Airbus Bets on New Propulsion Concepts

Farnborough Air Show » 2014
The Quadcruiser features four rotors for lift, and a pusher propeller.
July 16, 2014, 1:00 AM

The Bauhaus Luftfahrt aerospace think-tank in May unveiled a concept for a “propulsive fuselage” aircraft, opening a new possibility for fuel burn reduction. It is part of a European Union-funded project in cooperation with a number of research centers, as well as MTU Aero Engines and Airbus Group Innovations (OE13). The latter company is also studying a hybrid-power regional airliner with Rolls-Royce (Hall 4 Stand H3). Meanwhile, it is flying a hybrid-lift quadcopter demonstrator for unmanned military and civil missions, the Quadcruiser.

The Bauhaus Luftfahrt idea is to fully integrate a special engine design into the aircraft’s tapered aft fuselage. The latter is encircled by a so-called “fuselage fan” (see image). The accompanying gas turbine is located in the tail cone.

The main advantage of this “distributed” propulsion architecture (distributed meaning, in this case, that the thrust is spread around the fuselage) is the effective ingestion of the boundary layer. It is thus “re-energized,” as its decelerated airflow in close proximity to the fuselage is re-accelerated. In doing so, the “fuselage fan” compensates for a significant percentage of the fuselage’s viscous drag, the Bauhaus said.

Therefore, the two conventional engines producing the largest part of the overall thrust could be scaled down. It would enable fuel savings of up to 10 percent over projected technology improvements in 2035. “Dispensing with the classical separation of airframe and propulsion systems could open up new synergies and facilitate significant efficiency gains,” the organization said.

In another effort to distribute thrust, Airbus Group Innovations and Rolls-Royce, with Cranfield university as a partner, are jointly engaged in the Distributed Electrical Aerospace Propulsion (DEAP) project, which is co-funded by the UK’s Technology strategy Board. The first iteration of what the outcome could be is the E-Thrust or E-Airbus. It could enter into service in the 2030-2050 timeframe, as a 100-seat regional aircraft.

Rolls-Royce expects distributed hybrid propulsion will dramatically cut noise and fuel burn. “Distributed” here means a greater number of fans replace the usual two bigger, heavier turbofans. For example, the E-Airbus may have six electric fans. They will be distributed along the wingspan in clusters of three. The bypass ratio (or its equivalent number for such a system) is expected to be more than 20.

An additional efficiency gain appears possible if the boundary layer is ingested and accelerated by the fans, under the aforementioned “re-energizing” scheme.

One gas power unit (in short, a turbofan connected to a generator) will provide the electric power for the six fans and to re-charge the energy storage. The serial hybrid architecture offers the possibility to improve overall efficiency by allowing the separate optimization of the thermal efficiency of the gas power unit (producing electrical power) and the propulsive efficiency of the fans (producing thrust).

As the gas power unit is optimized for cruise, additional power for take-off will be provided electrically. In the cruise phase, the gas power unit will provide the cruise power and the power to recharge the batteries. During the first part of the descent, the E-Airbus will be a glider and the gas power unit will be switched off. Then, the fans will start windmilling, producing electricity, too. For the landing phase, the gas power unit is re-started to provide for redundancy.

The storage system’s energy density is expected in the order of 1,000 Wh/kg (Watt hours per kilogram). This will more than double today’s best performance. For the megaWatt range power levels required, Airbus and Rolls-Royce are counting on superconductivity.

In unmanned aircraft, too, Airbus is endeavoring to organize power differently. The Quadcruiser uses the popular four-rotor architecture and adds wings and a pusher propeller. The four-rotor architecture, commonly used in small unmanned rotorcraft, is paradoxically simpler than a design with a main rotor and a tailrotor. First, the aircraft gets rids of complex transmission gear. Second, the rotors have a fixed pitch, as control can be performed through differential speed variations. Third, the architecture is well suited to electric power, as each motor can be co-located with its rotor.

The Quadcruiser has two flight modes–rotary-wing for maneuverability and fixed-wing for speed and efficiency. During transition to the fixed-wing aircraft mode, the pusher propeller accelerates the aircraft until its wings provide sufficient lift. Subsequently, the lift motors are stopped and their propellers adjusted to a low-drag position. Before landing, the aircraft transitions back to the quadcopter mode using its four lift motors, enabling a vertical landing.

A proof-of-concept Quadcruiser made its fist flight at the Grabenstetten special airfield near Stuttgart, Germany, in December 2013. Further testing took place at the Airbus Defence and Space facility in Manching. The Quadcruiser aircraft now in flight test is capable of up to 50 minutes of horizontal flight in the fixed-wing aircraft mode.

According to Airbus, the 20-pound demonstrator vehicle represents a baseline for incremental developments. First flight of a 20-55-pound class Quadcruiser is envisioned for 2015, followed by a 330-880-pound class version in 2018-2019. The Quadcruiser project team includes light aircraft and UAV specialist Steinbeis Flugzeug und Leichtbau GmbH, which built the first demonstrator aircraft. It features five 1.1 kw motors and a total 185 Wh stored in the batteries.

Airbus sees potential applications in long-range missions as well as in urban environments. These could be surveillance and reconnaissance flights for the military and police, boarder patrols and fire brigades on the civil side.

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