Aeromechanical Response of a Distortion-Tolerant Boundary Layer Ingesting Fan

Boundary layer ingestion (BLI) is a propulsion technology being investigated at NASA by the Advanced Aircraft Transportation Technology (AATT) Program to facilitate a substantial reduction in aircraft fuel burn. In an attempt to experimentally demonstrate an increase in the propulsive efficiency of a BLI engine, a first-of-its-kind subscale high-bypass ratio 22″ titanium fan, designed to structurally withstand significant unsteady pressure loading caused by a heavily distorted axial air inflow, was built and then tested in the transonic section of the GRC 8′ × 6′ supersonic wind tunnel. The vibratory responses of a subset of fan blades were measured using strain gages placed in four different blade pressure side surface locations. Response highlights include a significant response of the blade's first resonance to engine order excitation below idle as the fan was spooled up and down. The fan fluttered at the design speed under off operating line, low flow conditions. This paper presents the blade vibration response characteristics over the operating range of the fan and compares them to predicted behaviors. It also provides an assessment of this distortion-tolerant fan's (DTF) ability to withstand the harsh dynamic BLI environment over an entire design life of billions of load cycles at design speed.