Wind Turbine Aerodynamics Part A: Basic Principles - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 327

wind turbulence and control system actions generally cause the mean test power of HAWTs
to fall below theoretical predictions, unless the effects of these two factors are specifically
added to the performance model. Below the knee of the power curve, strip theory predicts
the system power with acceptable accuracy.
Dimensionless Performance Parameters
Shown in Figure 5-29 is a dimensionless plot of power coefficient K P versus advance
ratio J , displaying performance test data for all three of the wind turbines in these examples.
Although the three turbines vary in rated capacity from 100 kW to 2.5 MW, their dimension-
less performance characteristics are seen to be remarkably similar. The Mod-1 HAWT, with
its highly-twisted blades, high tip speed, low solidity, and NACA 44XX-series airfoils, is
observed to have superior performance in below-rated winds.
Figure 5-29. Dimensionless performance curves, permitting comparison of test data for
the Mod-0, Mod-1, and Mod-2 HAWTs. [Wilson and Walker 1984]
HAWT Aerodynamic Loads
In general, the determination of the loads on a HAWT rotor involves the complex inter-
action of aerodynamic forces, structural deformations, and wind variability in both time and
space. However, the aerodynamic moment obtained from a rigid-body analysis (the first inte-
gral in Equation (5-41a)) is a fair estimate of the steady blade bending load . The peak (maxi-
mum) value of this steady load occurs at rated power for a pitch-controlled rotor. Using the
Glauert optimum rotor model, upper bounds can be established for steady bending loads, but
these will greatly overestimate the actual aerodynamic loadings when the blade design departs