Environmental Engineering Reference
In-Depth Information
Windmill Technology Development
Throughout history, windmill technology represented the highest levels of development
in those technical ields we now refer to as mechanical engineering, civil engineering, and
aerodynamics. The best technical minds of their day were constantly seeking to improve
the design and operation of windmills. A continuous series of modest changes introduced
and tested by builders and millers must have occurred that inally resulted in the reinement
and advancement of windmill technology. We will now consider some important technical
developments in three areas: sails, the heart of the windmill; control devices, particularly
those making the miller's work easier and safer; and technical analysis, which determines
the factors that inluence performance and eficiency.
Development of Windmill Sails
Windmill sails and their development are topics about which we would like more
information, since most of the early designs are known only from a few dated pictures of
a general nature. There is no adequate record of sail designs for the earliest years of the
horizontal-axis windmill. Through the ifteenth century, we have to rely on crude illustra-
tions and carvings that were quite small and showed little detail. Perhaps sails were at irst
lat boards, but these were soon replaced by
a cloth-covered wooden lattice consisting of
transverse and longitudinal battens. This
lattice was ixed symmetrically on a central
spar, forming two “ladders” through which
the sail cloth was laced over and under
alternate transverse battens. The sails of the
large post mills and tower mills developed
chiely in the countries of Northwest Eu-
rope; most of the discussion that follows is
concerned with the type called the com-
mon sail. Although it was replaced in many
instances by improved designs, it was still
in use at a very late date.
Figure 1-14 shows the structure of the
common sail diagrammatically. The main
structural element was the stock , which
either was mortised right through the winds-
haft or was itted into the iron canister or
poll end. It could be as long as 27 m;
although anything longer might have led
to structural problems, it was most likely
limited to this length by the availability of
the right kind of lumber. Along the length
of the stock were fastened narrower timbers
called whips , through which transverse sail
bars were mortised at intervals. To the sail
bars were nailed longitudinal laths, the
outside ones called hemlaths and the inter-
mediate ones (one or two) called uplongs.
In this way, a lattice was formed on which
Figure 1-14. Structural components of the
common sail.
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