Krueger Noise Generation and its Comparison to Slat Noise

Abstract

High lift system related noise is still the major airframe noise contributor during the approach and landing phase of today’s transport aircraft. The classical slat is highly maturated and thus widely used by airframers. Against this background, slat noise was investigated in deep and technologies to reduce slat noise were developed including alternatives like e.g. droop nose devices or the long chord slat device . Another alternative and a pre-requisite for the use of laminar wing technology is the Krueger leading edge flap. This device was investigated in the German national research project INTONE led by Airbus. The Krueger device was designed for a conventional non-laminar 3-dimensional wing and tested in the acoustic test section of DNW’s low speed wind tunnel in Braunschweig. The respective Krueger configurations are based on the same shape of the Krueger flap but different Krueger setting parameters like e.g. deflection angle or gap size. The basic findings can be summarized as follows. All tested Krueger configurations compare well to the slat in terms of the aerodynamic performance. Acoustically speaking, all Krueger leading edge devices represent the dominant noise source of the high lift system. The so-called reference Krueger configuration exhibits only slightly higher noise levels than reference slat. For this configuration the effect of the Krueger cavity on noise was investigated. It turned out that a closure of this cavity has only marginal influence on farfield radiated noise. Exemplary gap and overlap changes showed an influence of both the overlap and the gap on noise. Finally, it turned out that basic mechanisms between the high lift setting and the noise generations as known for the slat are generally speaking also valid for the Krueger. In particular the gap size turned out to be an effective means to influence the noise generation. As for the slat, the smallest gap size led to lowest noise levels. This again is similar to slat noise for which the same characteristics were documented in the EU-projects TIMPAN and OPENAIR. Furthermore, the INTONE project provides evidence that a Krueger leading edge flap is not silent by itself as was already demonstrated by Wild and Pott-Pollenske.