On Fri, 17 Mar 2006, Olin Lathrop wrote:

> Wouter van Ooijen wrote:
>> The 'eggbeater' style vertical rotors work on lift, not drag. The
>> rotors have a vertical-areoplane-wing form, creating lift when the
>> wind flows around. The advantage of lift is that the rotor can move
>> faster than the wind flowing around it, drag is limited to the wind
>> speed.
>
> Yes, I understand that, but there has to be a change in airfoil profile or
> angle of attack between the front and back.  Something has to preferentially
> push the blade one way when it's at front, then the other way when you flip
> it 180deg.  Unless they are counting on the wind shadow effect of the front
> blade on the back, but that doesn't sound too efficient.

The blade that faces the wind the 'right' way (aerofoil-wise speaking) 
generates lift. The other does not, but neither makes a lot of drag 
because they are aerodynamically efficient (thin chord). These are 
Darreius rotors and they do not start by themselves, they have a starter 
motor or a small Savonius turbine built into the axle to start them. The 
foils generate torque only for a part of the rotation.

But the most efficient turbine is the two-blade 'propeller' type, by 
far.

Peter
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