Aft Fuselage Boundary-Layer Ingesting Propulsion Systems for Turbo-Electric Aircraft

Abstract

This thesis focuses on a rigorous assessment of a tube-and-wing turbo-electric boundarylayer ingesting (BLI) aircraft, including (i) definitions of the aerodynamic attributes for the best fuel burn benefit, (ii) design guidelines to achieve these attributes, and (iii) a conceptual design of tail-BLI aircraft. The assessment of the BLI benefit relative to a baseline conventional aircraft is based on a TASOPT-CFD analysis framework, using a CFD body-force model for fan representation and a power balance for aircraft performance analysis. The aircraft mission is to carry a 17500 kg payload over 5500 km range at a cruise Mach number of 0.8. The baseline aircraft has two next-generation geared turbofan engines with fan pressure ratio (FPR) of 1.35. The tail-BLI aircraft has six integrated propulsors with electric fans of 1.40 FPR in addition to two underwing turbofans; it is estimated to achieve an 8.5% fuel burn benefit compared to the baseline. To achieve the benefit, the tail-BLI aircraft has a non-axisymmetric aft fuselage that creates axial vorticity in the tail-mounted propulsor inflow, providing co-swirl to reduce rotor incidence variations to improve the fan efficiency and stall margin. The best propulsor configuration, with six 24-inch fans, is established balancing the boundary-layer kinetic energy defect ingestion and propulsion system weight. The integrated propulsor aerodynamic design includes an upstream inlet extension, an inlet leading edge design tailored for each propulsor, a supercritical nacelle, an annular nozzle, an elliptic nozzle plug, and a non-axisymmetric tail cone, to minimize shock loss and achieve attached flow at cruise and takeoff. The BLI fan pressure ratio was selected based on a trade between engine propulsive efficiency and propulsion system weight. Non-axisymmetric stator inlet angles are also used to reduce the stator incidence variations. Fan forcing analysis suggests that the BLI rotor unsteady aerodynamic loading has negligible impact on blade life. Fan stability analysis shows that the BLI inlet distortion reduces the rotor stall margin by 4.5% and 2.7% relative to the rotor in uniform flow, at cruise and takeoff, respectively. The tail-BLI propulsors are thus estimated to have appropriate operability with BLI inlet distortion. In conclusion, the distributed aft fuselage boundary-layer ingesting propulsion system is suggested to offer a relevant pathway for tube-and-wing configurations with a potential major advancement in fuel burn reduction.