The rotor wake aerodynamic characterization is a fundamental aspect for the development and optimization of future rotary-wing aircraft. The paper is aimed at experimentally and numerically characterizing the blade tip vortices of a small-scale four-bladed isolated rotor in hover conditions. The investigation of the vortex decay process during the downstream convection of the wake is addressed. Two-component PIV measurements were carried out below the rotor disk down to a distance of one rotor radius. The numerical simulations were aimed at assessing the modelling capabilities and the accuracy of a free-wake Boundary Element Methodology (BEM). The experimental and numerical results were investigated by the Γ₂ criterion to detect the vortex location. The rotor wake mean velocity field and the instantaneous vortex characteristics were investigated. The experimental/numerical comparisons show a reasonable agreement in the estimation of the mean velocity inside the rotor wake, whereas the BEM predictions underestimate the diffusion effects. The numerical simulations provide a clear picture of the filament vortex trajectory interested in complex interactions starting at about a distance of z/R = −0.5. The time evolution of the tip vortices was investigated in terms of net circulation and swirl velocity. The PIV tip vortex characteristics show a linear mild decay up to the region interested by vortex pairing and coalescence, where a sudden decrease, characterised by a large data scattering, occurs. The numerical modelling predicts a hyperbolic decay of the swirl velocity down to z/R = −0.4 followed by an almost constant decay. Instead, the calculated net circulation shows a gradual decrease throughout the whole wake development. The comparisons show discrepancies in the region immediately downstream the rotor disk but significant similarities beyond z/R = −0.5.

The aim of the present work is the improvement of a free-wake model for the analysis of a small-scale two-bladed propeller in hover. The simulations are carried out using a BEM approach implemented in the medium-fidelity solver RAMSYS. An acoustic validation is also performed using the developed tool ACO-FWH. The work proves that even mild discrepancies in the propeller geometry must be accounted for as their influence is not negligible, especially on the aeroacoustics of the propeller. In particular, the proper modeling of the blades enables the correct identification of the sub-harmonics of the SPL spectra. An optimization procedure based on the application of the evolutionary Genetic Algorithm is followed to identify the values of the parameters describing the dissipative and diffusive properties in the Bhagwat–Leishman vortex core model, an upgraded version of the classical Lamb–Oseen one. On average, this approach enabled the further improvement of the accuracy of the numerical model in terms of acoustic signature evaluation with respect to the one obtained by only modeling blade dissimilarities. The results obtained demonstrate the promising capabilities of a fine-tuned free-wake medium-fidelity approach to simulate the aerodynamic and acoustic details of a small-scale propeller in hover, provided the accurate geometrical modeling of the propeller and the selection of suitable parameters to be used in the wake modeling.

The present work describes the investigation conducted by the GARTEUR Action Group HC/AG-26 into the acoustic and aerodynamic characteristics of tan- dem propeller configurations, with a focus on the impact of mutual interactions between propellers, which are significant for eVTOL aircraft. Numerical results, obtained through simulations with varying levels of fidelity (from panel methods to computational fluid dynamics), were compared with experimental data from a dual propeller test case developed by Politecnico di Milano. The comparison demonstrated the capabilities and limitations of the various numerical approaches in simulating a challenging test case, characterized by strong unsteadiness and significant viscosity effects. The simulations accurately reproduced the aerody- namic performance losses of the rear propeller and the variations in overall sound pressure level, which are correlated with the interaction with the front propeller’s flow, for both flight conditions (hover and airplane mode) and for the different lateral misalignments between the propeller axes.

The aerodynamic interactions in a quadcopter during forward flight reduce the efficiency of the back rotor(s) and increase generated noise levels. To alleviate interactions without compromising the vehicle’s compactness, a vertical offset between the front and back rotor(s) can be introduced as an alternative solution to increased hub spacing. The efficiency of a quadcopter in square and diamond configuration with a vertical spacing up to 0.38 rotor diameter is analysed experimentally and numerically using free-wake solvers. Forward flight conditions are considered, including high interaction cases with a nose-up tilt and close rotor hub spacing with overlapping blades. The results show that for the square configuration, a thrust comparable to four isolated rotors is achieved by the elevation of both back rotors. In contrast, introducing a vertical separation to the already underperforming back rotor in the diamond configuration further reduces its efficiency.

The Italian research project PROSIB (PROpulsione e Sistemi IBridi per velivoli ad ala fissa e rotante), is a 30-month initiative funded by the Italian Ministry of University and Scientific Research (MIUR) and coordinated by the Leonardo company. The project is aimed to investigate configurations for regional aircraft and rotary wing platforms and architectures for propulsion systems, and is dedicated to the identification of the best strategy for their use, given different on-board energy sources. The reduced environmental impact is the key for the success of the new hybrid/electric aircraft configurations. This not only considers the chemical pollution introduced in the atmosphere, but also the noise produced on the surrounding area of airports. The present paper describes the acoustic impact assessment resulting from the inclusion of new propulsion technologies and new configurations of regional aircraft (ATR42 pax) in a reference airport area.

This research seeks to improve a free-wake model for the study of the hover performance of a small, two-bladed propeller using the Boundary Element Methodology (BEM), implemented in the medium-fidelity aerodynamic solver RAMSYS, coupled with the tool ACO-FWH for acoustic validation. It emphasises the necessity of accounting for subtle geometric variations, which significantly affect aeroacoustic predictions, specifically sub-harmonic SPL spectra. An optimisation process, employing a Genetic Algorithm, determines optimal parameters for the Bhagwat-Leishman vortex core model, enhancing acoustic signature accuracy beyond geometric correction alone. The findings confirm the effectiveness of a carefully calibrated medium-fidelity approach for capturing the details of the aeroacoustic behaviour of small-scale propellers, contingent on accurate geometric representation and optimised wake modelling parameters.