Introduction to Modern Wind Turbines - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 53

The terms downwind rotor and upwind rotor denote the location of the rotor with respect
to the tower. An unconed rotor is one in which the spanwise axes of all of the blades lie in
the same plane. Blade axes in a coned rotor are tilted downwind from a plane normal to the
rotor axis, at a small coning angle . This helps to balance the downwind bending of the blade
caused by aerodynamic loading with upwind bending by radial centrifugal forces. Tower
clearance (the minimum distance between a blade tip and the tower) is inluenced by blade
coning, rotor teetering, and elastic deformation of the blades under load. Elastic deformation
can be signiicant for blades fabricated from composite materials, such as iberglass. Often
an axis-tilt angle is required to obtain suficient clearance. Axis tilt is kept to a minimum be-
cause of potential negative side effects, such as reduced swept area and a vertical component
to the rotor torque that can cause a yaw moment on the nacelle.
The two general types of rotor hubs are rigid and teetered . In a typical rigid hub, each
blade is bolted to the hub and the hub is rigidly attached to the turbine shaft . The blades are,
in effect, cantilevered from the shaft and therefore transmit all of their dynamic loads directly
to it. To reduce this loading on the shaft, a two-bladed HAWT rotor usually has a teetered
hub, which is connected to the turbine shaft through a pivot called a teeter bearing or a teeter
hinge , as shown in Figure 2-5(a). This bearing permits cyclic, rigid-body motion of the ro-
tor perpendicular to the plane of rotation through small teeter angles (less than ±10 deg) at a
frequency equal to the rotor speed ( i.e. one cycle per rotor revolution or 1P ). Teeter bearings
may contain rolling, sliding, or elastically-deforming elements such as elastomeric bearings
composed of alternate rubber and steel sheets.
Teeter motion is a passive means for balancing air loads on the two blades, by cyclically
increasing the lift force on one while decreasing it on the other. Teetering also reduces the
cyclic loads imposed by a two-bladed rotor on the turbine shaft to levels well below those
caused by two blades on a rigid hub. One-bladed rotors have been attempted, and these are
almost always downwind of the tower and teetered, because their inherently-large aerody-
namic imbalance causes large teeter angles.
A three-bladed rotor usually has a rigid hub. In this case, cyclic loads on the turbine
shaft are much smaller than those produced by a two-bladed rotor with a rigid hub, because
three or more blades form a dynamically-symmetrical rotor : One with the same mass mo-
ment of inertia about any axis in the plane of the rotor and passing through the hub. This
is fortunate, since practical gimballing (teetering about two orthogonal axes) is dificult to
achieve.
Medium- and large-scale HAWT rotors usually contain a mechanism for adjusting blade
pitch , which is the angle between the blade chordline (at a speciied reference radius) and the
plane of rotation. This pitch-change mechanism , which may control the angle of only the
outboard section of each blade ( partial-span pitch control , Fig. 2-1) or of the entire blade
( full-span pitch control , Figs. 2-3, 2-4, and 2-5(b)) provides a means of controlling start-
ing torque, peak power, and stopping torque. Pitch-change mechanisms are high-quality
structural/mechanical devices with strong actuators (usually hydraulic) and computerized
controls. Some small- and medium-scale HAWTs have ixed-pitch stall-controlled blades,
avoiding the cost and maintenance of pitch-change mechanisms by relying on aerodynamic
stall to limit peak power.
A simpliied form of aerodynamic control mechanism is a tip brake or a tip vane , in
which a short outboard section of each blade is turned at right angles to the direction of mo-
tion, stopping the rotor by aerodynamic drag or at least limiting its speed.
A wide variety of materials have been used successfully for HAWT rotor blades, includ-
ing glass-iber composites (both laid-up in molds and ilament-wound over mandrels), lami-
nated wood composites, steel spars with non-structural composite fairings, and welded steel
airfoils. The choice of blade materials is a system engineering decision involving consider-
ations of size, strength, stiffness, weight design and manufacturing expertise, maintenance,