Windmill brake.
 

I am building a windmill to aerate my pond. I have a blade from a thousand ton cooling tower, I took a air pump from an old school building and will link them together with a set of pulleys and a belt. My problem is when we get gusts of wind in 50's I don't to have to find pieces of my windmill in the neighbors yard. So I need a brake on it to prevent from spinning out of control. I have looked around and haven't really come up with anything.

I know nothing about this other than the name of the part you need--centrifugal brake - Google Search

Good luck----

You could try a Furling system (I had to look up the name, so don't take me as an expert). The turbine is essentially mounted on a hinge with a spring and when the wind blows too hard the turbine rotates out of the wind and slows down.