Blade Design, Manufacture, and Testing - Small Wind Turbines: Analysis, Design, and Application - page 134

grounds that this would ease the fitting of the fibreglass reinforcement if

composite blades were made by a closed mould process.

7.5 Blade Manufacture

There is a wide range of possible materials for, and methods of making small
blades. The suitability of each may vary with blade length. For example, timber is
an excellent material for small blades. Peterson and Clausen [ 4 ] documented the
material properties and fatigue behaviour of Radiata Pine and Australian Hoop
Pine which is extensively used for ultralight aircraft propellers. Both these timbers
grow well in plantation. Hoop Pine is the assumed blade material for the example
blade design in the previous section and the application of the IEC SLM in Chap. 9 .
Sinha et al. [ 5 ] provide similar data for Nepali timbers as well as information on
weathering of a number of surface finishes. Other timbers, including Sitka Spruce,
also widely used for propellers, and Douglas Fir, are suitable for wind turbine
blades, provided they are grown sustainably. However, the cost of laminating it is
high, and so the only practical way of using timber is to carve or machine blades
from solid blanks. As length increases, it becomes more difficult to obtain blanks
that are knot- and defect-free. Computer numerically controlled (CNC) milling of
timber, which was used for the 0.87 m long blades in Fig. 7.7 , is straightforward
but is probably too expensive for volume production. The most promising tech-
nique would appear to be the use of a copying router using a master blade possibly
cut from metal on a regular CNC machine. Development of this method is
underway at the University of Calgary and progress will be reported in the online
materials http://extras.springer.com . Whatever method is used, considerable care is
required to reproduce the design aerofoil profile. Figure 7.9 shows measurements
of the surface of one of the blades in Fig. 7.7 near its tip, in the region where
significant differences in blade shape could have large impact on the power output.
Unpublished calculations by Barbara van Bossuyt using aerofoil computational
programs XFOIL and RFOIL did not indicate a significant power loss due to the
change in shape (Fig. 7.9 ).
For longer blades some form of composite manufacture is preferable for
which there are many material and manufacturing issues shared with large blades,
e.g. Brøndsted et al. [ 6 ] and Dutton et al. [ 7 ]. For example, blades of all sizes

0.1

0.05

0

0 0.2 0.4 0.6 0.8 1

Distance along chord line, x/c

Fig. 7.9 Measured blade surface at r/R = 0.995 on one blade from Fig. 7.7 . The crosses show
the measured surface. The design aerofoil profile (SD7062) is the solid line