## 5 Conclusions  
In this paper, the aeroacoustic interactions of small rotors in hover were evaluated experimentally and computationally. The APC 8x4.5 MR rotor was tested in both single and dual rotor hover configurations to quantify the effect of self-induced interactions as well as rotor-rotor interactions. Experimental performance values for the single rotor  
Figure 19: Overall sound pressure level of the dual rotors in hover at 6000 RPM versus elevation angle. Experimental results are shown as lines. Total noise from Helios/PSU-WOPWOP combined with UCD-Quietfly is shown as squares. Standalone Helios/PSU-WOPWOP predictions are shown as circles. Figure 20: Overall sound pressure level of the dual rotors in hover at 6000 RPM versus azimuthal angle. Experimental results are shown as lines. Total noise from Helios/PSU-WOPWOP combined with UCD-Quietfly is shown as squares. Standalone Helios/PSU-WOPWOP predictions are shown as circles. were taken from reports available online. Experimental acoustic values for single and dual rotors were obtained at University Roma Tre in an anechoic chamber, using a linear array of microphones oriented vertically to obtain the polar directivity and horizontally to obtain the azimuthal directivity. Computational predictions were obtained, using the CREATE-AV Helios suite coupled to PSU-WOWOP and UCD-Quietfly. The CFD employed DES, a hybrid URANS/LES method, and the PSU-WOPWOP was used to perform an impermeable surface Ffowcs-William and Hawkings calculation. UCD-Quietfly accounted for trailing edge broadband noise. The predicted performance values and trends of the isolated rotor in hover were consistent with previously reported results. Differences between reported results are attributed here to manufacturing differences between the blades and possible experimental calibration errors. The predicted acoustic spectrum demonstrated reasonable first and second BPF for the in-plane mic when compared to the experiment, but did not predict the higher harmonics. All BPF harmonics were under-predicted for the above-plane observers. Due to the dominance of the loading noise effects above the plane, it was posited that the computations were not capturing the effects of recirculation. The UCD-Quietfly broadband modeling framework showed good prediction of the broadband shape and amplitude, although the in-plane results need further inspection. No dual rotor performance values were available for comparison. Examination of the computed dual rotor performance trends showed decreasing mean thrust and increasing torque with increasing proximity. Increasing proximity also created large thrust fluctuations. For counter-rotating rotors, an impulsive type thrust response was seen and attributed to the interaction of the rotor and disturbances in the gap between the rotors. An omega-shaped vortex appeared in the gap that qualitatively matched computations and PIV measurements of prior works. The dual rotor acoustic predictions showed higher harmonic noise, indicating that the effects of the interaction are more significant than those of the recirculation for dual rotors. The broadband shape of the combined Helios/PSU-WOPWOP and UCD-Quietfly results showed excellent agreement with the experiment, although the amplitude was low. The counter-rotating configuration was shown to have greater noise than the co-rotating configuration above-plane, reflecting the aerodynamic performance trend. The azimuthal directivity of the counter-rotating rotors demonstrated the effects of destructive interference due to rotor-phasing and was much stronger in the computation, where the phasing is perfect, than in the experiment, which did not control for the rotor phase. This work highlights how challenging it is to obtain experimental data for the validation of small rotor acoustics computations.