This chapter discusses several aspects related to engineering methods in wind turbine design codes. Current engineering models for rotor aerodynamics topic are built around the Blade Element Momentum (BEM) theory. The Blade Element Momentum theory in itself is very basic, e.g., it is derived for two-dimensional, stationary, homogenous, and non-yawed conditions. For this reason, several engineering models have been developed which overcome these simplifications and which act as add-ons to the basic BEM theory. This chapter describes the BEM theory, the most important engineering add-ons, and an assessment of BEM with engineering add-ons with results from higher fidelity models and measurements.
