-20170206-English.pdf (33 m span for only 230 kg).
And with a tether length of 2 km the potential is multiplied.
As it is a yoyo system, several unities (2 or 3 or 4, not more) would be required in order to use the gap to manage a more regular energy.
Among Dave’s videos wsikfwing (the second) represents a profile flying by autoration under a lifting kite.
I see two advantages by using a low radius loop: using less space (numerous times less than the usual figure-eight for an almost equivalent power) leading to using a far larger kite; then producing a relatively smooth force. A tether of 20 m length (like on the video) as the reel-in phase is starting, at 30° elevation angle, can allow a radius loop of about 15 m, so a soft kite of 5 m span and about 10 m², producing a force of about 4160 N at 10 m/s wind speed, leading to a power of 6160 W. Now with the same proportions (a soft kite of 250 m span and so on) and by using a tether of 1000 m, the force would be about 10 800 000 N, leading to a power of 16 MW. It’s acceptable.
Kite Networks enable the closest practical spacing of low-radius loops. For example, Rod’s Daisy Networks are comprised of low-radius looping wing groups (unit-daisy) set closely together under a lenticular mesh lifting layer.
Low radius loops minimize power bypass inside loops, and kite networks minimize bypass between loops, to allow aggregation of the largest number of loops possible, within a given airspace and land footprint.
What could be the Kite Network arrangement replacing my example of a single (too huge) wing of 250 m sweeping a loop of 750 m diameter? A single ring with numerous wings-blades sweeping the same? Numerous smallers rings? Please could you provide a sketch?
I think Dave is saying you can stack taller near the centre…
Not exactly like any of these drawings… but feel free to expand…with reference to…
Is there a lifting kite at the top of each cell? Or where is the “lenticular mesh lifting layer”?.
There are countless variations possible, and only a few known. KiteLab documented many lifter mesh concepts on the Old Forum. Rod has also covered many variations. Pierre could cut his lenticular tarp kite into a mesh of sub-sails, and it would fly.
There is theoretic advantage for different altitudes of lenticular harvest and lifter units, either by a higher center, or the whole lenticular network tilted up (AoA > 0) downwind. This seems the best way so far to host the most low-radius-loop units within a tight airspace and land footprint.
Again, a lot of early dense-network/low-radius-loop analysis is preserved in Old Forum archives, for those researchers to whom these ideas may be new.
Please use links or photos for actual transferable data comunications. We are humans not future all knowing bots with time to trawl through a massive ill ordered mess of old style online comms. (By which I’m referring (in the old sense without a link) to the old forum archives)
Pierre seems to have missed extended lifter mesh discussion over the years.
Thanks for the graphic above. We honor reference requests as best we can. Its just not humanly possible to provide every request as fast as demanded. It helps to at least state that there are sources known to exist in the public archives, that others can help find as well.
If only the AWEurope’s secretive ventures were so helpful with requested information. They make lots of claims with no public accountability.
Today I made some experiments and measures with the same 0.7 m² kite (Peter Lynn, Vibe) with its two lines of 20 m. This kite was used on Low radius loop. This time two electronic steelyards were used. The previous measures were confirmed. The wind speed was 5-8 m/s, at the best the traction was 8-9 kg with loops of 3-4 m diameter, in such a way that I was pulled forward; with large figures in eight the traction was 4.5 kg on each steelyards, so 9 kg in the whole, in such a way that I was pulled in a side then in the other side alternatively. Then with a wind-speed of 3-6 m/s I made small (10 m) figures in eight getting a similar traction, which dropped when the figures became very small (6 m), as for the loops below 3 m diameter.
As a result the low radius loop looks to be efficient thanks to being in the center of the flight window, optimizing the swept area more than other figures such like even small figures in eight that use more wideness. Perhaps also the repeated circular figure leads to an increasing traction as this appeared. So the small loop could advantageously be used for pumping mode, perhaps also for a flygen, allowing more scaling for a given tether length.
Great to hear about active testing, Pierre.
kPower experience with looping foils under pilot kites also confirms “low radius loop” effectiveness. Adding the pilot kite establishes basic stability without active automation. 100% COTS TRL9 parts-