For maximum extraction, on a drag machine, the turbine needs to be loaded to a TSR of 0.33 or 1/3 of the wind speed. The closer it gets to a TSR of 1 the less power it can extract.
For instance, if the blade, or drum, is moving away from the wind as fast as the wind is commng in then there can't be any power extraction.
On another note, the efficiency of the savonius type blade is very close to the Betz limit when loaded if you consider the actual wind it's extracting from. As an example, the wind is moving at 10mph the turbine is loaded to a TSR of .33 or moving away from the wind at 3.3 mph your actually extracting energy from a 6.7 mph wind. Unfortunately, after you subtract the up wind blade drag and other machine inefficiencies the power is considerably lower. Since your only extracting energy from one side of the turbine it makes it look like it's not very efficient overall when in reality the blade or drum is actually extracting energy more efficiently than alot of "good" turbines that are out there.
Put that 1/2 drum on a linear alternator and run it in a straight line for a mile or two it would have better efficiencies than almost all the turbines being built. You may need to run the return drums in a tunnel below the main run to reduce the losses.
Unfortunately they just don't do so well in a circle...
;O)
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Have Fun!
Windstuff Ed
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