Historical Development of the Windmill - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 31

Figure 1-16. Smeaton's laboratory apparatus for testing the performance of model
windmill rotors [Smeaton 1759].
work in its construction and then performed a series of considered, methodical tests that
would not otherwise have been possible.
Smeaton's model rotor had a sail-tip radius of 53 cm, a sail length of 46 cm, and a sail
breadth of 14 cm. The maximum “wind speed” developed appears to be about 2.7 m/s;
hence, the Reynolds number for these tests was very low, about 25,000. This may have
affected his conclusions quantitatively if not qualitatively. His results can be summarized
as follows:
(a) For lat, untwisted sails, the optimum angle to the plane of rotation should be 15 deg
to 18 deg.
(b) Weathered (twisted) sails should have a twist that gives a concave surface to the
wind, and a total twist of 15 deg with a tip angle of 7.5 deg.
(c) A 25 percent increase in the sail area toward the tip ( i.e. the model in Figure 1-16)
together with an increase in blade pitch of 2.5 deg yields 28 percent more power,
achieving optimum power per sail.
(d) Increasing the sail area beyond a certain point decreases the output.