cyclic loads into the drive shaft and
system during yawing.
Finally, aesthetics can be
considered one of the biggest factors
in determining blade count, since
most people find the three-bladed
rotor is more pleasing to look at as
opposed to a one or two bladed
rotor. All these are reasons why
modern wind turbines use three
Figure 10. Three-Phase AC alternator
Figure 11. Attaching blades to the
the last 50 years have almost
universally used either two or three
blades. Aerodynamic efficiency
increases with the number of blades,
but with diminishing return.
Increasing the number of blades from
one to two yields a six percent
increase in aerodynamic efficiency,
whereas increasing the blade count
from two to three yields only an
additional three percent in efficiency.
Further increasing the blade count
yields minimal improvements in
aerodynamic efficiency and sacrifices
too much in blade stiffness as the
blades become thinner. We will do
some experiments on this to confirm
if the WindPitch exhibits behavior
close to these same efficiencies.
Switching from blade theory and
design to the WindPitch power plant,
I mentioned at the beginning the
WindPitch uses a three-phase
alternator (Figure 10) rather than a
simple DC motor. The primary
advantages of alternators are no
commutator and increased power
output due to the three-phase
architecture (refer to Part 7 for
background on three-phase versus
The alternator is permanently
wired in the STAR configuration, so I
won’t be able to do any STAR versus
DELTA comparisons, but with a three-phase alternator ALL the electrical
power is delivered directly to the
rectifier circuit with nothing lost
through the mechanical and electrical
resistance of a commutator. In the
WindPitch, the alternator is inserted
inside the plastic fuselage with its
shaft protruding from the front and
attaching to the rotor hub (Figure 11).
Number of Blades
Three-bladed turbines are not a magic
number as these photos point out.
The 98 meter diameter (longer than a football
field), two-bladed NASA/DOE Mod-5B wind
turbine was the largest operating wind turbine in
the world in the early 1990s. Like the Spruce
Goose of the 1940s, it too is an exaggerated
example of what can be done with a particular
technology. Photo credit: Wikipedia.
The NASA Mod-0 research wind turbine at
Glenn Research Center’s Plum Brook station in
Ohio tested a one-bladed rotor configuration. Its
odd looks probably contributed to its lack of
popular acceptance even though its one blade
may have proven effective for its design goals.
Photo credit: Wikipedia.