# Assessment of a Mid-Fidelity Numerical Approach for the Investigation of Tiltrotor Aerodynamics
## 1 Introduction  
Tiltrotors combine the speed and range of a conventional fixed-wing aircraft with the vertical take-off and landing capabilities of helicopters. Their architecture is typically characterised by two powered rotors that are mounted on tilting nacelles located at the outer portion of the wing. The flight mission of tiltrotors involves different vehicle configurations characterised by several relative angles between rotor nacelles and wing. Indeed, tiltrotors take off as a helicopter with the rotors tilted vertically (see Figure 1a) and they turn the rotors forward in cruise conditions to behave as a fixed wing airplane (see Figure 1d). In the conversion phase between take-off and cruise flight conditions, the aircraft rotates the rotors providing different tilting angles between the rotors and wing axis (see Figure 1b,c). Consequently, tiltrotor aerodynamics are characterised by complex interactions between the rotor wake and wing that are peculiar of the different attitudes experienced by the vehicles during their flight mission. Several experiments have been conducted to investigate rotor-wing interactions, from the early stages of the JVX program [1, 2] to the present day [3, 4]. Recent advances in the field of high-performance computing favoured the use of high-fidelity Computational Fluid Dynamics (CFD) for a thorough investigation of thecomplex aerodynamic interactions that characterise tiltrotor architectures. Indeed, several numerical works employing the use of high-fidelity Navier-Stokes equations solvers for the study of tiltrotor aerodynamics can be found in the literature. Examples of computational studies of tiltrotor aerodynamics are the works by [PERSON] [5], [PERSON] and [PERSON] [6], and [PERSON] et al. [7], where hover configurations were simulated while using different implementations of the Navier-Stokes equations. Moreover, several high-fidelity CFD codes, such as elsA [9] by ONERA, FLOWer [10] by DLR and Airbus Helicopters Deutschland, HBM3 by the University of Glasgow [11], and ROSITA [12] by Politecnico di Milano, were developed in Europe for rotorcraft application studies. These codes, based on the block-structured grid, finite volume, and Chimera approaches for the simulation of rotating bodies, were used and compared for the study of the aerodynamics of the tiltwing aircraft ERICA [13,14] and, in particular, within the European project NICETRIP [15]. In the recent literature, several numerical works were carried out to simulate the aerodynamics of the XV-15 tiltrotor aircraft using high-fidelity CFD tools. Indeed, in these works, [PERSON] and [PERSON] et al. [16,17,18,19] investigated the main aerodynamic interactions between the rotor and the wing of the XV-15 tiltrotor. Figure 1: Examples of tiltrotor aircraft in different flight conditions: (**a**) Bell XV-15 in hover [8]; (**b**) Leonardo AW-609, in early conversion phase [[https://www.leonardocompany.com/it/products/aw609](https://www.leonardocompany.com/it/products/aw609)]([https://www.leonardocompany.com/it/products/aw609](https://www.leonardocompany.com/it/products/aw609)); (**c**) Bell Boeing V-22 in advanced conversion phase, [[http://www.military-today.com/helicopters/](http://www.military-today.com/helicopters/)]([http://www.military-today.com/helicopters/](http://www.military-today.com/helicopters/)); and, (**d**) Bell XV-3 in cruise [8]. Nevertheless, time-accurate URANS simulations of tiltrotor aircraft configurations still require a huge computational effort. Thus, high-fidelity CFD tools are usually employed for the detailed study of a limited number of tiltrotor configurations in steady-state conditions, as done in [16; 17]. Mid-fidelity aerodynamic solvers that are based on unsteady panel methods are considered of great interest in the field of rotorcraft simulations. As a matter of fact, the limited computational effort that is required by these numerical tools enables one to perform the large number of aerodynamic simulations required for the preliminary design of novel rotary-wing vehicles.