I thing this has not been covered.

I was thinking about the tether drag on a reeling system. Lets assume a wind speed w. The reeling speed r is independent on w but looking towards experience with other designs, and related to tether strength, we can assume that r should be at least a third of w but more is better. The eact ratio is determined by a gearing ratio. For now lets assume the speeds are equal, r \approx w

The apparent airflow on the tether is thus increased to \sqrt{w^2 + r^2} and the tether drag is pointing in a 45 degree angle relative to the initial wing.

Anyways, the tether drag will be possibly quite significant acting as a Â«frictionÂ» force on the reeling system. The faster you reel, the more of this effect.

Lets also try with some numbers; assume a system with elevation angle 90 degrees producing 1 kW in 7 m/s wind, and also r =7. There are two tethers, due to reeling, and the distance ground to kite l is 200 m. Tether diameter d is 2 mm.

The drag on the tether lengthwise, as a loss to reeled power would be

D = \frac{1}{2} \rho \left[ 2 w^2 \right] d l 2 C_D \sin^2{\frac{\pi}{2}} \approx 7 \mathrm N

The power loss would thus be 0.07 kW, or close to 7% of the systems power output.