The Farnborough air show highlighted a few new innovations this year. One of the exciting innovations that were announced comes straight from the IATA Aircraft Technology Roadmap to 2050 – which we have covered in part through this series of articles.
Airbus and CFM International announced at Farnborough that they are collaborating to flight test an open fan engine architecture on board an Airbus A380. This Flight Test Demonstrator is aimed to mature and accelerate the development of the open-rotor advanced propulsion technology.
The open rotor engine architecture is a hybrid between a propeller and a turbofan engine characterized by two counter-rotating, unshrouded fans. The advancement offered by this technology is a reduction of fuel burn and CO2 emissions in the order of typically 30%, compared to conventional turbofan engines.
Figure 1: The A380 Open Fan Demonstrator
To see a video of this demonstrator, click on the following link (Airbus Twitter narrative and video).
The IATA Technology Road map claims that the open-rotor concept is several decades old, but its development has slowed down due to noise levels. With the recent R&D focus on electric and hybrid aviation motive systems, manufacturers anticipate that the open-rotor engine will enter service before 2030.
But just what is the Open Rotor Engine?
An open rotor engine is essentially a turboprop with two rows of blades, or propellers, which can operate efficiently at higher speeds than a conventional turboprop. In an open rotor engine, the forward propeller pushes the air backwards, while the rear one sucks it. The net effect is that the blades of a turboprop tend to spin air out rather than push it back.
Figure 2: NASA Open Rotor Green Engine
The following is a simulation of the open rotor engine.
For comparison’s sake, a turboprop engine would also need blades twice the diameter of its open rotor equivalent to produce the same power.
In terms of lifecycle costs and other comparisons, the open rotor has no nacelle (engine covering), thrust reverser, or fan case. These design changes reduce production and maintenance costs.
An open rotor is intended to operate at constant torque, with the blades’ pitch and rotors’ speed controlled by a full-authority digital engine control which provides the thrust demanded by the pilot.
This configuration raises the interesting requirement for a gearbox. An industry specialist comments, “It’s an interesting debate at the moment, the open rotor architecture has deleted a lot, but you have added complexity at the back. And the main complexity that you’ve added is the gearbox. This gearbox is half the power of the [P&W] geared turbofan.” Currently, the gearbox is yet to be developed, but plans are underway.