Low flight speed fan noise from a supersonic inlet

A model supersonic inlet with auxiliary inlet doors and boundary-layer bleeds was acoustically tested in simulated low speed flight up to Mach 0 2 in the NASA Lewis 9x15 anechoic wind tunnel and statically in the NASA Lewis anechoic chamber A JT8D refan model was used as the noise source. Data were also taken for a CTOL inlet and an annular inlet with simulated centerbody support struts Inlet operation with open auxiliary doors increased the blade passage tone by about 10 clB relative to the closed door configuration although noise radiation was primarily through the main inlet rather than the doors. Numerous strong spikes in the noise spectra were associated with the bleed system and were strongly affected by the centerbody location. The supersonic inlet appeared to suppress multiple pure tones (MPTs) at or near the fan source Inlet length and the presence of support struts were shown not to cause this MPT suppression