The next step (building the blades,see my response to v.m.)will be very spectacular.

The aluminium cilinder with the bearings will sit in a steeltube,that will be attached to the tower.The pronybrake will be sitting on the ground(need for continuous adjustment)The shaft going down must be stiff and light (again),to avoid extra bearings.I'm thinking of bambou again.
If you know a bambou 'forest',bambou sticks have an unbeatable stiffness/weight/price ratio.
I hope the pronybrake tests will allow the decision if it's worth building a alternator,kind of systematic aproach.