Aeroacoustic Characteristics of Externally Blown Flap Systems

Externally blown flap systems are known to be one measure to augment the overall lift during take-off and approach. The potential gain of overall lift depends highly on the way the engine exhaust jet interacts with the flap system. The most common way to design an externally blown flap system for civil transport aircraft is to generate additional lift by means of a thrust deflection. A thrust deflection directly implies the interaction of the highly turbulent jet flow with the solid flap surface which finally results in an airframe noise source. Within the German national founded research project HIT (High Lift Innovative Technologies) the effect of externally blown flaps on farfield radiated noise was investigated by means of dedicated experiments in DLR’s Acoustic Wind Tunnel Braunschweig. The noise tests were performded with a 2D 2-element high lift system and an exhaust jet simulator. By means of pressurized air jet flow speeds of up to Ma=0.67 were realized. The obtained results show a medium to high frequency noise level increase of up to 4 dB due to jet–flap interaction noise for the tested 2-element high lift system which was equipped with either a single slotted or a double slotted flap system.