sin αst

ROTOR
6.2.3 Manufacturing
During the process of airfoil selection and blade design, manufacturing has been a high
priority. This section contains a manufacturing guideline that describes the import consid-
erations that must be made when hand-carving the blades. Use of manual power tools or
machining centres provides a good alternative to hand-carving.
The raw material is cut from a single, solid work-piece of wood. It is important that the
direction of the fibres is along the z-axis on the blade as illustrated on figure 6.11.
52
Figure 6.11: Direction of fibres on the blade (left picture modified from [30])
It is recommended to use heartwood (centre wood from a tree trunk) as blade material
due to its ability to resist fungi and attacks from insects. It furthermore increases the mois-
ture content stability. Heartwood is commonly found 40-60 mm from the bark of the tree
and into the pith [30, p. 2-2].
The centreline of the hub junction is angled 2� with reference to the chord of the airfoil, so
that the blade has a fixed pitch angle of 2� when mounted on the front disc of the genera-
tor, see figure 6.12.
Figure 6.12: Angling of airfoil with respect to the hub junction
The series of pictures on figure 6.13 illustrates the manufacturing process from raw mate-
rial to a finished blade. It is stressed that the carving must be carried out with highest pos-
sible precision in order to minimise the imbalance of the rotor.