In Search of Airworthy Trailing-Edge Noise Reduction Means

The EC co-financed project OPENAIR aims at a further development of both innovative and established aircraft noise reduction technologies. Among these activities an innovative two-component trailing-edge (TE) design, applicable at a slat TE, was evaluated through generic tests in DLR's Acoustic Wind-Tunnel Braunschweig(AWB). The proposed design combines a supporting frame structure (a coarse metal mesh or rib structure) with a thin, hydraulically smooth cover material (a fine metal mesh, micro-perforated sheet metal or cloth material) which are both permeable to the flow. The result is a mainly hollow, flow-transparent TE region. Variants of this concept were parametrically tested versus a solid reference TE at a 0.4-m chord generic airfoil. Material selection was based on airwor- thiness considerations. Detailed flow/acoustic resistance measurements were performed to characterise the used materials. TE noise was measured by a directional microphone system. The tested TE modifications achieve a broadband noise reduction of up to 4 dB, de-pendent of the configuration. However, the noise reduction effect diminishes with growing angle-of attack. This adverse behavior as well as geometrical dependences will have to be further studied prior to large scale testing. Pressure distribution measurements reveal a non-measurable to small effect of the TE modifications on the airfoil's suction peak. Basic noise scaling laws for transposition of the results to full-scale applications are suggested