###### Abstract  
A panel/vortex particle hybrid method is coupled with a computational structure dynamics code to predict helicopter rotor loads. The rotor blade surfaces and near wakes are modeled by the panel method, while the far wake is modeled by resorting to the vortex particles method. A fast summation method is introduced to accelerate the evolution of particle-particle-induced velocity and its derivative as well as panel-particle interactions. The developed vortex particle method code is coupled with the multibody code MBDyn to predict the rotor airloads. Numerical validations are carried, out and the results are compared with the experiments and simulation results in the literature. vortex particles method (VPM); computational structural dynamics (CSD); aeroelasticity 4.0.11.11.11.11.11.11.11.11.1.  
in order to solve the vortex wake transport problem. The viscous vortex particle method (VPM) solves the vorticity transport equation using a grid-free Lagrangian formulation. The VPM represents the wake with a cloud of vortex particles; each particle is associated with vector-valued vorticity, velocity, and position. Based on first principles, VPM captures, without introducing any artificial numerical dissipation, the fundamental fluid flow physics for both the vorticity stretching and the physical viscous diffusion. In VPM, the aerodynamic surfaces are treated as a vorticity source that emits vortex particles into the flow field. [PERSON] et al. [14] modeled the far-field wake with vortex blobs, coupled with a panel method, and incorporated Fowcs Williams-Hawkings (FW-H) equation to predict the blade-vortex interaction (BVI) and aeroacoustics of a rotorcraft; however, the viscosity of the flow was not accounted for in this model. [PERSON] et al. [15] developed the FastAero code, a fast, unsteady panel method with vortex particle wakes, and used the pre-corrected Fast Fourier Transform (FFT) approach and the fast multipole method (FMM) to accelerate the computation. Similar works have been conducted by [PERSON] and [PERSON] [16], [PERSON] and [PERSON] [17], [PERSON] and [PERSON] [18] and [PERSON] et al. [19], who combined the potential flow panel method or the lifting line model with the viscous vortex particle method. [PERSON] et al. [20] developed a flexible medium fidelity aerodynamic computational tool, called DUST, which enables different aerodynamic elements such as lifting line, vortex lattice, and vortex particles, all available in a single model. In the above works, either the fast multipole method (FMM) [21] or the Treecode method [22] were utilized to accelerate the evaluation of particle convection velocity as well as its gradient. However, when complex configurations are analyzed, the number of vortex panels increases and the computational cost from the vortex panels to the particles becomes nontrivial. For this reason, a fast summation method is required also for the vortex panels. [PERSON] and [PERSON] [23] developed a parallel viscous vortex particle method called FLOWVPM, based on the open-source code ExaFMM developed by [PERSON] and [PERSON] [24] and [PERSON] et al. [25], to evaluate the multi-rotor configuration performance. In the work of [PERSON] and [PERSON] [23], the load distribution was calculated by resorting to the blade element method (BEM) and was used to derive the circulation along the blade in order to perform all the computations using FMM, to model the lifting surfaces using embedded vortex particles, and thus to obtain an \(O(N)\) simulation. However, obtaining the surface pressure of the lifting surfaces, which is of great importance for aeroacoustics analysis, is not straightforward. In present study, a panel method is coupled with VPM. The panel method is used to model the rotor blade surfaces and near wakes, and VPM is used for far wake. Besides the classical FMM, a semi-analytical fast summation method is developed to speed up the computation of panels' mutual interactions and of the influence exerted from the panels to the particles.