Propeller/Rotor Phase Control for Reduction of Community Noise from Distributed Propulsion Vehicles

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.

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.

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.

Distributed propulsion and lightweight flexible structures on air vehicles pose a significant opportunity to improve mission performance while meeting next generation requirements including reduced fuel burn, lower emissions, and enhanced takeoff and landing performance. Flexible wing-shaping aircraft using distributed propulsion enable the ability to achieve improved aerodynamic efficiency while maintaining aeroelastic stability. Wing shaping concepts using distributed propulsion leverage the ability to introduce forces/ moments into the wing structure to affect the wing aerodynamics. This can be performed throughout the flight envelope to alter wing twist, hence local angle of attack, as the wing loading changes with air vehicle weight during cruise. Thrust-induced lift can be achieved by distributed propulsion for enhanced lift during take-off and landing. For a highly flexible wing structure, this concept could achieve a 4% improvement in lift-to-drag ratio, hence reduced fuel burn, as compared to a conventional stiff wing. This benefit is attributed to a reduction in lift-induced drag throughout the flight envelope by actively shaping the spanwise lift distribution using distributed propulsion. Vertical tail size could be reduced by utilizing differential thrust flight-propulsion control. This will result in weight reduction to achieve further fuel savings. Aeroelastic stability is addressed in the design process to meet flutter clearance requirements by proper placement of the propulsion units. This technology enables synergistic interactions between lightweight materials, propulsion, flight control, and active aeroelastic wing shaping control for reducing the environmental impact of future air vehicles.