Today, by wind speed of 5 m/s, I experimented my 1 m² two line power kite, 0.2 kg without lines, 0.3 kg with 20 m lines, in vertical trajectory from bottom to top, and face to the wind in the middle of the wind window. Then I compared with horizontal trajectories one by one then chained into figure-eights, then loop.
The traction developed during vertical trajectories was significantly lower than that developed during the other figures, half for the best. This impression of little power during vertical trajectories was confirmed by my steelyard: 10 to 30 N, against 40 to 70 N for the other figures.
The gravity can be an explain. I noticed that every time the kite deviated from its vertical trajectory, it immediately gained additional power.
Another explanation, which I had to mention here and there, is that the horizontal trajectory is twice as long as the vertical trajectory, articulating on either side of the power zone, unlike the vertical trajectory ( which cannot go underground).
A version of the chapter 12 is available on:
See the figure 1.24. See also the 1.7 Conclusions and Perspectives page 25, mentioning a theoretical value of 1.48 kW/m². The chapter 13 stated that the vertical trajectory is more powerful than the conventional trajectory (for Magnus effect-based balloon).
But theoretical value for crosswind flight with figure-eight (not purely vertical trajectory) would be still higher, as stated on:
Table 2, balloon 40 x 12.5 = 500 m²; 5. CONCLUSIONS :
This results in a mean power of the full cycle of 1.47 MW.
So 2.94 kW/m².