Cavity Noise Reduction Technology

NASA Langley designed the shape memory alloy slat-cove filler to provide significant broad-band noise reduction to any aircraft wing structure that has a leading-edge, high-lift device and that is distinct from the main-wing element. The design can be retrofitted to existing aircraft structures and can be easily incorporated into the existing or future designs for aircraft wing structures. The concept involves very few components, requires no additional mechanical support from pneumatic or hydraulic systems, and makes use of existing slat-actuation systems for retraction. The design is autonomous, simple, and constitutes low-weight addition. The concept is also considered fail-safe because the lift would not be diminished in the event that the slat cove filler failed to deploy. Several advancements have been devised to accommodate complex features encountered in application to practical airframe structures. Graphics from a computational model of a 2D physical demonstration system show the configuration and strain in the slat-cove filler in the deployed and stowed conditions. Features enabling stowage of a large curvilinear length (sliding hinge) and maintenance of the optimized outer mold line (auxiliary component) are highlighted. Other advancements for application to 3D, flight airframes are visible in the image from a model for one entire section of a slat-cove-filler treatment for a wide-body, transport-class aircraft. NASA Langley also offers a design for a deformable structure that is deployed from the leading edge of the main-wing element, termed the slat-gap filler. It closes and covers the gap between the slat and the main-wing element, but can be readily and autonomously opened in emergency to regain the baseline high-lift configuration and its corresponding lift performance at high angles of attack. This approach has similar benefits as the slat-cove filler device.

This technology is an evolution of acoustic liners developed for engine noise abatement that are typically located inside nacelles. The acoustic liners described here can be outfitted on external surfaces and in tight spaces. Three initial areas of the aircraft have been considered as part of an aircraft configuration incorporating an open rotor propulsion system. The three areas where the liner configurations were applied were (1) under the rotor, (2) on the upper surface of the elevon, and (3) on the surface of a strut.

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 drag penalty incurred by a conventional acoustic liner is dependent, to a large extent, on the perforate open area ratio (porosity) of the perforated facesheet. As the open area ratio is decreased, the facesheet behaves more like a solid surface and the drag is reduced. However, if the open area ratio is too small, the external acoustic field will be isolated from the resonators (in the liner), and the system will not provide noise reduction. The technology is a new type of variable-depth acoustic engine liner, which will reduce the drag and potentially manufacturing cost of this class of engine liner. Individual resonators within a conventional variable-depth liner are effective near resonance, but provide less acoustic benefit at other frequencies. In fact, at anti-resonance, a resonator behaves similar to a hard wall (i.e., the normal component of the particle velocity at the inlet is zero). Therefore, the proposed innovation couples neighboring resonators (tuned for different frequencies) together within the core of the liner. In other words, multiple resonators share a single inlet/port. Sharing inlets reduces the overall number of openings needed to maintain the acoustic performance of the liner by a factor of two or more. Reducing the open area ratio will in turn reduce the liner drag, and will reduce the number of holes that have to be machined into the facesheet, potentially reducing manufacturing cost. The functional operation of the proposed innovation will be identical to conventional engine liners. The innovation enables a reduction of the open area ratio of the perforated facesheet (by a factor of two or more) without degrading the acoustic performance. This will decrease the liner drag, and has the potential to reduce the manufacturing cost of the liner, since fewer holes need to be machined in the facesheet.

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.