Blade Design, Manufacture, and Testing - Small Wind Turbines: Analysis, Design, and Application - page 133

Fig. 7.7 Machined 0.87 m long timber blades for the turbine in Fig. 6.1 . These blades have
holes at the 2/3 radius point for static and fatigue testing
Fig. 7.8 Finite element
model of the Aerogenesis
2.5 m long blade for Load
Case H of the IEC simple
load model. The strain is
colour-coded with the
maximum occurring in the
constant chord region at the
hub. Image from Phil Clausen
reiterated at the start of this chapter. Most small blades are held in a rectangular
attachment section similar to that shown in Fig. 6.1 . The back plate of the blade
holder is attached to the generator shaft and then four bolts sandwich each blade
between the front and back plates. These bolt holes can be seen in Fig. 7.7 .
For commercial turbines it is advisable to make it difficult or impossible to mount
the blades backwards for protection against inexpert installation. The attachment
section often lies in the plane of rotation, although not always. The two blades on
the Aerogenesis 5 kW turbine shown in Fig. 7.6 are mounted at 20 to the plane of
rotation. Extensive finite element analysis (FEA) as demonstrated in Fig. 7.8
showed that this arrangement minimises the stresses in this region. The other
noteworthy feature of the small blade in Fig. 7.6 is that its leading edge is straight.
Recall that blade element analysis does not fix the element position along the
chord line so a wide range of tapers is possible for the leading and trailing edges
for the same net taper of the blade. The straight leading edge was selected on the