## 1 Introduction  
The Actuator Line (AL) method [1] is a renowned CFD technique widely used in the aerodynamic analysis of Wind Turbine (WT) rotors. In particular, it has been employed in studying the generation and convection of rotor wakes [2, 3]. There are also publications where it has been used in helicopter rotor aerodynamic analyses [4, 5, 6], particularly when costly simulations are involved, in which a detailed distribution of the blade loading is not an absolute requirement. The broad application of AL stems from the high fidelity level that the CFD context provides, alongside with the limited computational requirements of the methodology. The latter originates in the fact that the rotor blades are not fully resolved, rather than modeled as a set of blade elements along their axis. Just by a simple blade element analysis, together with the use of predefined airfoil polars, the blade forces can be calculated in a cost effective manner and then applied to the flow as source terms of the Navier-Stokes equations. In this way, the effect of the rotor on the flow can be accounted for in great detail [7,8]. Incorporating CFD in rotor analysis renders feasible the estimation of viscous and turbulence effects. These are significant in wind farm simulations, where rotor wake diffusion has an impact on the performance of the downwind turbines. Furthermore, viscous effects are important when studying the rotor wake interaction with the boundary layer developed on surrounding bodies (e.g., helicopter obstacle [9] or WT tower) or even the ground. Such interactions affect the rotor performance and the loading of the rotor [10,11]. Another possible aspect where CFD analysis may be needed is the rotor operation close to bluff bodies with a non-thin boundary layer. Separation of flow around the fuselage is known to induce vibrations on blade loads. Hence, resolving the vortex shedding from bluff bodies in the operating region of rotors is of great importance. The main shortcoming of the method is that numerous numerical parameters are involved that affect load predictions. In particular, the method used in the projection of the blade forces onto the CFD grid plays an important role in the intensity of the produced tip and root vortices [12]. As a result, in order to capture the blade loads in detail, a case specific tuning is needed for these parameters. A great deal of research effort has been devoted to determining the best practices of body force projection [13, 14, 15, 16]. A common requirement for the successful application of all these methods is the adoption of really fine grid resolution around the blades and in particular close to the blades tip. The alternative is to use a coarse mesh and account for the tip vortex effect through proper tip loss correction models [17,18]. In most of the above cited studies, AL results are compared against results produced by a Lifting Line (LL) model [19] or a Blade Element Momentum (BEM) model that uses [PERSON]'s tip correction [20]. Nevertheless, capturing blade loads in detail is still considered to be an issue under investigation for AL methodologies, which requires further development. As a result, aeroelastic simulations using AL have been published only by few researchers, mainly in the fields of WTs. In [8], the AL methodology implemented in EllipSys3D is loosely coupled with the aeroelastic code Flex5, where the structural analysis is performed in the context of modal reduction. The study focuses on wake characteristics investigation under turbulent free-stream flow. Moreover, a comparison against experimental measurements is performed regarding aerodynamic performance characteristics (power production and rotor thrust loading for various wind speeds) on a model scale turbine. In [21], an OpenFOAM implementation of the AL method described in [22] is tightly coupled with an in-house structural solver, which is again based on modal analysis.