On Sep 13, 10:21 pm, Jordan wrote:
....
The challenge is how to control the things so they don't spin too fast,
but still be able to start in a light breeze.
....
Jordan

I built several rock-tumbler windmills when I was a kid. They were set
up out back behind the garden where the noise bothered only the birds.
The blades were very thin sheetmetal attached at the leading edge and
they bent back as the wind increased. Typical life was about 3 months
before the "bearings" seized.

If this is a Savonius type, maybe you could shock-cord the sail at the
inner edge so a strong wind opens it up to bleed air and lose
efficiency.

jw