Two different modifications have been developed for the edges of aircraft wing flaps to reduce noise. Reactive Orthotropic Lattice Diffuser (ROLD) is a modification that consists of a honeycomb-like, interconnected structure added to the region of the flap that experiences the highest degree of sound-generating airflow instability. Flap Edge Noise Reduction Fins (FENoRFins) are modifications consisting of rigid fins on the edges of the aircraft flaps.
The open-structure nature of each flap modification (porosity of ROLD and the presence of gaps in FENoRFins) near the side edge allows the aeroacoustic environment outside of 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 flow and delay vortex formation that causes noise during aircraft landing and approach.
The effectiveness of these two concepts has been demonstrated by computational fluid dynamic simulations using Gulfstream jet aircraft geometry. Comparisons of fluctuating surface pressures between untreated and treated flaps showed an order of magnitude reduction of pressure fluctuation amplitude by 3 to 5 dB. These computer simulations have been corroborated by 18% scale wind tunnel testing.
By limiting the control action to the steady and fluctuating fields in a very small region near the flap edge, the gross aerodynamic characteristics of the flap are left unaltered. Hence, the expected aerodynamic penalty is small. The present inventions can be incorporated into both current and future designs of aircraft flaps. The limited treatment area makes these concepts well suited to retrofitting existing flaps.
This work was done by Mehdi Khorrami of Langley Research Center. LAR-18166-1