Flight test of the 747-JT9D for airframe noise

Abstract

This paper describes the method used to isolate the individual airframe noise components and determine their relative contribution to the total noise radiated by a 747 aircraft with JT9D-7A engines at approach. The individual components are isolated using ensemble averaged flight test data. The spectral data are normalized on the basis of altitude, airspeed, temperature, and relative humidity. The noise radiated at approach is reconstructed from addition of individual components and compares very accurately in level, spectral shape, and directivity pattern to the actual flyover. A comparison is made between each individual airframe noise component, their logarithmic sum representing the synthesized total airframe component, and Fink's prediction method. The results of this study show that the landing gear, when isolated from configurations having the flaps retracted, is the major contributor to airframe noise. If the effect of flap deployment was included in the isolation of the landing gear noise component, this result could be altered. No evidence of jet-flap interaction noise is found for the 747-JT9D aircraft.