Comparisons of Scale-Model and Flyover Jet Noise

Continued interest in commercial supersonic flight has highlighted the need for accurate jet-noise predictions because jet noise is expected to be a dominant noise source at takeoff for future supersonic transport aircraft. The current investigation compares acoustic spectra obtained with a scale-model nozzle system to that from flyovers with a Learjet 25D airplane. Reasonable agreement was achieved between scale-model and flyover source spectra in both peak level and frequency when the physical model scale factor was increased by 6%. The perceived noise level (PNL) and the effective perceived noise level (EPNL) were computed for a representative future supersonic-aircraft takeoff using the flyover and scale-model data. The PNL values were slightly higher for the flyovers than for the scale model, and the rolloff of the PNL with emission angle at large angles was different for the two datasets. However, the scale-model EPNL values fell within the uncertainty bands of the corresponding flyover results. The EPNL difference was within 0.6 EPNdB at the highest engine setting investigated. Applying distributed-source models for the shear-layer refraction corrections to the scale-model data only slightly improved spectral comparisons to flyover data but resulted in a limited range of emission angles that could be used in the EPNL computations.