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

[PierreB]

@PierreB

These last losses are due to the kinetic energy increase when the kite goes faster within the trajectory, and having to be dissipated when the kite slows down, at the cost of enormous energy consumption.

It’s as if, in a car, you were constantly accelerating and braking instead of maintaining the same pace, for an identical average speed.

To my knowledge, this point has not been taken into account in the various equations derived from Loyd’s model.

Can they be corroborated by other tests than mine, whether for flexible or rigid wings in crosswind flight?

Regarding flexible wings, some hints are exposed again on Annual Reviews: Autonomous Airborne Wind Energy Systems - Engineering / System Design - AWESystems Forum.

Regarding rigid wings, flying in circular path:

Multiple Wing Systems – an Alternative to Upscaling?

1: 47: “wing area of 1 m² generates 40 kW (at 13 m/s wind speed and L/D of 15).”

3: 08: “10 kW for 4 m² plane [Vander Lind 2013] at 8 m/s wind speed,i.e. 25% of potential.”

I wouldn’t even consider the example on the left side of the image, which is still far less favorable.

The 4 m² plane can have a L/D ratio of 15. If that’s the case, what leads to only 25% efficiency, apart from the irregularity of power (and perhaps tether drag in a lesser level), hence the losses of the power generated by the accelerations and decelerations during the path, assuming that the L/D ratio of 15 partially or fully includes the tether drag?

See the curve of a Makani wing on:

Makani Power Autonomous Flight. June 18, 2010 - HD

@PierreB

Then (with slightly lower wind) a regular (as expected) traction force of 3 kgf plus 1 kgf was achieved on very tight loops (about 3 m in diameter like in the old video of Low radius loop that showed the traction force on a single line, with the second handle having been released and the two lines wrapping around each other).

It is possible that large-radius circular trajectories, such as those of the Makani wing, result in fewer losses due to irregularities compared to figure-eights which likely produce more accelerations and decelerations.