How to measure the angle of a rope on a moving airplane?
I'm not entirely clear on the scale of the plane and the forces involved, but from the sound of it the first version is going to be something like a 1:12 or 1:24 scale plane? Also, do you need both axes or will one axis do?

I would like to present the "load cell" approach. You would need to work this into the mounting plate for the hook on the bottom of the plane; for example, four strain gauges, one in each mounting bolt. You might have to mount them "upside down" since they tend to prefer compression to tension. This will tell you the force applied, while not requiring anything outside the plane.

Finding the right parts may be a bit of a problem, but fundamentally they're the technology used in $10 digital kitchen or bathroom scales, so for a prototype you might just be able to grab them from there. They're also the tech used by the Wii Balance Board to determine how people are placing their weight.

In re the power generation to the skeptics: this is a variant on "kite power", or an answer to the mathematics that suggests that a one-blade wind turbine is most effective. All it requires is putting an aerofoil into the wind stream and extracting energy from it through lift. The available power increases with wind speed, which increases with altitude.