Abstract: Combination bulk absorber-honeycomb acoustic panels particularly adapted for use in the inlet and fan air duct of a fan jet engine are disclosed. Each panel comprises a broadband noise-suppressing bulk absorber layer mounted between a back sheet and a perforated septum; and, a narrow band noise-suppressing honeycomb layer mounted between the septum and a perforated face sheet. The panels are mounted in an engine such that the perforated face sheet interfaces with airflow in the inlet or the fan air duct, as the case may be, whereby the face sheet, honeycomb layer and septum protect the bulk absorber from deterioration by isolating the high speed inlet and fan air duct airflows from the bulk absorber layer.

Abstract: This invention is an improved turbofan engine having a variable geometry fan duct which reduces stator source noise emitted from the engine. Attached to the inner wall of the fan casing near the fan stators is a pneumatically actuated annular membrane having two operating positions. In the cruise position, the membrane is forced radially inward to press against the radially outermost tips of the fan stators. In the second position, used during approach and takeoff, the membrane is drawn against a recessed portion in the fan casing, thereby creating an annular passageway between the stator tips and the casing. When the membrane is in this position, turbulent flow from the rotor blade tips which would normally impinge on the stators is directed through the annular passageway.