Still out of ideas for my ball machine, I constructed this K'nex windmill in about 2 hours yesterday. It stands 9 feet tall and uses about 700 pieces. The thing that separates this windmill from my previous attempts (which I did not post on the blog) and others that I have seen is that it uses no non-k'nex pieces. The blades are made from 20 k'nex panels, which seem to give me the best weight-to-wind-resistance ratio. Though they are full of small holes, the wind still catches the blades pretty well.
The windmill will turn steadily in winds as low as 3 miles per hour, and can easily reach 60 RPM on a normal day. The shaft of the windmill is connected to a small gearbox, which generates a surprisingly large amount of torque. I may even end up powering a small mechanical device with this if I feel up to it.
The blades of the windmill are about 1.25 feet long, and it sits atop an 8 foot tall tower which is stabilized at its base by 4 bricks.