Today I tested my Peter Lynn Vibe of 0.6 m² at 8 m/s wind speed instead of 4 m/s wind speed, with large crosswind figures or tight loops, obtaining about 9 kg traction (instead of 3-5 kg with 4 m/s wind speed according to several measures) which was measured with a steelyard at each handle. But the pull should have been more, about 12-20 kg, knowing also that 12 kg value compared to 9 kg falls within the margin of error.
If we analyse the data on the table 8 page 44 of https://repository.tudelft.nl/islandora/object/uuid%3A56f1aef6-f337-4224-a44e-8314e9efbe83
we can see that the efficiency of the (1, 2, 3, 4, 5 = Mutiny of 25 m², 6 = Hydra of 14 m²) kite looks to be higher at low wind speeds, examples: 1) 2.8 m/s wind speed, 4.06 kW average generation power phase; 6) 10 m/s wind speed, 13.8 kW average generation power phase.
And also the figure 15 of the pdf seems represent a rather low value of average 92 kW by taking account of the 12 m/s wind speed.
Perhaps some study could be made in order to know if the efficiency remains constant or if there is a progressive coefficient of loss according to increasing wind speed.
Some remark; even at low wind speed, the apparent wind will be generally higher than the real wind speed for a static (lifter) kite. And also I observed that the efficiency of a Sharp rotor increases as the real wind speed increases, at less until 12 m/s.
An explanation from @rschmehl that seems plausible:
I think the stakes can be high. Perhaps some (crosswind) flexible kites can be studied in order to lead to suitable deformation at high wind speed, keeping its efficiency at high wind speed.
The concern is different for my small Sharp rotors (see above) as they are rigid. But surely larger inflatable versions could undergo deformation by high load, which also explains why the power consumption of a Magnus-effect balloon is so high.
We have seen our 25 m2 kites flattening due to higher loading, which would increase the resultant aerodynamic load because of the larger effective area. On the other hand, the wing is also billowing more which increases the sweep angle. All these effects are linked to the wing design and the bridle system layout. More quantitative results to come soon…
Could it be that the tether force depends on the **effective wind velocity ** (vector sum of actual wind velocity and crosswind velocity). Crosswind velocity does not change much with increase in wind velocity and that is why the tether tension does not increase exponentially. Do you notice much higher crosswind velocity at higher wind velocity?
I think that the real cause of the lowering of efficiency with increasing airspeed is both the drop in glide ratio from an optimum speed (see the link below) as well as the rather limited speed that a flexible wing can achieve compared to a rigid wing (Best Glide and Speed to Fly for Paraglider Pilots).
Crosswind velocity does increase with the wind velocity. From Loyd’s theory, we know
\lambda = \frac{L}{D} (1 - f),
where
\lambda = \frac{v_{k,\tau}}{v_w} is the tangential velocity (aka crosswind) factor, \frac{L}{D} is the lift-to-drag ratio, and f = \frac{v_{k,r}}{v_w} is the reeling factor.
You can see that the tangential (aka crosswind) kite speed v_{k,\tau} depends linearly on the wind speed v_w, as opposed to your statement.
Yes @rschmehl , but a flexible kite cannot fly very fast. As an example if its lift-to-drag (glide, L/D) ratio is 5 (with tether drag), it could perhaps fly at 25 m/s airspeed with a wind speed of 5 m/s, but not at 100 m/s airspeed with a wind speed of 20 m/s.
Cosine parameter was added to Loyd’s formula. Perhaps (for flexible kites) a variable coefficient of loss could be added, by taking account of increasing wind speed, knowing both the limited (air)speed of a flexible kite and the drop of the L/D ratio from an optimum speed.