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Conventionally, fan stages are designed with more stator vanes than rotor blades. These fan stages provide a tonal noise advantage as the blade passing frequency tone is cut-off. Contrary to conventional cut-off designs, low-count OGV (Outlet Guide Vane) designs are fan stages with fewer stator vanes than rotor blades. Low-count OGV designs benefit from lower broadband noise due to the reduced number of stator vanes. However, the blade passing frequency tone may no longer be cut-off. Therefore, we assess tonal noise reduction mechanisms for subsonic low-count OGV fan stages. We use an analytical and a numerical method to predict the rotor–stator interaction noise for different blade count pairings. The results indicate that the mode phase propagation angle and the cut-on factor are two acoustic parameters that dominantly influence the noise excitation. For certain blade count pairings an acoustic benefit exists if the mode phase propagation angle is congruent to the stator leading edge angle and / or if the mode is away from the cut-off limit. Moreover, reducing the rotor tip speed allows, for some configurations, a cut-off design even with fewer stator vanes than rotor blades (inverse cut-off). Overall, the study shows promising possibilities to reduce tonal interaction noise for low-count OGV fan stages.

System noise has been integrated as an additional design objective within conceptual aircraft design. The DLR system noise prediction tool PANAM accounts for individual noise sources depending on their geometry and operating conditions. PANAM is integrated into the existing aircraft design framework PrADO from the Technical University of Braunschweig in order to realize a design-to-noise simulation process. In addition, a ray-tracing tool from DLR, SHADOW, is incorporated into the simulation framework in order to account for structural engine noise shielding. The overall simulation process is then applied to identify promising low-noise aircraft concepts. The presented application aims at fan noise reduction through shielding. For the selected reference aircraft, the fan is a major noise source during both landing and takeoff. It is demonstrated, that the aircraft designers influence on the environmental vehicle characteristics is significant at the conceptual design phase. Usually, a trade-off between extensive engine noise shielding and economical flight performance is inevitable. The new design-to-noise process is well suitable to assess all four measures of ICAOs balanced approach.