I have been watching the discussion on blade design and the physics
of lift and power. I have a few responses:

Laurie asked how the wind is slowed by the turbine. The simple
physics answer is that energy is taken out of the air by the
turbine. Since power is 1/2*density*velocity^3*area and
energy=power*time, for a given area and incompressible flow (much
less than Mach 1) Velocity has to be decreased in order to get energy
from the wind. Remember that energy can only be converted from one
form to another, not created. (remember that law from high school
physics) A simple physical explanation is seen in propellers. The
"slipstream" behind a propeller has a higher velocity than the air in
front. In that case, the engine is putting energy into the flow by
spinning the propeller. A wind turbine generates energy so the air
must be slowed.

If questions arise with a particular blade design, one can analyse
its performance before building anything. The same analysis
techniques for propellers also apply to wind turbines. This was
shown in 1926 by H. Glauert and his method of propeller and windmill
blade analysis has come to be known as "Glauert Blade Element
Theory." A good presentation of his method is available at
http://aeroserver.aero.usyd.edu.au/aero/propeller/prop1.html. The
method is presented for aircraft propellers but with a little
tinkering one can see how it applies to wind turbines. Remember that
coeff. of power = (coeff. of torque)*(tip speed ratio)

______________________
Matthew Duquette
Clarkson University