EADS has set itself an ambitious target for its E-Thrust hybrid propulsion concept, a joint effort with British engine maker Rolls-Royce that went relatively unnoticed when it was revealed at the Paris Air Show in June. The E-Airbus, under its new moniker, is to enter into service in 2030 as a 100-seat regional aircraft. The announcement of the schedule and market segment came during a conference held in Paris in September, where the French government outlined its latest industrial policy.
Rolls-Royce (Stand 1845) expects distributed hybrid propulsion will dramatically cut noise and fuel burn. “Distributed” means a greater number of fans replace the usual two bigger, heavier turbofans. According to EADS chief technology officer Jean Botti, the E-Airbus will 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 in excess of 20.
A major benefit of the distributed propulsion system is that it can be integrated into the airframe’s structure allowing the airflow around it to be optimized, while also reducing the aircraft’s weight, drag and noise level, according to Rolls-Royce. Botti referred to Europe’s Flightpath 2050 goals, which call for cuts of 75 percent in CO2 emissions, 90 percent in NOx emissions and 65 percent in noise, compared to 2000 levels, although he didn’t clarify where the E-Airbus would position itself in 2030.
An additional efficiency gain appears possible if the boundary layer is “ingested” and accelerated by the fans. Such a re-energizing process reduces the aircraft’s wake and hence its drag. However, the airflow into the fans is not uniform, therefore, the fan blades must be able to withstand the distorted, unsteady intake. Rolls-Royce is currently developing this type of blade at its University Technology Centre in Cambridge.
One gas power unit (a turbofan connected to a generator) will provide the electric power for the six fans and for recharging the energy storage. Initial results from the Airbus study indicate that a single large gas power unit has advantages over two or more smaller gas power units, reducing noise and allowing the filtering of particles in the long exhaust duct.
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). It also makes it possible to downsize the gas power unit and to optimize it for cruise.
The additional power required for takeoff will be provided by the electric energy storage. 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, also producing electricity. For the landing phase, the gas power unit is restarted to provide redundancy (backup thrust in case the aircraft needs to go around).
The storage system’s energy density is expected on the order of 1,000 Wh/kg (Watt hours per kilogram). This will more than double today’s best performance. Lithium-air batteries are seen as the most promising solution as they are lighter, using oxygen from the air as the oxidizer (and thus air acts as the cathode). They are not yet commercially available, however.
For the megawatt range power levels required, EADS (Stand 410) and Rolls-Royce are counting on superconductivity. This phenomenon of zero electric resistance occurs in certain materials when they are cooled below a critical temperature, and allows electric system components–such as wiring–to be much smaller, lighter and more efficient. On the E-Airbus, cryogenic cooling may have to reach -252C (about 21 Kelvin, that is, above absolute zero, which is -273.15 C). One candidate technology is available today for space applications, said EADS.
In addition to the wires, the hub-mounted motor also will be superconducting. The operating principle starts with a superconducting stator, which generates a powerful electromagnetic field that rotates around the circumference. Electromagnetic torque is created by effectively aligning the rotor’s and the stator’s magnetic fields. The superconducting machine replaces the copper and iron stator structure of a conventional machine. It is a much more powerful, lighter and low-loss design.