NASA’s Langley Research Center has developed aircraft flap tip modifications that reduce noise produced during approach and landing. The modifications are rigid open lattice (honeycomblike) and fin structures (closely packed columns) that delay the formation of a noise-generating vortex at the flap side edge. The designs are low-profile, with control action limited to the steady and fluctuating fields in a very small region at the flap edge. By limiting the control action to a very small region, aerodynamic characteristics of the structure remain nearly unchanged. The noise-reducing structures may be used in a variety of applications where tip vortices produce undesirable noise. These applications may include helicopter or wind turbine rotors. The technology has been validated using computational fluid dynamics analysis and wind tunnel testing. NASA is seeking partners who are interested in co-development or licensure of the technology for a variety of applications.
The open structure of the flap modifications allows the aeroacoustic environment outside the flap to communicate with the perforations or gaps within the flap. The openings embedded within the volume of the flap change the boundary condition at the surface of the flap, significantly reducing the steady pressure differential experienced by the edge. These modifications reduce turbulent fluctuations and delay vortex formation that causes noise during aircraft landing and approach. Comparisons of fluctuating surface pressures between untreated and treated flaps showed an order of magnitude reduction of pressure fluctuation amplitude. Each concept is effective over the entire auditory frequency range, reducing noise amplitude by 3-5 decibels — up to a 50% reduction. Computer simulations have been corroborated by 18% scale wind tunnel testing at NASA Langley.
Although the aerodynamic penalties associated with the concepts are predicted to be very small, minimizing them to negligible levels will require further optimization of the invention. Further considerations of cost, manufacturability, and maintainability are forthcoming.
Potential aerospace applications include use in aircraft flap side edges, helicopter blade tips, and jet turbine fan blade tips. The technology can also be used in wind energy for wind turbine airfoil tips or trailing edges.