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Assuming smaller propellers would spin even in very low wind, whereas a large propeller would stall;

Would it be efficient to use a bunch of propellers from axial fans geared to a single motor instead of a single large propeller?

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""Assuming smaller propellers would spin even in very low wind, whereas a large propeller would stall; "" No, this assumption is (and will stay) Yours alone. Guess why wind propeller generators became ever bigger and higher during the last 20 years. –  Georg Sep 19 '11 at 16:50
    
Erm. Because it isn't efficient to gang multiple propellers to drive a larger one? –  Everyone Sep 19 '11 at 17:28
    
It's not a bad question, but it actually would help the question if you can provide some of the reasoning behind the assumption you're making. –  David Z Sep 19 '11 at 17:47
    
Not a lot of reasoning ... more like letting my thoughts freewheel. I was thinking about how easily a whirlygig spins away merrily. So ... we mount the axial fans (minus the motor) on the larger blade, and use their spin to make the larger blade move. –  Everyone Sep 19 '11 at 18:05
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The power output of a wind turbine is proportional to the square of the blade length $L$ (specifically, the "swept area" enclosed by the tips, which will be $\pi L^2$). A single, large turbine is more efficient because it has fewer moving parts than several smaller ones. If you added up the friction from every bearing and gear in all of the smaller turbines, it would be greater than the equivalent from one large turbine.

Thus, for almost any given situation a single large turbine is preferable to multiple smaller ones. As an added bonus, a large turbine will have to be higher above the ground where wind speed is generally higher; capturing even more energy.

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