Method for Predicting In-Flight Noise on Jet Aircraft Structure

Abstract

The principal noise sources from jet aircraft can be summarized as jet noise, boundary layer noise, and the shock wave-turbulence interaction. The noise production of jet aircraft during the in-flight condition is different from the ground static operation due to changes in relative jet velocity, aerodynamic background, and the distribution of sound waves. Basically the in-flight sound pressure levels on a jet aircraft structure are derived from the ground static levels by adding corrections under various in-flight conditions. The directional distribution correction is due to a lateral acoustic quadrupole in motion at various Mach numbers. The relative velocity correction takes into consideration the moving air as a result of an airplane in motion. The altitude correction accounts for the changes in the density of air and the speed of sound at different altitude. All the corrections are expressed in decibels relative to sea level static condition. Boundary layer noise correction is applied in cases where such noise is comparable to or dominates other noises. After the corrections are applied to a number of positions on the aircraft structure, new contours of equal sound pressure level are drawn to represent the in-flight condition.