Tonal Noise Prediction of a Distributed Propulsion Unmanned Aerial Vehicle
Noise is expected to be a major barrier of unmanned aerial vehicles (UAVs) to public acceptance. A noise prediction scheme is introduced in this paper and applied to a specific vehicle configuration, namely, the Greased Lightning-10. Results herein will be used to demonstrate the feasibility of incorporating a noise constraint within the flight control system of a distributed electric propulsion vehicle by modifying commands for low-noise operation over sensitive areas, e.g., communities, schools, etc. Steady loading and thickness noise signatures of single propellers are computed using the Propeller Analysis System of the NASA Aircraft NOise Prediction Program. The individual signatures are then superposed at far field observers after applying corrections for spherical spreading and phase delays based on straight ray propagation. Two-propeller source fields are verified using analytical directivity patterns of monopoles. Notional effects of rotation rate, rotation direction, and relative propeller phase are given. Under ideal circumstances and equivalent RPM, random phasing, which occurs in most small UAVs, can produce up to 20 decibels uncertainty in the tonal sound pressure level at a given ground observer. Additionally, directivity modification via relative propeller phase control is shown to have great potential as a noise reduction technique. This paper will focus on the forward flight mode but will also briefly discuss the vertical flight mode.