Individual Propeller Source Noise Assessment from Full-Scale eVTOL Flight Test Measurements

Aircraft equipped with distributed propulsion, e.g., those utilizing many propulsors for lift and thrust, radiate complex acoustic signatures with strong dependence on the vehicle state. In an effort to better interpret acoustic measurements and characterize the major noise source mechanisms, the Vold–Kalman order-tracking filter is applied to full-scale acoustic flight test measurements of the Joby Aviation eVTOL aircraft. Using synchronized acquisition of the aircraft position, time-varying rotation rates of each propeller, and any given single-channel acoustic signal, harmonic and nonharmonic acoustic content can be separated. Furthermore, this time-domain technique can also separate harmonic content among individual propellers, providing additional physical insight into the total acoustic field. A 60 kt level flyover and hover are used to exemplify the effectiveness of the method. High-fidelity simulations were performed under similar aircraft conditions from the flight test, indicating very good agreement with measured overall aircraft noise levels and filtered levels of individual propellers. Results clearly demonstrate the ability to rank propellers in terms of their relative importance without the use of phased microphone arrays. Frequency- and order-domain results are provided, as well as noise hemispheres, to illustrate relative source contributions and emission angle dependence. Given the similarities in propeller states for each flight condition, acoustic differences can be associated with interactional or installation effects, highlighting the shortcomings of isolated propeller models.