a couple of points to past posts,
re: helix,it doesn't matter what shape of blade is downwind, it is still down wind, or in the shadow of the blade in front of it. With a lift blade though you get some power from the down wind side depending on the tsr and profile you use.
you can find a lot of usefull info on builds at this site for vawts and mills in general: www dot vawts dot net this is where 5-25 and the caleb I know lives
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