Airbus applied for an intriguing engine patent (hat tip AeroPatent.com). They describe it like this: A propulsion engine comprising at least a first propulsion unit including a first fan encased by a geared ring and a gas turbine engine driving a first shaft connected to the first fan, at least a second propulsion unit including a second fan encased by a geared ring connected to a second shaft operatively coupled to an electric machine and at least an electric storage device connected to the electric machine. The geared rings are con figured to transmit torque between the fans so that they can rotate in conjunction (directly or through an intermediate gear) when they are driven by at least one of said first and second shafts. The propulsion engine is arranged for control ling the torque to be supplied to the assembly of the first and second fans by the gas turbine engine and/or by the electric machine acting as a motor or as a generator.
The application comes from the Getafe facility in Spain. The application provides insight not commonly heard about. The maximum power of conventional turbofan engines is generally determined by take-off or end of climb requirements and therefore the engine core is oversized for cruise. The idea seems to be leading us to conceive of engines that might be hybrids. This is an area the US Patent office has to deal with before (U.S. Pat. No. 8,324,746, U.S. Pat. No. 8,596,036).
Airbus notes that engines are now coming with massive bypass ratios making them large and this impacts airframe design. Hybrid engines are also likely to be large as described in the aforementioned patents. Airbus notes that : A disadvantage of these hybrid engines is that they require a specific and powerful generator on the jet engine to drive the electric motor. Another undesirable characteristic is that the electric motors must be able to produce all the thrust even for the flight regimes requiring the maximum power and are, therefore, oversized for the cruise condition.
The image below describes how Airbus envisions the engine layout. They describe the invention providing a propulsion engine compromising at least a first propulsion unit including a first fan encased by a geared ring and a gas turbine engine driving a first shaft connected to the first fan, at least a second propulsion unit including a second fan encased by a geared ring connected to a second shaft operatively coupled to an electric machine and at least an electric storage device connected to the electric machine The geared rings are configured to transmit torque between the fans so that they can rotate in con junction when they are driven by at least one of said first and second shafts. The electric machine of the second propulsion unit is arranged for actuating, in combination with the electric storage device, whether as an electric motor extracting energy from the electric storage device to drive the second shaft or as an electric generator extracting the mechanical energy of the second shaft when it is driven by the second fan to charge the electric storage device. The propulsion engine also comprises a controller connected to the gas turbine engine, to the electric machine and to the electric storage device for controlling the torque to be supplied to the assembly of the first and second fans by the gas turbine engine and/or by the electric machine acting as a motor and for controlling the charge of the electric storage device by the electric machine acting as an electric generator.
In the embodiment shown, the propulsion engine (10) is a multi-fan engine housed in a nacelle (20) comprising: A propulsion unit 11 in the right side having a first fan 13 encased by a geared ring 19 and a combustion engine 15 housed in a fairing 12 including at least a compressor, a combustion chamber and a turbine driving a first shaft 17 connected to the first fan 13. A second propulsion unit 21 in the left side having a second fan 23 encased by a geared ring 29 connected to a second shaft 27. The second shaft 27 is also coupled to an electric machine 25 housed in a fairing 22 and connected to an electric storage device 41, such as a battery, located externally to the fairing 22.
The solution proposed seems to us will also be large since it is in essence a double engine. Even with smaller fans, the combination has to be very wide. The location of such engines on an airframe will be a design challenge – probably they will be somewhat embedded at the rear between a split tail as seen in this image from Airbus.
With engine intakes below the aircraft, water ingestion on the ground is an issue that would be interesting to overcome. Airbus has been releasing intriguing layouts and ideas for some time. The thinking is novel.