About seventeen years the airline industry is expected to produce a 17% saving in fuel burn from step-change improvements in engine and, to a lesser extent, airframe technology.
Our current crop of super-efficient engines is the product of steady evolutionary development. This happened around 2010 with the advent of the extra-large diameter high-bypass Pratt & Whitney geared turbo fan (GTF).
Five years later the GTF’s current competitor, the CFM LEAP engine first flew. It uses both hotter hot sections and bigger fans to achieve a 15 – 17% fuel burn improvement. These ‘next generation engines’ gave rise to the Airbus A320 NEO (for new engine option) and the Boeing Max.
With these ultra-high bypass jet engines there’s little room left for further development. Thus there is a widespread belief that any step changes will struggle to produce even a 10% fuel efficiency increase. This is due not only to the physical restrictions on the size of the turbofan but also to the cost and unavailability of the complex compound ceramic heat resistant materials (‘unobtanium’) required to achieve further efficiencies in the core of the engine. And the fans have got as big as they can get for underwing installations on the Boeing 737.
Given the enormous cost, and the 10 – 20 years of development required to certify and deliver a step-change in engine technology, the next generation engine is already overdue. What makes the need for a revolutionary engine all the more compelling is environmental demands such as the CORSIA Programme and IATA’s net zero emissions by 2050 promise.
The delay is because the next step change is going to be, not just a big step, but in many ways a step backwards. The front runner for the new technology is the CFM open fan RISE engine. The CFM RISE (derived from the clunky acronym; Revolutionary Innovation for Sustainable Engines) goals include reducing fuel consumption and CO2 emissions by more than 20% compared to current generation engines and ensuring compatibility with sustainable aviation fuels (SAF) and hydrogen.
CFM has signed a deal with Airbus, which will lead to a demonstrator that’s expected to fly on an Airbus A380 ‘in the mid-2020s.’
The RISE engines will look completely different. They feature huge unducted fan blades that look more like a ship’s propellor than a jet engine. Having unguarded blades slashing about like a kung-fu fighting movie means that the engines must be far from the vulnerable parts of the aircraft like wing fuel tanks or the passenger cabin. And they have to be mounted high enough for people not to walk into. This will require all-new airframes to accommodate these engines. The most likely solution is to mount the RISE high up on the tails, like the old DC-9, or bizjets.
The surprising thing is that the RISE is an old idea reheated. In the quest for efficiency there is little that has not already been tried. And so it is with the RISE engine.
GE Aviation began working with NASA on the unducted fan in the early 1970s. They called it the not very catchy name of ‘Quiet Clean Short-Haul Experimental Engine (QCSEE)’ demonstrator, and it was the first high-bypass geared turbofan engine. In the mid-1980s, it had evolved into the GE36 or Unducted Fan (UDF). French engine builder Safran, (then Snecma), was part of the development and testing.
The GE36 first flew on a Boeing 727 and on both a DC-9 and an MD-80, with the latter aircraft flying at the 1988 Farnborough Airshow.
Around the same time Pratt & Whitney-Allison also tested their open rotor propfan concept. Initial results were not promising. It was considered too large, too heavy and too expensive. And, thanks to supersonic fan blades, it howled like a Stuka dive bomber. The big challenge was to keep the fan blade tips below supersonic speeds. This was largely achieved in 2007, when the Progress D-27 propfan engine developed by Ivchenko in Ukraine met FAA / ICAO Chapter 4 noise standards.
This achievement re-invigorated the unducted fan and in 2012 a study projected that noise would be 10–13 decibels quieter than allowed by the Stage 4 regulations. The study also projected that open rotors would be nine percent more fuel-efficient but remain 10–12 decibels louder than turbofans. Undaunted, Snecma claimed that its propfan engines would have about the same noise levels as its CFM LEAP turbofan engine.
It was not just noise levels, but blade failures which were another huge concern, especially due to that old bugbear flutter, induced by the complex airflow patterns to and through the fans and the broad airspeed and tip speed range the fans would have to cope with.
But still the big issue was the bottom line. After the oil crisis of the seventies, fuel prices dropped dramatically in the eighties. What with the huge engineering challenges, an unholy howl, and low fuel prices, the unducted fan was quietly shelved. But now – under sustained pressure from climate-change activists, the need for reduced carbon emissions is just as compelling, if not more so, than just plain old fuel efficiencies.
So why now, 40 years later, is the unducted fan making a comeback? The big idea is that the unducted fan combines the fuel efficiency of a turboprop with the speed and performance of a turbofan. CFM says; “the RISE is nothing like a turboprop engine. It will be able to fly at up to Mach 0.8, with a noise signature that will meet anticipated future regulations. An open fan configuration can achieve improved noise levels even relative to today’s state-of-the-art engines. We have also been able to reduce both the complexity and weight of the engine while maintaining significant performance benefits.”
The current RISE design is for 30,000 lb thrust which aims it squarely at the competitive single-aisle market dominated by the A320 and Boeing 737 families. However, CFM points out that there is plenty of scope for up-scaling both the fan blades and power core.
The new engine will use a compact high-pressure core and a recuperating system to preheat combustion air with exhaust heat along with ceramic matrix composites in the hot section. Lightweight resin-transfer-moulded composite fan blades are the big innovation. The design includes a nonrotating set of variable-pitch stator blades that act as flow recovery vanes. The design increases the fan-pressure ratio and reduces rotor loading, increasing airspeed.
To keep tip speeds down, the fan is driven by a high-speed compressor mated to a low-pressure-shaft-driven front gearbox. The whole power unit will be certified as an “integrated engine” instead of the traditional propeller and engine because of its airframe integration complexity.
Thanks to the smaller engine core, the CFM RISE should have a bypass ratio of a hitherto unheard of 75. CFM claims that open fan architecture enables a much higher bypass ratio and, as a result, delivers much higher propulsive efficiency. The engine maker hopes that “achieving 20% better fuel efficiency has the potential to be the single greatest generational improvement in fuel efficiency that we’ve ever achieved.”
Unlike the complex contra-rotating fans of the earlier GE36 UDF, the RISE features a single rotating fan, with variable pitch carbon fibre blades. Ahead of the fan (in a pusher configuration) is a row of static guide vanes.
Since the 1980’s the development in engine technologies, particularly in the power core, has carried through to the unducted fans. CFM says this enables open fans, “but at very low noise levels and smaller dimensions to make it easier to integrate with the airframe.”
The investment in the RISE engine is big, even by aerospace standards. CFM’s parent companies, GE Aerospace and Safran Aircraft Engines, have more than 1,000 engineers globally supporting the development of RISE technologies.
“Since we launched the RISE programme, we made significant progress to validate the conceptual design review and launch the industrialisation of the first demonstrator parts. We are on track to do the ground and flight tests around the middle of the decade with a thorough test plan that includes open fan aeroacoustics modelling this year,” said Michel Brioude, Vice President Engineering and R&T at Safran Aircraft Engines.
“The industry cannot reach its net zero ambition by 2050 with status quo incremental improvements in fuel efficiency. Revolutionary technologies are needed. That is why we believe the time for open fan is now, an advanced engine architecture that could unlock the single greatest jump in generational engine efficiency that CFM has ever achieved. This is supported by our most comprehensive testing roadmap yet to prove out and mature these technologies for the future of flight,” said Mohamed Ali, vice president of engineering for GE Aerospace.