A trade-factor-based system study has been carried out to identify fuel burn benefits associated with boundary layer ingestion (BLI) for generation-after-next (N+2) aircraft and propulsion system concepts. The analysis includes detailed propulsion system engine cycle modeling for a next-generation, Ultra-High-Bypass (UHB) propulsion system with BLI using the Numerical Propulsion System Simulation (NPSS) computational model. Cycle modeling was supplemented with one-dimensional theory to identify limiting theoretical BLI benefits associated with the blended wing body reference vehicle used in the study. The system study employed low-order models of engine extractions associated with inlet flow control; nacelle weight and drag; fan performance; and inlet pressure losses. Aircraft trade factors were used to estimate block fuel burn reduction for a long-range commercial transport mission. Results of the study showed that a 3-5% BLI fuel burn benefit can be achieved for N+2 aircraft relative to a baseline high-performance, pylon-mounted, UHB propulsion system. High-performance, distortion-tolerant turbomachinery, and low-loss, low-drag inlet systems, were identified as key enabling technologies. Larger benefits were estimated for N+3 configurations for which larger fractions of aircraft boundary layer can be ingested. NomenclatureA = area (in. 2 ) AR = inlet aspect ratio (w / h) c, C = aircraft chord (ft or in.) D = amount of aircraft viscous drag ingested by propulsion systems (lbf) F n , F N = engine net thrust (lbf) FB = fuel burn (lbs) h = inlet height (ft or in.) H = boundary layer shape factor (δ* / θ) k = boundary layer pseudo-energy thickness (in.) K = boundary layer pseudo-energy factor (k / θ) M = Mach number n = unit surface vector 2 P = pressure (psi) P T , P t = total pressure (psi) R = wake recovery factor (1-Δ j /Δ 0 ) T = thrust (lbf); temperature (°R) U = velocity (ft / s) V = free stream velocity (ft / s) V x = axial component of free stream velocity (ft / s) w, W = inlet width (ft or in.) x, X = axial coordinate or dimension (in.) y = transverse or vertical coordinate or dimension (in.)Greek: δ* = boundary layer displacement thickness (in.) Δ = wake velocity defect relative to freestream or jet velocity condition ρ = density (slug / ft 3 ) τ = wall shear stress (psf) θ = boundary layer momentum thickness (in.) Subscripts / Superscripts:∞, 0 = freestream condition j = propulsion system jet velocity condition MA = mass averaged quantity s = static condition T = stagnation condition x = axial component

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The ability to control fan nozzle exit area is an enabling technology for next generation high-bypass-ratio turbofan engines. Performance benefits for such designs are estimated at up to 9% in thrust specific fuel consumption (TSFC) relative to current fixed-geometry engines. Conventionally actuated variable area fan nozzle (VAN) concepts tend to be heavy and complicated, with significant aircraft integration, reliability and packaging issues.The goal of this effort was to eliminate these undesirable features and formulate a design that meets or exceeds leakage, durability, reliability, maintenance and manufacturing cost goals. A Shape Memory Alloy (SMA) bundled cable actuator acting to move an array of flaps around the fan nozzle annulus is a concept that meets these requirements. The SMA bundled cable actuator developed by United Technologies Corporation (Patents Pending) provides significant work output (greater than 2200 in-lb per flap, through the range of motion) in a compact package and minimizes system complexity.Results of a detailed design study indicate substantial engine performance, weight, and range benefits. The SMA-based actuation system is roughly two times lighter than a conventional mechanical system, with significant aircraft direct operating cost savings (2 -3%) and range improvements (5-6%) relative to a fixed-geometry nozzle geared turbofan.A full-scale sector model of this VAN system was built and then tested at the Jet Exit Test Facility at NASA Langley to demonstrate the system's ability to achieve 20% area variation of the nozzle under full-scale aerodynamic loads. The actuator exceeded requirements, achieving repeated actuation against full-scale loads representative of typical cruise as well as greater than worst-case (ultimate) aerodynamic conditions. Based on these encouraging results, work is continuing with the goal of a flight test on a C-17 transport aircraft.

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