Structural Dynamic Considerations in Wind Turbine Design - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 589

loss of effective disk area. The rigid interconnection of two blades restrains coning while
permitting the rotor to teeter freely with respect to the shaft. Blade pitch angles may or
may not be coupled to the teetering motion. A passive means for accomplishing this
coupling is the d 3 hinge , in which the teeter axis is not perpendicular to the blade axis. The
d 3 -angle effect is discussed in more detail later.
Unsteady air loads are attenuated as a result of the upwind-downwind motion that the
teeter hinge permits. This motion causes passive changes in blade angles of attack to
oppose and eliminate the major once-per-revolution (1 P ) component of these air loads. For
example, a steady vertical wind shear , which would create large fluctuating air loads on
blades rigidly attached to the shaft, will create only a small teetering motion which in turn
creates a cyclic fluctuation of angle of attack and eliminates the fluctuating air loading. The
fluctuating loads that would result from a side wind component or yawed flow on a rigid-
hub rotor are similarly eliminated by a teeter hinge.
A teeter hinge also attenuates, but to a lesser degree, the air loads that result from
small-scale turbulence (spatial turbulence on a scale smaller than the rotor diameter) and the
wind speed deficiency in the tower shadow , which is important for rotors operating
downwind of the tower. In summary, the advantage of the teeter hinge is that no major out-
of-plane bending moments from the rotor can enter the rotor shaft, whatever their source.
Some or all of this advantage can be lost if teetering is restrained by dampers, springs, etc.
Number of Blades
The third major influence of rotor type is the choice of the number of blades. If the
rotor is not hinged there will be major blade bending loads caused by wind shear, small-
scale turbulence, yawed flow, and possibly tower shadow. These loads will bend the blades
cyclically once per revolution of the shaft. If the hingeless rotor has two blades, these
cyclic blade loads will shake the nacelle heavily at two cycles per revolution (2 P ) . If there
are three or more blades, the nacelle will not shake as heavily, but each blade will still
suffer the cyclic bending strains. Either a teeter hinge or individual flapping hinges will
eliminate both the blade bending and the nacelle shaking. A three- or four-bladed rotor
could be provided with the equivalent of the teeter hinge effect by a concept called
gimbaling , but this would result in an excessively complex and costly hub construction.
Based upon the above fundamental aspects of rotor configuration influence, the two-
bladed, teeter-hinged rotor emerges as a highly attractive choice except for one additional
factor. The mechanics of the teeter motion introduce cyclic in-plane accelerations of the
blades because of Coriolis effects. If the power train is torsionally stiff and the lateral
retention of the hub bearing is stiff, there will be substantial cyclic torque in the shaft and
cyclic force on the nacelle at a 2 P frequency. However, low shaft stiffness and low lateral
nacelle stiffness are usually adopted for other reasons, so these Coriolis effects appear as
very small motions rather than significant cyclic loads. Therefore, the load-attenuating
behavior of the teetered rotor on a compliant nacelle structure becomes nearly ideal.
A final rotor feature that influences cyclic loads and prospective fatigue life is the
modulus of elasticity of the material used in blade construction. Low blade stiffness
enhances the cyclic displacements and velocities that can occur at a given vibratory stress
level. This in turn increases the aerodynamic damping and the inertial impedance that the
blade experiences in response to transient loadings. Thus, the selection of rotor blade
materials for high ratios of fatigue stress allowables to modulus of elasticity can reduce
loads and increase fatigue life. These ratios are generally favorable in wood and some
composites and generally unfavorable in metals.