Interessant konsept under utredning - AW&ST

Europe Eyes Fuel-Saving Boundary-Layer Propulsion

Graham Warwick

Airliners with fans embedded in the tail to reenergize the fuselage wake and reduce drag are attracting interest as a potential next step in cutting the fuel burn of commercial aircraft. But there is debate over whether such designs are also a promising initial application of electric propulsion to large aircraft.

NASA’s single-aisle turboelectric aircraft with aft boundary-layer propulsor concept, or STARC-ABL, is drawing attention thanks to studies indicating a 7-12% reduction in fuel burn in a configuration that is otherwise similar to conventional airliners flying today.

The improvement is achieved by ingesting and accelerating the slow-moving fuselage boundary layer using an electrically driven fan mounted on the tail. This fills in the wake and reduces drag. In STARC-ABL, the tail fan is powered electrically by generators on the underwing engines, which can be smaller than in a conventional airliner because of the reduction in drag.

But there are other ways of achieving boundary-layer ingestion (BLI). Aurora Flight Sciences’ D8 design has its turbofans embedded between the twin tails, where they ingest the flow over the upper fuselage. French research agency ONERA’s Nova concept is essentially similar, but with the engines embedded in the aft fuselage either side of the single tail.

The Propulsive Fuselage Concept developed under the Dispursal project uses a turbine-powered, mechanically driven fuselage fan embedded in the tail. Credit: Bauhaus Luftfahrt

 

Bauhaus Luftfahrt’s Propulsive Fuselage Concept (PFC) is similar to STARC-ABL in that the propulsor is mounted on the tail, breathing through an annular inlet that runs round the aft fuselage. But where the NASA concept is turbo-electric, the German design house uses another gas turbine in the aft fuselage to mechanically drive the BLI fan. Bauhaus, a research institute in which Airbus is a shareholder, developed the PFC concept under an EU-funded project called Dispursal, completed in January 2015. This study indicated a fuel-burn reduction of 9-14% compared with an equivalent 2035 entry-into-service conventional single-aisle airliner. Since completing Dispursal, Bauhaus has continued to refine the concept, conducting a more detailed analysis comparing a conventional 340-seat, 4,800-nm-range widebody twin to a PFC aircraft with a tail-mounted, turbine-driven fuselage fan for wake-filling and underwing geared turbofans for residual propulsion. A two-shaft turbine engine is installed in the aft fuselage, the low-pressure spool powering the fuselage fan via a planetary reduction gear system. Air enters the fan via a 0.54-m-high (1.8-ft.) annular intake duct. The fuselage boundary layer is about 1 m deep at this point, says Bauhaus. Inside the nacelle, aft of the fan, an S-duct supplies air to the core engine. Results of the study were presented at the American Institute of Aeronautics and Astronautics (AIAA) SciTech conference in January, Bauhaus reporting a 12.1% reduction in fuel burn over the advanced, 2035-time-frame conventional design. Aircraft empty weight increases by 4.6% because of a 16% higher propulsion system weight due to the fuselage fan, but maximum takeoff weight is essentially unchanged. For the baseline study, Bauhaus assumed a common core is used in the fuselage fan and underwing engines to minimize cost. Removing this constraint and individually optimizing the size and thrust split between the fuselage and underwing propulsion systems resulted in a further 1.3% reduction in block fuel, to be traded against higher operating cost. Where the conventional reference aircraft requires a maximum climb thrust of 12,880 lb. per engine, this was reduced to 8,450 lb. in the PFC design, the fuselage fan producing a net thrust of 3,690 lb. When the common-core requirement is removed, the fuselage fan shrinks to 2,610 lb. thrust while the underwing engines grow to 8,880 lb., but block fuel burn reduces by 13.1% compared with the reference aircraft.

 

The PFC configuration that will be wind-tunnel-tested under the Centreline project uses a  turboelectric tail fan, avoiding the S-duct losses of a buried gas turbine. Credit: Bauhaus Luftfahrt

 

Unlike Dispursal, the aft-fuselage propulsor in the PFC concept to be evaluated under Centreline will be driven electrically from generators on the underwing engines, resulting in a turbo-electric configuration similar to NASA’s STARC-ABL. Centreline will perform proof-of-concept and initial experimental validation of the PFC, raising its maturity to a technology readiness level (TRL) of 3-4 from its current TRL of 1-2. The aim is to achieve TRL 6, ready to enter product development, by 2030. Bauhaus’ partners in the Centreline project include Airbus Defense and Space, MTU Aero Engines, electric drive specialist Siemens and French R&D consultancy Arttic, as well as universities in the Netherlands, Poland, Sweden and the UK.