I thought I bought 6" round duct pipe, but apparently it's actually 5" in diameter. Anyway, I played with it a bit and figured I could make a neat rotor using a 2×4. I put it in the CAD software to get the dimensions (note also that I'm using a 10% gap between blades at the axis which is recommended by most people.
The idea was to notch the ends of the pipe halves and bend them into tabs to screw onto 2×4's. I wanted to ensure that one tab extended to screw into the end of the 2×4 and that the arc of the pipe fell tangent to the edge of the 2×4 so I could put another screw in the side through a tab. What I found was to cut the 2×4 to 9.2 inches long.
I learned that it was important to ensure the bent tabs form parallel planes. My first rotor came out crooked … not by too much, but enough that it would vibrate like mad in even modest wind. (By the way, that's the axle from a baby stroller wheel that I used as a pivot point to test it.) That, and I should alternate my cut tabs inward and outward so they don't overlap on the inside of the curve. Oh, and that you can only screw into tabs that are on the outside of curve — I guess unless you've got a short screwdriver or make the pipe segments longer than I did.
The best part, though, is that the rotor is very rigid. I'll be more diligent about the next design, and I sure hope it will survive strong winds.
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