Environmental Engineering Reference
In-Depth Information
Figure 17 : Blade design elements and pitch positioning.
aspect ratio of 3.5”/1.5” or 2.33 and a 2
8 is 7.25”/1.5” or 4.83. In either case it
is easier to defl ect the span in the fl at or shorter direction as opposed to the longer
dimension. For wind blades these load directions are referred to as fl apwise and
edgewise, respectively.
Blades are constructed to be as light as possible yet still providing the strength,
stiffness and life required by the system. This is achieved with internal structures
that incorporate either a box spar or shear webs. The box spar is a radial beam that
the aerofoil skins are bonded around. The shear web approach uses the aerofoil
skins as part of the spanwise structure with the internal shear webs transferring
loads from one side of the aerofoil to the other to form a defl ection resistant box
structure. Box spar construction has an advantage for longitudinal strength and
uniformity, but it is heavier and structurally ineffi cient relative to the shear web
approach. Both techniques are successfully used in today's GFRP blades.
Figure 17 illustrates a number of typical blade design features as viewed by an
observer looking up from the base of the tower with the wind approaching the
turbine straight-on and travelling from left to right. Sketch /A/ shows the blade in
the 85
×
90° or fully feathered pitch position. In this position the blade is primarily
experiencing edgewise loading from the wind. Sketch /B/ shows the blade pitched
to the intermediate blade angle of 65
−
68° (as will be explained in subsequent sec-
tions, this is the position used to start and reinforce rotor rotation for the case of
increasing winds above cut-in wind speed). The blade in this position is experienc-
ing a combination of edgewise and fl apwise loads from the oncoming wind and
wind gusts. Sketch /C/ shows the blade pitched to the full operational angle of
around 0° - the position maintained throughout the variable speed region of the
power curve. In this position the blade primarily experiences fl apwise loads due to
the wind.
The combination of blade prebend, cone, drivetrain tilt and overhang can also
be seen in Fig. 17. These design features are not of signifi cant consequence for the
pitch positions of sketches /A/ and /B/, where the blade is lightly loaded and span-
wise defl ections are small. However, blade spanwise defl ections are greatest for
−
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