Horizontal axis wind turbines (HAWTs) experience three-dimensional rotational and unsteady aerodynamic phenomena at the rotor blades sections. These highly unsteady three-dimensional effects have a dramatic impact on the aerodynamic load distributions on the blades, in particular, when they occur at high angles of attack due to stall delay and dynamic stall. Unfortunately, there is no complete understanding of the flow physics yet at these unsteady 3D flow conditions, and hence, the existing published theoretical models are often incapable of modelling the impact on the turbine response realistically. The purpose of this paper is to provide an insight on the combined influence of the stall delay and dynamic stall on the blade load history of wind turbines in controlled and uncontrolled conditions. New dynamic stall vortex and nonlinear tangential force coefficient modules, which integrally take into account the three dimensional rotational effect, are also proposed in this paper. This module along with the unsteady influence of turbulent wind speed and tower shadow is implemented in a blade element momentum (BEM) model to estimate the aerodynamic loads on a rotating blade more accurately. This work presents an important step to help modelling the combined influence of the stall delay and dynamic stall on the load history of the rotating wind turbine blades which is vital to have lighter turbine blades and improved wind turbine design systems. K E Y W O R D Sblade element momentum (BEM) model, dynamic stall, stall delay, tower shadow, turbulent wind, unsteady aerodynamic loads | INTRODUCTIONThe prediction of wind turbine performance requires efficient aerodynamic models. This is, in particular, true for the extremely demanding calculation of a wind turbine design load spectrum which leads to more than a million time steps, each requiring a full aeroelastic calculation of inflow, rotor, and wake. This is explained in the work of Schepers, 1 where it is moreover mentioned that the main "workhorse" in wind turbine design is the so-called blade element momentum (BEM) theory. This model is basically 2D and steady and then engineering models (i.e., efficient and transparent models) are added amongst others to account for dynamic stall and 3D rotational effects. The combined effects of these phenomena and other unsteady sources yield to the blade exhibiting unsteady aerodynamic loads, which play an important role for the wind turbine aerodynamic performance and the fatigue life of its structure.