Boundary Layer Ingestion (BLI) -- passing a portion of an airframe's wake through the engine -- has been suggested as a means of reducing civil aircraft energy consumption. An aerodynamic performance benefit results from the reduction of viscous dissipation in the airframe wake and in the propulsor jet downstream of the aircraft. A number of challenges exist in the design and performance assessment of BLI aircraft configurations. One is that definition of the propulsion system requirements becomes more difficult because the concepts of thrust and drag, conventionally associated with the engines and airframe, respectively, become ambiguous with a tightly integrated propulsion system. Further, the engine performance itself may be adversely affected by the presence of inlet distortions arising from the ingested airframe boundary layer. The goal of this computational and experimental project is to establish a strategy for the design of propulsors for operation in BLI aircraft configurations.
Propulsion system requirements are determined using the power balance method. Thus, rather than considering the forces on the aircraft, the performance of the airframe and engine are characterized in terms of sources and sinks of mechanical energy. Quantities analagous to thrust, drag, and propulsive efficiency are defined, and it is seen that the system-level benefit of BLI can be explained as the combined effect of a decrease in effective airframe drag and an increase in propulsive efficiency. Viewing the system performance in this way also provides insight into appropriate propulsion system scaling for powered wind tunnel models and for comparison between integrated and conventional podded propulsion system configurations.
The impact of BLI inlet distortion on fan stage turbomachinery performance is assessed using a non-axisymmetric throughflow model. A parametric study to determine the sensitivity of the distortion response and fan efficiency to various design parameters is being carried out to illuminate design features for propulsors more tolerant of BLI type inlet distortions. The performance of selected designs will be assessed both computationally and experimentally, as part of wind tunnel tests of a scale model BLI aircraft configuration to be conducted in the MIT Wright Brothers WindTunnel and in the NASA Langley Research Center 14'x22' Wind Tunnel.