Multirotor Aircraft Noise Reduction
aerospace
Multirotor Aircraft Noise Reduction (LAR-TOPS-325)
Phase-locked rotors reduce noise
Overview
Multirotor aircraft typically create a significant amount of tonal noise from each rotor. Groups of rotors operating at the same rotation rate with an appropriate phase offset can be used to reduce the tonal noise of the aircraft when there are multiple rotors on each side of the aircraft.
Reducing tonal noise, depending on the aircraft design, can effectively reduce the total noise output for a given flight scenario. Aircraft can be designed with this technology to prioritize quieter take-off and landing, or can be designed for decreased noise in cruise. Existing designs may benefit from this technology as well, but what type of benefits can be gained depend on the design of the aircraft.
Using this method, multiple rotors can be significantly quieter than a single rotor, without sacrificing thrust.
The Technology
Noise produced by multirotor vehicles may be objectionable to some, especially as industry moves toward drone deliveries and potentially air taxi operations.
However, noise reductions can now be attained by synchronizing the rotation rate and then controlling the phase offset between neighboring rotors. The optimal phase offset is uniquely defined based on the number of blades and the relative location of each rotor and does not depend on the rotation rate, blade geometry, or other aspects of the vehicle design.
This technology is not viable for all multirotor aircraft but is applicable if the rotation rate of neighboring rotors can be synchronized mechanically or electronically.
Benefits
- Quieter aircraft operations
Applications
- UAV Deliveries
- Air Taxis
- Professional and hobby drone pilots
Similar Results
Anti-Phase Noise Suppression Rotor Technologies
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.
Propeller/Rotor Phase Control for Reduction of Community Noise from Distributed Propulsion Vehicles
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.
Device for Providing Real-Time Rotorcraft Noise Abatement Information
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.
Application of Leading Edge Serration and Trailing Edge Foam for Undercarriage Wheel Cavity Noise Reduction
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.
Cavity Noise Reduction Technology
Attached to the interior edges of the wheel well and covering the entire cavity opening, the stretchable mesh promotes growth of three-dimensional flow structures within the free shear layer. The fine flow structures generated by the mesh effectively reduce shear layer roll-up and eliminate span-wise coherence of the large-scale flow structures immediately downstream of the landing gear cavity leading edge that generate cavity noise. Consequently, the generation of high amplitude acoustic waves and subsequent cavity resonance is significantly diminished. The mesh has been tested in a high fidelity
18% scale model in NASA Langley Research Centers 14- by 22-Foot Subsonic Wind Tunnel. Measurements of acoustic far field noise were collected using a phased microphone array. The stretchable mesh concept is able to reduce the gear cavity noise in excess of one to three decibels from 100-500 Hz, and by about one decibel in the 500-800 Hz range. Sound reduction efficacy of the stretchable mesh construct was compared with rigid mesh and the stretchable mesh has proven more effective in landing gear cavity noise reduction.
Determination of a final embodiment of the stretchable mesh will require design and optimization of the cavity mesh support and attachment fixtures. Further considerations of cost, manufacturability, and maintainability are forthcoming.