One of the more critical constraints to the continued growth of air traffic is the related concern regarding aircraft noise. This concern has resulted in increasingly stringent noise restrictions for airports, both nationally and internationally. One of the major stretch goals for NASA is to develop noise reduction concepts that will allow the objectionable noise to be contained within the boundaries of a normal airport.
During aircraft takeoff, the dominant aircraft noise source is generally from the engine (propulsion). During aircraft approach and landing, airframe noise becomes a prominent component on par with engine noise. This airframe noise is caused by the interaction of the unsteady and typically turbulent airflow with the aircraft structures. Sound radiated from the side edge of a partial-span flap is one of the major contributors to airframe noise during aircraft approach and landing.
Acoustic liners have long been used in the walls of aircraft engine nacelles to absorb or redirect engine propulsion noise. However, they have not been previously considered for use in an aircraft flap side edge. The unique feature of this invention is the use of acoustic liner-based technology to absorb airframe noise generated in the vicinity of the flap side edge, and to inhibit the process of airframe noise generation at the surface of the flap side edge.
The flap side edge liners described in this disclosure can, to a large extent, be designed using similar tools to those used for the design of conventional liners mounted within the walls of turbofan engine nacelles. Given the significant packaging constraints, the design of the interior chambers of the flap side edge is challenging. Knowledge gained about the use of liners with serpentine chambers is valuable for the other potential noise-reduction applications, as this design concept allows the depth, and hence the weight, of the liner to be significantly reduced. Also, the limited treatment area makes this concept well suited for retrofitting on existing flaps.