Subject: Re: Any Experince with Savonius Rotor Windmills?

From: Kerry Miller <%cmsa.Berkeley.EDU:[email-masked]>
Date: Sun, 21 May 1995 04:47:14 GMT

Hugh wrote (citing Joergen),
> |>   Advantages is low cost, standard equipment/materials, and the
> |> interesting possibility to build a Savonius into the existing design of a
> |> standard house, even in a city (chimney-mount if not to heavy)....
>
> Another claimed advantage is that the gearbox and generator
>  are at the bottom of the rotor [often on the ground], with easy
>  access for installation or maintenance.  The more reliable the
>  generator the less advantage.
>
> |> ... For an experiment, start up with Polyethylene
> |> bottles or ping-pong balls cut in half. You will then be able to do some
> |> experiments.

If you want an anemometer, go with bottles. But a rotor of any size
reveals that
it is essential to have *both* bearings securely anchored. Guys won't do it.
The problem is, you're dealing with a three-dimensional dynamic balance
problem (bad enough in ~2D: ever try to balanvce your own tires?) and
the rectangular cross section of a cylindrical Savonius is made to order for
a design that throws itself out of balance once its rotating.

Take your oil drum, cut it in half,  fasten the halves to hubs (offsetabout a
foot) and string it up on a shaft. Then wait and watch the central space
*open up* and youll be able to see the shaft .  What this tells you is that
centripetal forces are deforming the surface -- in the section, from a
vertical line to a catenary curve. But the distortion is different on the
downwind side from the upwind side: i.e., out of balance.  Once the thing
starts throwing out, it's only a matter of time before the guys give way, or
the shaft fatigues or the whole thing topples over -- taking your chimbley
with it.

Having damped your enthusiasm, here's a
possible solution:  hang the axis *horizontally*.: both bearings are equally
rigid (or floppy if you want to look at it that way) plus you can draw your
power off from *both* ends. The axis is of course transverse to the
prevailing winds. (Dont worry about omnidirectionality. That was S's big
idea, because he thought it would work as a sail for water craft which go
all directions - but show me a place that has good consistent wind from all
directions and I'll show you a  place called Oz.)  The rotor will respond
well to wind even 45 off-axis, in any case.

 Secondly, forget the drums; make your own windcatcher with (doubly)
curved skins.  Fabric over ribs is one way to at least approximate this, with
the further advantage that when the wind finally takes it (nothing manmade
yet that's put up against nature stays put forever) its only a few yards of
cloth blowing into the neighbors yard rather than a serious piece of metal
plate.

The comments about loss of efficiency in gearing up for generation are
true enough -- but there are  other things to do with power (e.g. pumped
water storage or hydraulic swashplate compression) and anyway,
whatever happened to the Orbidrive (TM) cam type boxes that were
marketed as having 1000:1 ratios at 97% eff?

If you really want electrics, wind your own rotor: at a radius of two or 3 ft,
something will come of it (providing of course the stators are on the same
floppy/rigid mountings...), tho at this point I'm not speaking from
experience ;-)

kerry miller         [email-masked]
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Re: Any Experince with Savonius Rotor Windmills?

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From: Kerry Miller <%cmsa.Berkeley.EDU:[email-masked]>
Date: Sun, 21 May 1995 04:47:14 GMT