Flight Study of Induced Turbofan Inlet Acoustic Radiation with Theoretical Comparisons

This paper presents a study of acoustic tone radiation patterns from a small turbofan engine in flight and compares results with similar static test stand data and a recently developed radiation theory. An interaction tone was induced for test and evaluation purposes by placing a circumferential array of inlet rods just upstream of the fan blades. Overhead and sideline flight directivity patterns showed cut-on of a dominant single-mode tone occurred at the predicted fan speed, and there was an absence of any other significant circumferential or radial modes. In general, good agreement was found between measured flight and static data, with small differences being attributed to inlet geometry and/or forward speed effects. Good agreement was also obtained between flight data and theory for directivity pattern shape, however, the theory consistently predicted higher values for peak radiation angle over a wide range of frequency