Historical Development of the Windmill - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 15

Lift Replaces Drag
Both arguments, however — that for the diffusion theory and that for straightforward
development by upending the driven end of the ordinary water mill — require consideration
of another, very innovative step that warrants somewhat more attention than it has received.
Although the right-angle gear mechanism allowed the rotor axis to be transposed from
vertical to horizontal, the action of the sails also had to be turned through 90 deg. This was
revolutionary, because it meant that the simple, straightforward push of the wind on the face
of the sail was replaced by the action of the wind in lowing smoothly around the sail,
providing a force normal to the direction of the wind. As a concept, it is indeed a
sophisticated one that was not fully developed until the advent of the airplane at the end of
the nineteenth century and the engineering science of aerodynamics.
In fact, although they were not aware of it, the irst builders of the horizontal-axis
windmill had discovered aerodynamic lift and had used it to achieve a greatly improved
design over that of drag , which is the force that powered the Persian windmills. That the
concept of lift is dificult to comprehend, except in precise terms of low over an airfoil or
an edgewise lat plate with an angle of attack , is illustrated by a modern example quoted
in a small topic of short essays accompanied by ink-wash renderings of some European
windmills by Sir Frank Brangwyn, RA, and Hayter Preston [1923]. In an introduction,
there is a discussion of the comparative power of the horizontal- and vertical-axis types, and
another knight of the realm, Sir David Brewster (1781-1868), is quoted thus:
“The planes of these sails of the horizontal-axis windmill are placed obliquely
to the plane of the revolution; so that, when the wind blows in the direction of
the axle, it impinges on the surface obliquely, and thus the effort of the sail to
recede from the wind causes it to turn upon its axle.”
This picturesque, if somewhat humanized, description is based on drag and not lift, but
apparently Brewster's nineteenth-century explanation was acceptable to the twentieth-
century authors of the topic.
Because understanding the concept of lift and its application to the horizontal-axis
windmill does not seem to have been possible in the twelfth century, it is argued that the
form came about empirically. However, historians disagree as to the steps that were tak-
en along this empirical path. There seems still to be a logical gap in understanding the
transition from the drag to the lift concept; hence, we have no convincing explanation the
sudden ubiquity of the horizontal-axis windmill in Northwest Europe to the exclusion of the
vertical-axis type until much later on.
The upshot is that we don't know the answer. Shall we ever do so? The odds might
be against it, but Forbes has said (with respect to water mills) that many of the ancient
manuscripts have not yet been translated or studied. Likewise, Wailes, in connection with
the many earlier references that had proved false before those of c. 1180, states that “there
is always the hope that some new and fully authenticated ones may be found.” One wonders
also if all of the manuscripts studied have come under the eye of a scholar who is aware
of the importance of references to molinology, and whether heretofore they might have been
passed over unremarked. We might remember, too, that it took some 2,000 years for the
Dead Sea Scrolls to come to light.
Windmills spread rapidly throughout Europe in the thirteenth century, more or less from
west to east, although in view of the relative paucity of authenticated records the
appearances we know about are certainly not necessarily the earliest uses. Notebaart
provides worldwide coverage, with information on the type of windmill and other details
when they are available. It must be emphasized that attested dates of use must have been