Within the group, computational tools of varying fidelity and wind tunnel experiments were employed with the aim to unravel the complex flow field and characterize the loads of a helicopter rotor flying at low altitude, in close proximity to a well shaped obstacle in addition to ground effect. A number of wind tunnel test campaigns were performed as part of this action group [11,12], while one of them, was conducted in the atmospheric flows test section of POLIMI's wind tunnel, addressing the pure ground effect problem. In the present paper, numerical activities addressing the rotor-ground interference problem are presented, part of which were contributed to this GARTEUR group. The work was motivated by an earlier study by the authors [13] in which the loads of a hovering helicopter main rotor in IGE were computed using a potential free wake solver [14]. The predictions were found in very good agreement with the POLIMI measured data [12] at altitudes higher than 1.5 rotor diameters. However, accuracy degraded as the distance from the ground decreased. At lower altitudes the potential solver underestimated the increase in the loads caused by ground blockage. This led to the conjecture that at low altitude flights the displacement effect induced by the development of the ground boundary layer which is absent in potential methods, becomes important and may explain the deviation of the predictions. Similar observations were also made by other participants of the same [PERSON] group. [PERSON] et al. [15] investigated the effect of the ground and of a nearby obstacle on the loading of the rotor using the UPM panel code of DLR. He compared results of UPM code to data from the same experimental database and he arrived at the same conclusion, that panel codes underestimate the increase in thrust IGE at low altitude hovering conditions by \(\approx\)2%. Based on the above, in the present analysis a hybrid Computational Fluid Dynamics (CFD) model is applied that can accurately predict the formation of viscous boundary layers. The corresponding flow solver, couples a standard Eulerian finite volume solver with a Lagrangian particle approximation [16; 17]. In the Eulerian part, separate grids of limited width are defined around every solid body, on which the flow equations in Eulerian description are solved. In particular for the rotor-ground interaction problem different Eulerian grids are built around the rotating blades, extending by 1.5\(\times\) radius in the radial direction and by 0.5\(\times\) radius around every cross section. The blades are simulated as rotating actuator lines. Although the existing CFD framework is capable of treating the blades in a fully resolved manner [16], given that the present work emphasizes on the effect of the ground boundary layer on the rotor surrounding flow, it is considered that the simple and cost effective actuator line modeling approach is sufficient for capturing rotor loads correctly without aggravating computational cost. A separate grid is defined on the ground which extends 0.5\(\times\) the radius above the ground level. In order to interconnect the independent domains defined by the various grids, the entire flow is described in Lagrangian coordinates and the corresponding flow equations are solved via particle approximations in a fully coupled mode with the solutions within the Eulerian grids. The Eulerian part solves the compressible flow equations in density-velocity-pressure formulation and uses pre-conditioning at low Mach while the Lagrangian part is based on the density-dilatation-vorticity-pressure formulation. The main advantage of the hybrid model is that it can effectively deal with flow problems which involve several bodies, that move independently the one from the other while the same time resolves viscous phenomena.