Structural Dynamic Considerations in Wind Turbine Design - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 588

Philosophy of Load Control
A second fundamental design philosophy is that of controlling or attenuating the
aerodynamic loading transients at their point of application: on the blade surfaces.
Softening of the supporting structure can provide some reduction of these cyclic air loads
(through damping effects), but features such as the rotor teeter hinge are more effective in
eliminating major components of the cyclic air loads. Active pitch control could also
contribute to direct alleviation of cyclic air loads. All control methods of air load
alleviation involve some form of feedback from the external disturbance to the airfoil angle
of attack. Some, therefore, present possible dynamic instability problems. These must be
understood through adequate models in the system simulation.
Aerodynamic Stall Philosophy
A third choice that has divided HAWT designers into two schools is that of
encouraging aerodynamic stall as a means of control or of intentionally keeping the rotor
out of stall. While control through stall permits mechanical simplification, it presents
difficult additional problems in predicting and controlling periodic and transient loads
arising from stall hysteresis. By selecting stall control the HAWT designer introduces not
only a new source of fatigue loads, but also the need to account for almost unpredictable
unsteady aerodynamic effects in the system simulation.
The VAWT designer doesn't have the option of avoiding periodic stall and is therefore
confronted with the difficult problem of accounting for it in system simulations. This
explains the greater interest among VAWT designers in stall-hysteresis effects and the
development of special airfoils to reduce undesirable stall loads.
Rotor Geometry
The choice of the basic geometry of a wind turbine rotor has a major influence on the
way the structural system responds to air and acceleration loadings and develops internal
loads. The configuration elements that can be used to control these loads are blade coning
angle, blade-to-shaft hinges , and number of blades.
Blade Coning Angle
The steady thrust component of blade air loading can be alleviated as a dominant
source of blade bending stress by providing a suitable fixed coning angle , in which the
blades are inclined downwind from the plane of revolution. Coning combined with
centrifugal force acts to counter a selected portion of the downwind bending that would
otherwise be created by the thrust air loads. Both the steady air loads and centrifugal loads
can be simply and accurately calculated and simulated, so this aspect of rotor design is
straightforward. It can and should be accomplished independently of other features that
involve dynamic behavior of the system.
Blade-to-Shaft Hinges
A second fundamental influence of rotor type on loads encountered is the presence or
absence of a teetering hinge that will permit suitable freedom for the blades to move
upwind and downwind under the influence of unsteady air loadings. Individual flapping
hinges accomplish the same result but are impractical (except perhaps on small rotors)
because the centrifugal force will be insufficient to prevent excessive free coning and the