Time Varying Rotor Noise Computations And Analysis Of Electric Vertical Take Off And Landing Aircraft PDF Download

Electric Vertical Takeoff and Landing (eVTOL) aircraft are known to vary greatly in their configurations and operating conditions from conventional helicopters. For one, they rely on multiple fixed pitch rotors, or those equipped with collective pitch controls, to maneuver and maintain steady flight, whereas, cyclic pitch controls are generally utilized on main rotors of helicopters for the same purpose and to also compensate for the lateral asymmetry in lift when operating in steady forward flight. Therefore, eVTOL rotors individually sustain hub forces and moments that are balanced across the entire aircraft. This, in turn, affects the aerodynamic state of these rotors, which directly influences their noise emissions. This study demonstrates how analytical noise prediction tools can be used in conjunction with experiments to investigate how the low-frequency tonal noise emissions of an isolated rotor vary when they generate steady hub moments. These hub moments are imparted by using 1-per-rev cyclic pitch variations. A novel post processing technique is introduced that enables the in and out-of-plane noise components to be separated from the total measured acoustic emissions based on the corresponding radiation pattern. This phase separation technique gives an indirect indication of the relative contributions of thickness and loading noise. Furthermore, an in-house implementation of Farassat's integral formulations of the Ffowcs Williams and Hawkings equation enables the predicted equivalent acoustic noise source distributions to be visualized to assess where the loading noise emissions are primarily generated on the rotor disk. This phase separation and equivalent acoustic noise source visualization techniques are then employed to determine that thickness noise is dominant noise source for the considered rotor geometry, operating in a trimmed hover condition. In this condition, unsteady loading noise radiates symmetrically above and below the rotor plane. However, when the rotor generates hub moments, unsteady loading noise is amplified considerably and radiates out-of-phase about the rotor plane, as a vertically oriented dipole. The distributed electric propulsion (DEP) systems of eVTOL present unique opportunities for active noise cancelation and overcome some of the disadvantages associated with such strategies when they are employed on helicopters. Since eVTOL rotors can individually sustain hub moments, they can be purposely generated to result in destructive interference between thickness and loading noise. It is demonstrated that the interference pattern between the two noise sources could be steered using combinations of longitudinal and lateral cyclic pitch inputs. The predicted results indicate that the most substantial destructive interference occurs below the rotor plane and resulted in approximately a 20 dB reduction in the total sound pressure levels at this location in comparison to the baseline trimmed hover condition. Since the noise generation mechanisms of thickness and loading noise are inherently different only partial destructive interference is evident in and above the rotor plane. In all of these cases the total noise emissions are amplified outside of the target noise reduction region.