Comparison of the traction force of a static parachute kite and a crosswind kite, both with the same projected surface

[PierreB]

@dougselsam

I’d say Pierre has scrutinized this one as much as anybody, and his question about the height of the anemometer is appropriate - that’s a corollary to that first basic question of even having a power curve.

Skysails present their power curve at 10am today (22 March 2024 CET)

Could it be the same curve as in the figure 15? If yes, the deduced efficiency would be far higher if the reference height was the pattern height instead of the mast height as specified in the figure 15, given the wind gradient as suggested in the text (see the quote on the commentary on which this post is based).

@PierreB

This would be a big difference from the wind speed measurement at mast height as indicated in figure 15 (link also above). An excerpt on 4.1.1.:

A specific measuring campaign achieved an uninterrupted automated flight of 42 h that featured day and night flight with an average power production of 62 kW, in wind conditions varying between 4 and 13 m/s (1-min average measured at a height of 10 m) and estimated wind speeds at flight altitude of 6–19 m/s, showing the robustness of the implemented flight and power cycle automation algorithms and all system components at relevant load conditions.

A value of 12 m/s wind speed at mast height of 10 m) would lead to a value of approximately 18 m/s, if we take into consideration the high range of the two evaluations mentioned in the above quote or, in a lesser degree, the difference between the reference height of wind speed at 200 m (on the video) and that at 10 m.

If the kite keeps its efficiency at high wind speed (this may not be the case), its power at 18 m/s wind speed would be about 3 times its power at 12 m/s wind speed or also, to be more careful and keeping the same proportions of the wind gradient, 12 m/s versus 8 m/s.

Now, 3 times is also the (low) ratio of the traction force that I measured between the crosswind kite and the parachute kite in the photo above. The traction force is used to determine the power in reeling mode, by multiplying it by the reel-out speed (2/3 of the wind speed to simplify), and then by 4/9 due to the loss of 1/3 of the wind speed by going downwind.

My home experiments could better agree with the measurements in Figure 15 with the wind speed at 10 m from the ground, although with the reference to flight height, we would be closer to the theoretical maximum with an L/D ratio of 4, i.e. an increase in the traction force by 4².

My home experiences have highlighted the huge losses of crosswind kites. To be convinced of this, it is sufficient to take a look at the power curves in real time, for example the curves in Figures 15, 17 and 18 which zigzag.