NASA's X-57 "Maxwell" flight demonstrator incorporates distributed electric propulsion technologies in a design that will achieve a significant reduction in energy used in cruise flight. A substantial portion of these energy savings come from beneficial aerodynamicpropulsion interaction. Previous research has shown the benefits of particular instantiations of distributed propulsion, such as the use of wingtip-mounted cruise propellers and leading edge high-lift propellers. However, these benefits have not been reduced to a generalized design or analysis approach suitable for large-scale design exploration. This paper discusses the rapid, "design-order" toolchains developed to investigate the large, complex tradespace of candidate geometries for the X-57. Due to the lack of an appropriate, rigorous set of validation data, the results of these tools were compared to three different computational flow solvers for selected wing and propulsion geometries. The comparisons were conducted using a common input geometry, but otherwise different input grids and, when appropriate, different flow assumptions to bound the comparisons. The results of these studies showed that the X-57 distributed propulsion wing should be able to meet the as-designed performance in cruise flight, while also meeting or exceeding targets for high-lift generation in low-speed flight.
this technology. These drawbacks have thus far limited market penetration in all but the smallest vehicle classes. Current approaches to airborne electricity generation revolve around storage of the energy in batteries onboard the aircraft, which greatly limits range and endurance, or generation of electricity by combustion of stored hydrocarbon fuel as part of a hybrid-electric architecture, which suffers from mass and efficiency challenges.Borer [1] outlined these approaches and recommended pursuit of an additional option to the portfolio of solutions available to store energy for airborne electric propulsion. Borer proposed the use of a hybrid-electric solution that generates electricity primarily from a Solid Oxide Fuel Cell (SOFC) fed from an onboard fuel reformer, which converts heavier hydrocarbons into products suitable to be consumed by the SOFC. To be viable, this power system needs to operate at a specific power level of at least 300 W/kg and an efficiency of 60%, referenced to the lower heating value (LHV) of the stored fuel. These characteristics represent significant advances in the state-of-the-art for hybrid-electric SOFC architectures -a recent National Academies report indicated that such architectures are capable of a specific power of 100 W/kg and an efficiency of 30-40% [2]. More recent research suggests that higher specific power and efficiency levels are achievable [3].This paper is one of a number of concurrent papers related to NASA's Fostering Ultra-Efficient, Low-Emitting Aviation Power (FUELEAP) project. The authors present a detailed application of a hybrid-electric, heavy-fuel, SOFC power system developed under FUELEAP. Specifically, this paper describes the integration of a 120 kW power system in place of the battery system used on NASA's X-57 "Maxwell" distributed electric propulsion (DEP) technology flight demonstrator. Section II provides background on FUELEAP and the X-57 program. Section III describes the integration of the hybrid-electric SOFC power architecture onto the X-57 and provides performance estimates for two different design spirals, as well as comparison to gasoline-fueled and battery-powered alternatives. Finally, Section IV summarizes the results of this research. Other companion papers include: a more detailed description of the power system design [4], electrical integration of this power system into the X-57 [5], thermal cycle testing of SOFC hardware to assess suitability for aviation operations [6], system safety analysis of the FUELEAP X-57 demonstrator concept [7], and the use of model-based systems engineering frameworks to aid in the development of future demonstrator concepts [8].
Electrically-powered aircraft can enable dramatic increases in efficiency and reliability, reduced emissions, and reduced noise as compared to today's combustion-powered aircraft. This paper describes a novel flight demonstration concept that will enable the benefits of electric propulsion, while keeping the extraordinary convenience and utility of common fuels available at today's airports. A critical gap in airborne electric propulsion research is addressed by accommodating adoption at the integrated aircraft-airport systems level, using a confluence of innovative but proven concepts and technologies in power generation and electricity storage that need to reside only on the airframe. Technical discriminators of this demonstrator concept include (1) a novel, high-efficiency power system that utilizes advanced solid oxide fuel cells originally developed for ultra-long-endurance aircraft, coupled with (2) a high-efficiency, high-power electric propulsion system selected from mature products to reduce technical risk, assembled into (3) a modern, high-performance demonstration platform to provide useful and compelling data, both for the targeted early adopters and the eventual commercial market. Nomenclature
As the aviation vehicle design environment expands due to the influx of new technologies, new methods of conceptual design and modeling are required in order to meet the customer's needs. In the case of distributed electric propulsion (DEP), the use of highlift propellers upstream of the wing leading edge augments lift at low speeds enabling smaller wings with sufficient takeoff and landing performance. During cruise, however, these devices would normally contribute significant drag if left in a fixed or windmilling arrangement. Therefore, a design that stows the propeller blades is desirable. In this paper, we present a method for designing folding-blade configurations that conform to the nacelle surface when stowed. These folded designs maintain performance nearly identical to their straight, non-folding blade counterparts.
A growing demand for higher-power electrical equipment on aircraft is driving the exploration of alternative electrical power generation devices. "Hybrid-electric" aircraft architectures that leverage multiple or alternative power sources are also becoming more popular with rising concerns over efficiency. Fuel cells are particularly interesting as auxiliary or secondary electrical power generation devices, due to their relative efficiency and longevity, and could be used as a "power pod" for vehicles without enough on-board electrical generation capability. This paper examines the feasibility and potential value of using an externally-mounted, hybridelectric, solid-oxide fuel cell (SOFC) power system on a Class IV unmanned aerial vehicle (UAV) for the purpose of power generation in excess of the baseline electrical load. Moreover, the paper will demonstrate that a SOFC auxiliary power unit (APU) provides significant value to an aircraft by producing a relatively large amount of additional electrical power for a relatively low initial investment when compared to producing additional power by scaling the engine.
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