This chapter summarises the key aerodynamic theory of horizontal-axis wind turbine rotors. The actuator disc concept leads to the relationships between induced velocity, axial thrust, and power extraction. The theory is extended to multiple streamtubes, which, combined with 2D wing theory, establish the basis of blade-element momentum (BEM) theory. A straightforward mathematical treatment of BEM theory is included, with an iterative procedure suitable for coding. Measurements from a full-scale rotor illustrate the applicability of BEM theory but also its fundamental limitations: the latter are described, and measures are outlined to compensate for them in practical BEM codes. Simple relationships are given for the axial and tangential load distributions on an optimal HAWT blade. The structure of the rotor wake is described, leading into a description of vortex-wake theory, which provides a more physically realistic description of the airflow. Vortex wake codes are described in non-mathematical terms. The chapter includes wake measurements from full-scale wind turbines and small models. Vorticity maps from the latter verify the underlying mathematical model of a helical vortex wake.

Chapter 6 considers wind turbine control, including supervisory control, power limiting, starting and stopping, electrical power quality, and sector management. The importance of accurate yaw control is discussed in terms of energy capture and cyclic loading, and an active yaw system is illustrated. The main focus of the chapter is real-time power control, and the chapter builds on the aerodynamic and electrical concepts covered previously in Chapters 3–5. The differences between stall and pitch regulation are explained, in the latter case in the context of both constant and variable-speed operation. Power measurements from constant-speed and variable-speed pitch controlled machines illustrate the superior accuracy of the latter. Control block diagrams are given for both methods, with qualitative explanation of the principles. The procedure for starting and stopping different wind turbine types is explained, and the advantages of pitch control in this context are illustrated. The chapter includes a short description of sector management, a control strategy based on external factors such as wind speed and direction, and used for noise reduction, shadow flicker prevention, or fatigue mitigation.

Chapter 4 extends the aerodynamic discussions of the preceding chapter to show how the rotor net loads (power, thrust, and torque) are developed. The dimensionless power coefficient (Cp) curve is introduced, and the relationship between rotor tip speed ratio and optimum solidity is explained. The variation of thrust loading with wind speed on an ideal pitch-controlled rotor is explained from simple theory and illustrated with measurements from a full-scale wind turbine. Equations governing the chord and twist distributions for an optimised blade are given and discussed in the context of some historic blade types, with illustrations. Rotor aerodynamic control is explained with reference to fixed-pitch stall regulation and variable blade pitch in both positive and negative senses. The influence of blade number is examined, with discussion of the advantages and disadvantages of one-, two-, and three-bladed wind turbines. The chapter concludes with a brief overview of alternative aerodynamic control devices, including tip vanes and ailerons, and the downwind rotor configuration (with examples).