Historical Development of the Windmill - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 32

(e) The maximum load that can be lifted is nearly proportional to the square of the
wind speed.
(f ) The tip speed of the sails in lifting the maximum load is nearly equal to the wind
speed ( i.e. the optimum tip-speed ratio is unity). Therefore, the rate of lifting the
maximum load is nearly proportional to the wind speed.
(g) Hence, as a consequence of (e) and (f ), the maximum power of the sails is nearly
proportional to the cube of the wind speed.
Smeaton's work, however imprecise in measurement, gives basic insights into the process
of wind energy conversion, and much of it is still valid today.
In spite of all Smeaton's good work in analyzing performance, the builders who
continued with their established designs and practices seem to have taken little notice of it.
Of course, some of the labor-saving improvements we discussed earlier were made, but
otherwise, there were only minor changes. The steam engine put a brake on the use of
wind power, although the established design of the windmill did hang on for a long time,
continuing to be built in the early twentieth century. However, in the second half of the
nineteenth century a new form emerged, the multivane or annular windmill , which was
usually small but capable of satisfying the needs of a farm or ranch with respect to pump-
ing water. In the United States, every farm had one or more of the multivane mills, and the
type has come to be called the American windmill. This is a useful characterization in the
same sense as the term Dutch windmill , and thus it is so designated here.
Figure 1-17. Some of the many designs of the American windmill, which was used for
pumping water [Baker 1985]. (Reprinted by permission of University of Oklahoma Press;
©1985, the University of Oklahoma Press)