European design house Bauhaus Luftfahrt, in which Airbus is a shareholder, has revealed new details of its Propulsive Fuselage hybrid-electric concept and is looking to potentially fly a scaled demonstrator as part of the pan-European Clean Sky 2 aerospace research program.
Developed under a European Union-funded Framework 7 research program called Dispursal (distributed propulsion and ultra-high bypass rotor study at aircraft level), the concept mounts a third gas turbine in the tail, where it serves the purpose of filling in the wake behind the fuselage, reducing drag. Conventionally installed turbofans under the wing provide the thrust required for flight and redundancy needed for safety.
The turbine engine in the tail drives a fuselage fan which ingests air through an annular inlet that runs around the circumference of the fuselage. This takes in the boundary-layer airflow, re-energizes the momentum deficit in the wake caused by skin friction and profile drag on the fuselage and reduces the wasted kinetic energy in the jet.
In the concept studied, the fuselage fan is driven by the low-pressure turbine via a reduction gearbox, but future work will include investigation of a turbo-electric powertrain in which generators on the underwing engines drive the fuselage fan electrically, says Julian Bijewitz, an advanced motive power researcher within Bauhaus’s visionary aircraft projects group.
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The Dispursal study focused on an Airbus A330-type twin-aisle aircraft for potential entry into service in 2035 or beyond, with 340 passengers and a 4,800-nm design range. A conventional-configuration year 2035 reference aircraft was developed with bleedless ultra-high-bypass-ratio (UHBR) engines, all-electric systems and a fuel-cell auxiliary power unit. This showed a 32% reduction in block fuel burned compared with an A330-300.
Bauhaus’s initial study indicated a further increase in efficiency of about 10% for the Propulsive Fuselage concept over the advanced reference aircraft. Further refinement of the design following completion of Dispursal showed a 9.2% reduction in block fuel relative to the reference design, or 38.3% less than the baseline A330, Bijewitz told the American Institute of Aeronautics and Astronautics SciTech conference in San Diego in January.
Resizing the Propulsive Fuselage aircraft for an optimum long-range cruise speed of Mach 0.78, rather than with the Mach 0.80 cruise used in the original study, increased the benefit over the advance reference aircraft, showing 11% lower block fuel when both were resized. The lower cruise speed would only add 16 min. to an 11-hr. flight at the 4,800-nm design range, Bijewitz says.
The aft propulsor ingests fuselage boundary layer and fills in wake, reducing drag. Credit: Bauhaus Luftfahrt

The biggest constraint on the performance of the Propulsive Fuselage concept is the weight of the third engine, which provides about 23% of total thrust in the “best and balanced” design. The underwing engines are smaller (9.1-ft. dia. versus 10.8 ft.) because of the reduced drag, saving weight; but the longer fuselage, T tail and aft propulsor all add weight.
The weight penalty is offset by the reduction in block fuel. Operational empty weight for the Propulsive Fuselage concept is almost 6% higher than for the advanced reference configuration, but maximum takeoff weight (MTOW) is just 1.3% higher. However, studies showed the structural weight penalty is sensitive to fuselage fan efficiency in distorted flow, which results in MTOW penalties up to 4.6%.
The next steps for the Propulsive Fuselage include more detailed conceptualization of the design, encompassing improved prediction of airframe-propulsion interaction effects and related losses. Fuselage fan performance, particularly surge margin and efficiency, at low speed on takeoff and in abnormal conditions to assess the impact of flow distortion effects will be vetted.
An opportunity for testing the concept could occur as part of plans being hatched under Clean Sky 2, which aims to mature technology for an advanced airliner to enter service after 2030. In its third call for core partners, the research program is seeking participants to perform “divergent aircraft” configuration studies focused on hybrid electric propulsion and to help validate flight testing of a dynamically scaled model as a lower-cost way of identifying the performance of the full-size aircraft.
In 2019, researchers will decide from among several options that range from the Bauhaus concept to aircraft with airframe-embedded electric-driven propulsors such as the E-Thrust concept studied by Airbus and Rolls-Royce. 
A €4 billion ($4.5 billion) government/industry research program that will run from 2014 to 2024, Clean Sky 2 has identified distributed propulsion using hybrid power transmission—gas turbines driving electrically powered fans tightly integrated with the airframe—as the likely optimum solution to meet Europe’s Flightpath 2050 environmental targets.
Demonstration of radical aircraft designs falls under the umbrella of Clean Sky 2’s Large Passenger Aircraft program and involves Airbus as well as Dassault, Saab and Snecma. A core partner is being sought to perform conceptual design of novel configurations with different aircraft and propulsion systems. In addition to more radical hybrid propulsion concepts, Clean Sky 2 is looking at the tighter integration of contrarotating open rotor (CROR) engines and UHBR turbofans. Plans call for a Snecma-developed CROR to be flown mounted on the aft fuselage of an Airbus A340-600, with the first engine test in flight scheduled for late 2020.
Airbus and its partners also plan to demonstrate a fully integrated UHBR engine, Rolls-Royce’s UltraFan large geared turbofan, on a flying testbed in 2018. But the more highly integrated hybrid propulsion configuration would be vetted using an unmanned subscale aircraft, to reduce cost, time and risk. The schedule calls for flight testing by late 2022.