Autonomous Slat-Cove Filler Device
Aerospace
Autonomous Slat-Cove Filler Device (LAR-TOPS-87)
Reduction of aeroacoustic noise associated with the leading edge of aircraft wings
Overview
NASA's Langley Research Center developed a deployable and stowable mechanical design for filling the cavity behind the leading-edge slat (i.e., slat cove), when it is extended upon landing approach of an aircraft. Aerodynamic flow over an unfilled cavity typically exhibits strongly unsteady behavior that is a source of aeroacoustic noise. Conventional leading-edge slat devices for high lift are a good example of such geometric and flow conditions and are a prominent source of airframe noise. Experimental and computational results have shown that a slat-cove filler device could significantly reduce the noise produced by slat structures without aerodynamic penalty. The proposed structural concept will enable autonomous achievement of the desired deployed shape. The design will facilitate a clean cruise configuration with minimal weight addition to the aircraft. NASA is seeking development partners and potential licensees.
The 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.
Benefits
- Provides significant broadband noise reduction: ~4 effective perceived noise decibels (EPNdB) reduction
- Incorporates easily into existing aircraft structure or future designs of aircraft wing structures
- Requires no additional mechanical support from pneumatic or hydraulic systems
- Constitutes low-weight addition
Applications
- Commercial Aerospace - Aircraft that incorporate a leading-edge, high-lift device that is distinct from the main-wing element
- Launch vehicle or rockets
- Automobiles
Similar Results
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.
Low, Drag, Variable-Depth Acoustic Liner
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.
External Aircraft Noise Reduction Liners
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.
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.
A Method for Reducing Broadband Noise
This NASA technology is ideally suited to absorb sounds below 1000 Hz (at the low end of human auditory range), which commercially available materials have struggled to absorb effectively. NASA innovators designed the acoustic liner to mimic the geometry and the low-frequency acoustic absorption of natural reeds. To provide excellent noise absorption that endures even in a variety of challenging conditions, researchers have created and tested prototypes of acoustic filters using thin and lightweight parallel-stacked tubes one-fourth to three-eights of an inch in diameter. The assembly can feature a porous or perforated face sheet positioned on one or more sides of the acoustic absorber layer to increase noise-reduction capability as needed. These filters have demonstrated exceptional acoustic absorption coefficients in the frequency range of 400 to 3000 Hz. Results indicate that these assemblies can be additively manufactured from synthetic materials, generally plastic; however, ceramics, metals, or other materials can also be used. The reeds can be narrow or wide, hollow or solid, straight or bent, etc., giving this acoustic liner remarkable flexibility and versatility to meet the needs of virtually any application. This technology effectively demonstrates that a new class of structures can now be considered for a wide range of environments and applications that need durable, lightweight, broadband acoustic absorption that is effective at various frequencies, particularly between 400 and 3000 Hz.
