Aircraft noise is a significant problem with both economic and public health implications, especially for communities near airports. As a result, increasingly stringent constraints are being placed on aircraft carriers worldwide to reduce this noise. The current disclosure focuses on airframe noise generated at or near the surface of the flap-side edge.

The technology is a concept for applying noise reduction fins to the edges of aircraft wing flaps. These nearly rigid fins do not carry the performance penalty of prior art solutions, such as soft brushes. The fins reduce noise generation by reducing the interaction of unsteady flow with the flapside edge surface. The particular configuration of the fins also facilitates the communication of the airflow outside the flap and the inside of the fin bundle. This allows for a reduction in the steady pressure differential at the flap edge, which is key to weakening or delaying the vortex formation at the edge. At that point, the weakened or delayed vortex can no longer generate as much noise by scrubbing its unsteady flow over the side edge surface.

Noise produced by unsteady flow around aircraft structures, termed airframe noise, is an important source of aircraft noise during landing approach. Sound radiated from the side edge of a deployed conventional partial-span flap is one of the major contributors to airframe noise during aircraft approach and landing.

In this approach, Flap Edge Noise Reduction Fins (FENoRFins) are applied to the tip of an aircraft flap to reduce the amount of airframe noise. Unlike the ordinary soft brushes, the current concept specifically addresses the process of noise generation due to the interaction of unsteady flow with the flap-side edge surface, while simultaneously minimizing the aerodynamic penalties. By limiting the control action to the steady and fluctuating fields in a very small region near the edge, the gross aerodynamic characteristics of the flap are left unaltered and, hence, the expected aerodynamic penalty is small or none at all. Because of the gaps between the fins protruding from one or more segments of the flap tip surface, the aeroacoustic environment outside the flap can communicate with the complex and elaborate passages associated with the fin embedded within the solid structure. The intricate passages alter the effective boundary condition at the surface of the flap, significantly reducing the steady pressure differential experienced by the edge. As a result, the vortex formation process at the edge is either delayed or substantially weakened.

This work was done by Mehdi Khorrami and Meelan Choudhari of Langley Research Center. LAR-18301-1

NASA Tech Briefs Magazine

This article first appeared in the September, 2014 issue of NASA Tech Briefs Magazine.

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