A Savonius is a drag-based VAWT that was invented yada yada yada, blah blah blah, go here. This is a modified version with five blades. Why five? Because there are five bumps on the bottom of a two-liter bottle and it's easiest to follow the trend.
Basically all I had to do is cut along the pattern shown marked on the bottles. Lift-based VAWTs are easy from any cylinder, just cut flaps, fold back, staple, and wonder how plastic is going to turn fast enough to get good lift without tearing apart. I doubt they would work very well on this small scale anyway, but I'd love to hear about if if you've tried. Anyway, the Savonius took a wee bit more effort to figure out how to keep the bottle in one piece.
The angle in the pattern isn't really important, just as long as the end of each short line is at least 3/4" vertically from the end of the next one. Also the pattern should not go too far into the curved top or bottom of the bottle. The right bottle shows how I melted through at these points to increase durability -- that is where there will be the most stress on the plastic so unless I do this it is likely to tear over time. It also gave me a place to get a knife in to make a starter cut.
While my pen was hot, I also melted a two holes through the neck and two in the bottom. These places on the bottle are very thick and almost impossible to cut. I'll get to what these holes are for when I next update this page (still experimenting.)
A template, made from another bottle, helps make things nice and even. Just mark through the cuts on the template, and then rotate the template to the next notch. The bumps on the bottom of the bottle lock in place and provide a perfect 72 degree turn. I'm making more than one, so this also keeps them more or less identical.
When cutting I was careful not to cut through the other side of the melted hole.
Then I just folded vertically between the melted spots.
I made a bunch and strung them together with a bottlecap/wire u-joint between each, then hung them from a branch in my backyard with a hard-drive bearing at the top:
It trails out nicely like a windsock, and people seem to like it despite being obviously made of soda bottles. I think the color choice helps. I just wish there was a way to get the remaining glue from the labels off neatly to pretty it up a little more.
Without any load, it spins in the slightest breeze. I did hook it up to a steppor and it could jerk some light into a LED in a squall, but due to the tricky coupling making it hard for it to spin, it ended up being more impressive to just let it fly.
Anyway that's been out in my backyard all summer/fall, including some rather severe squalls, and as long as you don't let anything stop it from spinning for days on end, the bearing seems to shake off any corrosion nicely and remain smooth. The bottles show no sign of wear, but they have collected a good amount of icky vegetable matter. From a distance you can't tell though.
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