All-electric airplanes are already in service in limited applications like trainers and for towing gliders. These small planes boast low energy use and low maintenance, but battery capacity and weight remain limiting factors for longer range aircraft.
Airbus, Rolls Royce and Siemens are thus employing the same strategy used by automobile manufacturers before them – hybridization – as an intermediate step while battery technology catches up.
Their E-Fan X project will swap one of four turbofans on a testbed British Aerospace 146 with an electric fan with similar thrust to the engine it replaces. Electric power will be supplied by a turbogenerator in the plane’s fuselage.
Turbojets run air through a compressor section, add fuel and burn it, then run that hot air first through turbine section and then out the nozzle to produce thrust. The turbine powers the compressor on the same shaft spinning in the centre of the engine.
Turbofans draw more power from the shaft to spin an additional fan which also provides thrust. This is more efficient at airliner cruising speeds because the power recovered from the exhaust is used to create extra thrust.
The challenge with turbofans is that the turbojet core runs efficiently at high rotational speed while the fan operates better at lower RPMs.
Pratt and Whitney produces a turbofan with a gearbox between the turbojet core and the fan for this very reason, promising reduced noise and fuel consumption. Principle 35 – Parameter Change
Using a serial hybrid arrangement like in the E-Fan X permits the fan to run at entirely different speeds from the turbojet attached to the generator, promising greater efficiency and flexibility. It also eliminates the noise caused when turbofan fan exhaust meets the turbojet core exhaust (as discussed in a previous post).
Segmenting the turbojet from the fan this way, using an electric generator on the the turbojet and an electric motor on the fan, offers some exciting possibilities.
Take a four-engine jet like the British Aerospace 146 testbed used by the E-Fan X project. Replace the outer turbofan engines with turbojets with generators attached (thus merging the roles of turbogenerator and thrust turbojet – Principle 5 – Merging). Replace the inboard turbofans with electric fans.
Depending on flight mode (Principle 15 – Dynamics) the turbojets could be unburdened by the generator and operate purely as thrusters, or they could be used to produce electricity for the fans. This would also provide a high degree of redundancy in the event of failure. One turbojet could power two fans.
The electric fan could even be run backwards for reverse thrust on landings. (Principle 13 – The Other Way Round)