Europe-based global major aerospace group Airbus, and international aeroengine company CFM International (a 50:50 joint venture between GE of the US and Safran Aircraft Engines of France) announced, at the Farnborough International Airshow on Tuesday, that they were collaborating to test, in flight, the leading-edge open fan engine architecture being developed by the latter. This will see the engine, developed under CFM’s Revolutionary Innovation for Sustainable Engine (RISE) programme, flown on a modified Airbus A380 Superjumbo, during the second half of this decade.

An open fan, or open rotor, engine, previously also known as an unducted fan or as a propfan, is related to the turbofan engines that today power the vast majority of the world’s jet aircraft, civil and military. But while the fan blades of a turbofan are contained within a nacelle, the fan blades in an open rotor design are ahead and outside of the engine nacelle, effectively acting as advanced-design propellors. But the core of the engine is the same type of gas turbine found in a turbofan. Ideally, the open fan should provide the fuel economy of the propellor-using turboprop (which the open fan resembles in appearance but not in its engineering) and the speed of a turbofan.

Open fan engines have been under consideration by various manufacturers since the early 1980s, if not even before that. However, the big problem until now has been the noise generated by open fans: unducted fan blades are much noisier than ducted fan blades.

The CFM RISE engine, however, takes a totally new approach to open fan design. Instead of the established concept of each open fan having two sets of contrarotating fan blades, RISE has a single set of rotating blades, backed up with a second set of fixed blades (more accurately, stationary outlet guide vanes). These allow the airflow to be directed and allow the engine to achieve speeds compatible with those delivered by turbofan engines.

Open fan engines reduce weight in comparison to turbofan engines, because they do not need nacelles for their fans, only for their turbine cores. Further, the RISE engine reduces the complexity of open fan designs. And the designers have significantly reduced the noise emitted by the engine to a level that is now certifiable by aviation regulators. (GE engineers have been working on open fan concepts for about 40 years now.)  

Even using conventional Jet-A fuel, RISE promises to deliver a 20% decrease in carbon dioxide emissions compared with the most efficient engines in service today. Using sustainable aviation fuel (SAF), the reduction would be even greater. Confirming these benefits are among the objectives of the planned flight test campaign. Others include increasing understanding of the integration of the engine with the aircraft wing and the consequent aerodynamic performance; the engine’s compatibility with SAF; and establishing the noise levels actually emitted by the engine when it is operating on an aircraft. The flight test campaign will be conducted at Airbus’ Flight Test centre in Toulouse in France.

This is the second partnership between the two companies aimed at reducing aviation’s carbon emissions. In February, they announced a flight test project to validate the use of hydrogen as an aviation fuel.