An observation on tone cut-off in static test data from jet engine fans

The far-field acoustic data for eight full-scale fans tested at a NASA-Lewis outdoor facility were analyzed for the effect of a cut-off design feature where the number of rotor and stator blades are proportioned so that the blade-passage tone will not propagate. Inlet flow distortions, prevalent during static testing, interact with the rotor to generate tones that generally mask this cut-off phenomenon. Distortion tone properties previously evaluated are used to analyze the tones propagating from the aft fan duct. The analyzed data clearly differentiated the fans that are and are not cut-off