Evolution of Modern Wind Turbines Part B: 1988 to 2008 - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 180

ties of the blade using the unique attributes of composite materials, the blade can be built in
a way that couples the bending deformation of the blade to a twisting (pitching) deformation.
This is referred to as lap-pitch or bend-twist coupling and allows the outer portion of the
blade to twist as it bends as illustrated in Figure 3-45. Flap-pitch is accomplished by design-
ing the internal structure of the blade ( e.g. by orienting the iberglass and carbon plies within
the composite layups) in such a way as to make the blade twist as it bends. This twisting
changes the angle of attack over much of the blade. If properly designed, this change in angle
of attack will reduce the lift forces caused by wind gusts and therefore passively reduce cyclic
fatigue loads.
Figure 3-45. A twist-lap coupled blade fabricated from composite material with a
layup pattern designed to alleviate fatigue loads. Note that the bending delection of the
blade causes the pitch angle at the tip to change, thereby reducing cyclic lift loads from wind
gusts. [Lobitz et al. 2001]
Another approach to achieve lap-pitch coupling is to build the blade planform in a
curved shape so that aerodynamic load luctuations apply twisting movements to the blade
that will vary local angles of attack [Lobitz et al. 2001, Paquette et al. 2007]. These new
blade design concepts are complex and must be developed, tested, and optimized so as not to
adversely impact energy production or result in unstable vibrations.
Concepts such as on-site manufacturing and segmented blades are also being explored
to help reduce overland transportation costs. It may be possible to segment molds and move
them into temporary buildings close to the site of a major wind installation so that over-the-
road transportation is not necessary.
Limiting Factors on Onshore Turbine Size
As mentioned previously, land transportation constraints can pose limiting factors on the
future sizes of wind turbines installed in inland power stations. Cost-effective transportation
can only be achieved by remaining within standard, over-the-road trailer dimensions of 4.1 m