In small-sized unmanned air vehicle (UAV) configurations, rotor–beam interaction noise is an important contributor to the tonal noise. The emergence of harmonics is enhanced by the presence of the beam in the very near wake downstream of the rotor. Distinct and coupled noise source mechanisms occur. Both the rotor blades and the beam radiate sound as both experience unsteady loading due to the presence of each other. Rotor noise can be described by a blade-potential interaction model; i.e., the blades undergo a potential distortion when passing close to the cylindrical beam. Similarly, beam noise is radiated because of interaction with the potential field of the blades. The unsteady loading on the beam appears to be dominantly responsible for the increase of tone levels in the present study. Both unsteady loading noise contributions radiated by the blades and the beam are well reproduced with simple two-dimensional uncoupled analytical models based on the potential flow theory. At the blade passing frequency (BPF), the directivity is explained by the dominant contribution of the thickness noise and the steady-loading noise radiated by the rotor. At higher BPF harmonics, the unsteady loading noise radiated from the blades and the beam prevails, producing a torus-like directivity pattern. Despite their limitations, aerodynamic and acoustic results obtained from the models cross-validate with those obtained from numerical simulations and experiments. The models give a low-cost approach to quantify and rank the main contributions to the tonal noise radiated by UAVs.
Vella et al. (Mon,) studied this question.