Historical Development of the Windmill - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 28

improvements took place concurrently with the Industrial Revolution, a time rich in
mechanical invention.
The irst device we consider, for the setting of the sail, was the spring sail invented by
Andrew Meikle in 1772, in which a number of hinged wooden shutters mounted
transversely across the stock replaced the sail cloth. These shutters operated like vene-
tian blinds tied together by a longitudinal bar and iron cranks that opened and closed them
simultaneously. At full power, the shutters were closed and presented a lat surface to
the wind. Excessive wind speeds caused the shutters to open and spill the wind. The
movement was controlled by a spring; the initial setting was made with the mill at rest, and
once it was adjusted, the speed of the mill could be kept reasonably constant. Spring sails
were somewhat less eficient than the common sail because the shutters could not be
weathered satisfactorily. Thus, sometimes a mill would have two common sails and two
spring sails, as a compromise between output and steady running.
The second device was that of Stephen Hooper, who by 1798 had replaced each of the
stiff slats in the spring sail with a lexible roller-reeing sail. His main contribution to the
state of the art was a rotor control device in which furling was activated by a long striking
rod that passed right through a hole bored centrally down the entire length of the windshaft.
A cross or spider was afixed to the poll end of the striker. Each of the four “legs” of the
spider was connected by a lever to a bell crank ,,which operated a longitudinal rod along the
stock of a sail. In turn, this rod operated bell cranks ixed to each roller or shutter. In this
manner, a linear motion of the striking rod caused simultaneous opening or closing of all
the individual segments of the sail.
At the tail end of the windshaft was a rack-and-pinion mechanism attached to the
striking rod together with an endless chain that reached to the ground or a convenient lower
level. Pulling the chain turned the pinion, thus moving the rack and hence the striking rod.
As Wailes notes, the hole for the striking rod was drilled from the tail end of the windshaft
to the poll with the mill turning, the stationary drill being lengthened 0.5 m at a time. This
must have required excellent craftsmanship as well as courage on the part of the
millwrights. There were no laser-guided drills in those days!
Apparently, the roller-reeing sail itself was not successful, but the spider control
mechanism was retained by William Cubitt, who amalgamated it with the shutters of the
spring sail to form his 1804 patent sail. A further improvement was to attach weights to
the endless chain to keep the sail shut until the wind speed increased and exerted sufi-
cient pressure to open the shutters. Thus, the patent sail effected automatic sail setting and
relieved the millers of much arduous labor.
In France, Berton introduced a different orientation of the shutter around 1840. Instead
of being in the transverse position, the shutters were itted longitudinally for the whole
length of the stock and were pivoted so that they could close to make an almost lat surface.
When open, they were positioned one behind the other, as seen from the leading edge of
the sail. They had to be operated manually from inside the mill and were used extensively.
In addition to their use as sail elements, shutters were also utilized as aerodynamic
brakes. There were several varieties, with one, two, or three shutters itted longitudinally
at the outer end of the sail in remarkably similar fashion to the aileron control surfaces of
some modern experimental wind turbine blades. In his 1860 design, Catchpole used two
such shutters placed at the leading edge of the outboard third of the patent sails. These
longitudinal shutters were operated by the same mechanism as the transverse shutters.
When closed, they provided additional sail area. When opened, they were at an angle to
the plane of rotation of the sail assembly and hence, being outboard, could rapidly produce
a strong braking action. They were called Catchpole's skyscrapers. Another aerodynamic
brake design consisted of two or three shutters again arranged longitudinally near the tip but
located on the trailing side of the whip in place of some four or ive transverse shutters.