Multirotor Aircraft Noise Reduction

Rotor noise and vibration are two sources of operational challenges for all aircraft operating with open rotors such as helicopters, unmanned aerial vehicles (UAVs), urban air mobility personal air vehicles, drones, and aircraft operating with ducted fans such as passenger aircraft. One disadvantage of convention rotor design is the noise due to noise-induced shed vortices generated by rotor blades. The unique problem with rotor noise and vibration is the periodic blade passage that causes a harmonic reinforcement and causes the rotor blades to vibrate and generate noise sources. This technology from NASA Ames seeks to optimize the implementation of anti-phase trailing edge designs and asymmetric blade tip treatments for rotor noise suppression and integrated aircraft noise solutions by incorporating the anti-phase rotor design concepts into an aircraft flight control system to reduce noise footprint. There are several embodiments of the invention, which include the following: (1) an anti-phase trailing edge design whereby the trailing edge pattern of the leading rotor blade is offset by a phase shift from the trailing edge pattern of the following blade; (2) an anti-phase rotor design implementing asymmetric blade tips with inverted airfoil; and (3) other anti-phase enabled concepts such as unequal blade length, ducted rotors with non-radial unequally spaced struts, and multi-axis tilt rotor design incorporating the anti-phase rotor design.

This innovation comprises a method of adjusting the relative angular positions of the propeller and/or rotor blades from a distributed propulsion system to favorably modify the spatial distribution of noise emanated by the vehicle, that is, the directivity pattern, for the purpose of reducing community noise. Adjusting these angular positions shows a great ability to act as a noise-canceling technique by way of destructive wave interference. Effectively, the acoustic energy can be steered away from noise-sensitive areas, e.g., schools, communities, etc. In the initial implementation, the phase angles can be calculated prior to flight. These depend on the propeller/rotor rotation rate, observer location, and relative propeller/rotor spacing, the latter being constant for a given vehicle. Optimization techniques determine the set of phase angles over the parametric space.

The magnitude and direction of rotor noise radiation is determined by the aerodynamic operating state of the rotor commonly referred to as the "Blade-Vortex Interaction" which occurs when the wake vortex trailing from a preceding rotor blade interacts with the front edge of the following rotor blade. The wake vortex causes a rapid change in the blade loading, which results in the generation of high amplitude, impulsive, and highly directional noise. The occurrence, magnitude, and directionality of Blade-Vortex Interaction noise is very sensitive to the rotor operating state because it is dependent on the relative positions of the rotor and its vortex wake. By providing the rotorcraft pilot with information about annoying noise levels currently being emitted by the rotorcraft and its effects on the ground, corrective action can be taken to change the operating state of the vehicle to minimize or avoid annoyance due to such rotor noise sources. During operation, the pilot would activate the device before or during operation of the rotorcraft. The device displays the noise abatement information through a display unit, informing the pilot about the current acoustic state of the vehicle and providing guidance on how to change the vehicle performance and acoustic state to avoid objectionable blade-vortex Interaction noise. Annoyance footprint information can then be used by the pilot to change the flight path of the vehicle such that the annoyance footprint will not extend into noise sensitive areas.

Among the tests, landing gear cavities, a known cause of airframe noise, were evaluated. These are the regions where the landing gear deploys from the main body of an aircraft, typically leaving a large cavity where airflow can get pulled in, creating noise. NASA applied two concepts to these sections, including a series of chevrons placed near the front of the cavity with a sound-absorbing foam at the trailing wall, as well as a net that stretched across the opening of the main landing gear cavity. This altered the airflow and reduced the noise resulting from the interactions between the air, the cavity walls, and its edges.

A method for predicting the audibility of an arbitrary time-varying noise (signal) in the presence of masking noise is described in "An Algorithm for Statistical Audibility Prediction (SAP) of an Arbitrary Signal in the Presence of Noise" published in the Journal of the Audio Engineering Society (Vo. 69, No. 9, September 2021). The SAP method relies on the specific loudness, or loudness perceived through the individual auditory filters, for accurate statistical estimation of audibility vs. time. As such, this work investigated a new hypothesis that audibility is more accurately discerned within individual auditory filters by a higher-level, decision-making process. Audibility prediction vs. time is intuitive since it captures changes in audibility with time as it occurs, critical for the study of human response to noise. Concurrently, time-frequency prediction of audibility may provide valuable information about the root cause(s) for audibility useful for the design and operation of sources of noise. Empirical data, gathered under a three-alternative forced-choice (3AFC) test paradigm for low-frequency sound, has been used to examine the accuracy of SAPs. Future work should involve additional studies to examine the performance of SAP with realistic ambient noise and signals with higher-frequency content.