Environmental Engineering Reference
In-Depth Information
cloth could be attached. The sail bars were initially placed symmetrically on either side of
the stock, but in later times the common sail had the stock positioned as shown in Figure
1-14. We would now call this a
fractional chord position
with respect to the sail bars, at
1/3- or 1/4-chord, for example. This chordwise location of the stock is discussed in more
detail later.
The forward ends of the sail bars supported a leading-edge
wind board
that directed
the wind onto the sail and helped to hold the cloth irmly against the frame. At the poll
end there was a transverse iron bar onto which the end of the sail was attached by rings and
eyelets in the fashion of a present-day curtain. Ropes were attached along both lengthwise
edges of the sail so that it could be drawn radially outward and fastened at the tip. Note
that the tip had to be within reach of the miller, standing on or near the ground, or on the
tower stage, for those mills that had one.
Furling
of the common sail acted to control power and rotor speed much as the
variable pitch control does for the modern wind turbine.
Pointing lines
,
additional cords
near the tip, allowed the outer part of the sail to be furled back to the whip. When the mill
was not operating, the sail was unfastened at the tip, twisted into a roll, and cleated to the
whip. This was a considerable improvement over the earlier sail design, in which the spar
was at mid-chord and separate cloths on each side of it would over and under alternate sail
bars, making furling a very awkward task. However, the design of the common sail still
required that the mill be brought to rest when the sail area had to be rearranged, so the
miller had to be able to draw the brake very tightly to ensure that the sails did not sweep
him away in a gust of wind.
The earliest sails were inclined at a constant angle to the plane of rotation of about 20 deg,
whereas the common sail was given a
twist
from root to tip to vary the inclination
continuously along its length. This was called
weathering
the sail, and it was done by
mortising the sail bars through the whip at different angles which, according to Wailes,
might vary from 22.5 deg at the root to less than zero at the tip [1954]. This was undoubtedly
an empirical discovery, because it is unlikely that the millwrights were aware of the
concepts of
relative velocity
and
angle of attack.
Perhaps weathering was prompted by
observations of the behavior of the stretched cloth along its length “catching the wind” or
“illing the sail.” What seems somewhat surprising is that it was carried out as far as
placing tip sail bars at a negative angle to the plane of rotation. Although negative blade
pitch at the tip is now recognized as being theoretically correct in some instances, in those
days it must have looked wrong when the mill was at rest.
Jan Drees put forth some observations on the design and performance of windmill sails
based on his studies of sixteenth- and seventeenth-century paintings, etchings, and engrav-
ings of rotors, comparing these with the features of modern rotary wings, i.e. of helicopters
and the like [1977]. As a rotary-wing engineer, he was amazed to ind that such modern
design features as
nonlinear blade twist, leading-edge camber
(“droop snoot”), and
fractional-chord position of the main spar (stock) could all be found on the sails of large
windmills of the seventeenth century. Although many of the examples he quotes have been
recognized for some time, Drees' contribution is to emphasize a pattern of continuous
historical development of windmill sail technology in relation to modern concepts.
Drees developed a diagram of what he considers took place in windmill sail design
between the thirteenth century and today, and this is shown in Figure 1-15. Some of his
statements are conjectural, and he acknowledges that a great deal more research is necessary
to establish the validity of his contentions. According to Drees, a 1550 engraving by Pieter
Breughel the Elder shows for the irst time a windmill sail with the stock moved forward
from the 1/2-chord to the 1/3-chord position. This approaches the optimum position of 1/4-
chord for the aerodynamic center and the center of gravity of modern airfoils, which mini-
mizes twisting moments. This eventually allowed a threefold increase in rotor diameter
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