There is no power for an AWES if the lines are static such as this, The arrangment as recoreded would be perfect for downwind sailing. Exceptional actually. For an AWES however power from the kite is associated with the tension on the lines being let out. So the question here is; what is the optimum shape/flight path as the lines extend under tension?
Are we going for a long cycle where we let say 100m of line out then recover the kite? or is it better to have a kite that oscillates between 50-60 meters and recoups/resets after every loop or so? Perhaps there can be a scheme that takes advantage of gusts and subsequent lulls?
My tests were aimed at measuring the force I indicated, not directly the power in reeling mode, which can however be easily deduced by multiplying the force (N, so here about 40 N) by the reel-out speed (1/3 wind speed) and then by 4/9 because of a lower apparent wind. Result with 4 m/s wind speed: 23.7 W. (L/D)² was about 7. We then come back to the classic formula, here without counting cosine losses: kite area (about 0.6 m²) x 1.2 (air density) x 2/27 x wind speed³ x (L/D)².
I’ve been diving deep into your tests on kite force and the way you’ve approached power in the reeling mode. Your results, especially the 23.7 W power output at 4 m/s wind speed and the L/D squared value around 7, got me thinking.
I’m playing with an idea, kind of like how a clock’s escapement mechanism works. Imagine a system that adjusts its resistance when the kite’s tension goes beyond a set threshold. So, if we hit a certain torque, we’d let the line out at 0.2 meters every second.
Let:
T = tension
T_threshold = our set threshold
Rate of line release (R):
R = 0, if T < T_threshold
R = 0.2 meters/second, if T >= T_threshold
When the tension (T) increases, motors would need compensate to keep the line payout consistent. This isn’t about spinning faster, but adding resistance.
Breaking it down:
Motor voltage is determined from a raw value and scaled.
Motor current is calculated as: Motor voltage divided by Motor’s internal resistance.
Line tension is then derived from this motor current.
The motor’s effort adjusts based on the difference between the current tension and our set threshold. It’s a feedback loop that keeps the line payout consistent, no matter the pull on the line.
Just like how cruise control in a car maintains a consistent speed regardless of uphill or downhill terrain, the motor system ensures the kite line is pulled out at a steady rate. Even if the kite’s pull varies, similar to a car facing different slopes, the system adjusts itself to keep the line’s release consistent, ensuring the kite ascends at the rate.
When orbiting, the difference between all the control lines remains relatively static, in the video below you see that during a loop only slight inputs need to be made to the tether line line while the other line orbits around
I measured only the force. You can see on the video (0:26) that the force peaks at 50 N at the top and then gradually decreases as the kite descends, certainly because the wind (being measured at 4 m/s at my height) was stronger at the top, which more than compensated for the cosine loss.
The power is only deduced by calculation, and has not been tested as such.
Measuring force was a prudent move. To emulate drawing energy from the system, one could just walk downwind with the kite. I’ve observed that when looping the kite, it’s smoother if there isn’t a twist build up in the line. The mathematical rationale behind this is still something I’m trying to decipher.
I don’t understand how the tether tension/power is only a function of the kite area and the wind velocity. What if the kite rotates faster and sweeps more area? Doesn’t the power increase?
I don’t see how spinning the kite faster or increasing the swept area causes the L/D ratio to increase. Does this mean that the ratio varies with the effective wind velocity?
The more regular power generated by low radius loop can be an advantage in addition to the advantage of using less space.
That said in yo-yo mode, given the current art, a pod is required to achieve the control of different phases and of steering without losing altitude if possible. But a pod can be too heavy, while a lighter system aloft can be sufficient thanks to the simplicity of the loop path. Perhaps some device like OKE Precision Winch "Reel and Rotate" Technology with an adaptation for reeling-mode to generate electricity would be suitable.
Test today 09/18/2024 with the same Vibe of 0.6 m² rather than 0.7 m² (projected area 0.5 m²), east-southeast wind 8-9 m/s on my new anemometer, a steelyard for each of the 2 handles: 6 + 3-4 kg maximum for large amplitude eights, approximately the same with large horizontal trajectories, 5 + 4 kg for large loops, 5 + 2-3 kg for low radius loops. That said the proportion between the low radius loops and the large loops or figure eights seems more or less preserved:
Just like previously, the low radius loop pulls less, but not much less.
That said, the accuracy of his tests is relative because the wind changes all the time, from one second to the next. So sometimes a measure can be lightly contradicted.
The wind was twice as fast as previously, but the traction was doubled, not quadrupled, for both loops and eights.
Flexible wings seem to lose performance as the wind increases. And the flight speed is limited for a paraglider.
Cool @PierreB
What’s your ratio of low loop radius Vs blade span?
Roughly 1.5 from the early videos I’d guess.
Which seems to me like a very tight loop for a soft kite to get strong traction from
I’m posting again the old initial video made with the same 120 cm wingspan Vibe kite.
The tests yesterday were with the same kite but twice as fast wind, and the diameter of the low radius loops was also around 2.75 to 3 m. It’s very tight, but still around 2.5 times the span of Vibe: that’s what I saw, and which can be measured approximately on the video itself.
We would expect a greater drop in traction because the inner part of the kite, although not stationary, flies significantly slower.
My explanation for maintaining the relatively high traction is that the kite is located more toward the center of the flight window, where the traction is higher.
Another thing: as it is a very small power kite, I think it peaks more quickly and does not deliver as much relative power for large and faster trajectories as a larger power kite that is designed for this purpose.
We can therefore predict that a SkySails or Flysurfer with additional rods to increase performance wings could be relatively less efficient flying tight loops than large figure-eights or loops or horizontal trajectories. In other words, the difference between tight and large loops or eights that I obtained with the small Vibe could increase on better and larger wings which are designed to deliver relatively more traction when flying at higher speeds.
On the video, we can see that the traction decreases from 5 kgf to 2.5 kgf as the kite descends while rotating and the cosine losses decrease, indicating a steep wind gradient. The wind was much stronger at 20 m than at ground level (4 m/s).
