# Assessment of a Mid-Fidelity Numerical Approach for the Investigation of Tiltrotor Aerodynamics
## 1 Introduction
To cite few examples of numerical works involving the use of unsteady panel methods for rotorcraft applications, the University of Roma Tre developed a direct panel method based on a novel boundary integral formulation for the velocity potential that was used both for the investigation of blade vortex interaction of helicopter rotors [20] and for the study of wing-proprorotor aerodynamic interaction of tiltrotors [21]. More recently, DLR developed an unsteady panel, free-wake code that was used for the aerodynamic study of a complex rotorcraft configurations, such as the Airbus RACER compound helicopter [22]. In particular, recent works used the vortex particle method (VPM) [23; 24] for wake modelling, showing a quite accurate representation of the aerodynamic interactions among several bodies that are typical of complex rotorcraft configurations. To cite few examples, [PERSON] et al. [25] developed a method for the optimisation of the aerodynamic layout of a helicopter using an aerodynamic model based on viscous VPM combined with an unsteady panel hybrid method to simulate the interactions between the various components of a helicopter. The National Technical University of Athens developed the GENeral Unsteady Vortex Particle (GENUVP) software based on a panel method coupled with a VPM solver that was used for both aerodynamic and aeroacoustic simulations of rotorcraft [26]. [PERSON] and [PERSON] investigated multi-rotors configurations using a VPM-based code [27], showing good agreement with experimental data. Moreover, [PERSON] et al. used a vortex-based approach coupled with a viscous boundary model to investigate the complex rotorcraft-to-rotorcraft interference problems that occur during shipboard operations--i.e., the flow field and unsteady airloads of a tiltrotor affected by the wake of an upwind tandem rotor [28]. In recent years, a new flexible mid-fidelity computational tool, called DUST (www.dust-project.org), was developed as the result of a collaboration between Politecnico di Milano and A\({}^{3}\) by Airbus LLC, aiming to obtain a fast and reliable numerical tool for the aerodynamic simulation of complex rotorcraft configurations, such as eVTOLs aircraft. DUST is an open source code that was developed under an MIT license, simultaneously involving different aerodynamic modelling techniques, such as thick surface panels, thin vortex lattices, and lifting lines for solid bodies, while vortex particles were used for wake modelling. In particular, the implemented vortex particle model for wakes provides a stable Lagrangian description of the free vorticity field in the flow, which is suitable for numerical simulations with strong aerodynamic interactions. Indeed, for instance, DUST was used to simulate the aerodynamics of the full Vahana vehicle that was developed by A\({}^{3}\) by Airbus LLC [29] characterised by two rows of four rotors in a tandem configuration, showing a quite good agreement with both the flight test data and high-fidelity CFD results [30]. Moreover, thanks to the low computational effort that is required by the code implementation, DUST was recently employed to perform a parametric study of the rotor-rotor aerodynamic interaction that characterises the cruise flight conditions of an eVTOL aircraft [31]. Consequently, DUST can be considered in a mature state to be used for the investigation of challenging rotorcraft configurations that are characterised by complex aerodynamic interactions between rotors and wings. The present work aims to assess the accuracy of the mid-fidelity approach that was implemented in DUST for the simulation of tiltrotor aerodynamics with respect to CFD. In particular, numerical simulations were performed with DUST to show the ability of a mid-fidelity solver to capture the effects of the complex aerodynamic interactions between the rotor wake and the wing typical of the different stages of a tiltrotor flight mission.