Aircraft

GE resurrects the propfan aircraft engine, cutting fuel burn by 20%

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CFM's ultra-efficient open-rotor Rise engine will fly aboard an Airbus technology demonstrator within 5 years or so
CFM
CFM's ultra-efficient open-rotor Rise engine will fly aboard an Airbus technology demonstrator within 5 years or so
CFM
Open rotor designs place a large number of long, pitch-controllable blades in the air, combining the speed and performance of today's turbofans with the efficiency of a turboprop
CFM

GE once thought super-efficient propfans were the future of air travel, until low fuel prices in the late 1980s moved fuel consumption down the list of priorities. Now, it's bringing them back with the CFM Rise, promising 20% fuel savings.

Unducted fans (UDFs), also known as propfans or open rotor engines, first popped up in the 1970s. They look a little odd to the modern eye, with smooth nacelles and air intakes at the front, and two sets of fan blades poking out into open air at the back.

Typically, there's a lot of blades, and they're swept back, and heavily twisted. The rear blades are twisted in the opposite direction to the front ones; sometimes they counter-rotate, others don't rotate at all, acting as variable-pitch stator blades to help with flow recovery.

"The history of aviation propulsion," reads a GE whitepaper, "tells us that all previous breakthroughs in efficiency were achieved by new technologies that allowed for a larger fan size and a higher bypass ratio." Propfans do both, in a way that combines the fuel efficiency of a turboprop with the speed and performance of a turbofan.

The history of General Electric, meanwhile, tells us that this company had a propfan engine just about ready to rock back in 1989, capable of cruising at Mach 0.84 and promising a remarkable 30% fuel efficiency advantage over the popular turbofan engines of the day. The GE36 UDF, built in partnership with Snecma, made it to the prototype and flight testing phases, but once OPEC dropped the oil embargo, fuel prices crashed, and the design was shelved.

Fuel prices have very much uncrashed in recent times, and as aviation faces the challenge of decarbonizing a highly energy-intensive business, propulsive efficiency is very much back in vogue. Batteries and hydrogen can't fully replace kerosene as an energy carrier, so any extra mileage you can squeeze out of your electrons has become critical.

So GE is back on the propfan train again, and through CFM International, a 50/50 joint venture with Safran Aircraft Engines (formerly Snecma) that sadly doesn't produce boots, it announced a new UDF airliner engine called the Rise back in 2021.

Open rotor designs place a large number of long, pitch-controllable blades in the air, combining the speed and performance of today's turbofans with the efficiency of a turboprop
CFM

Efficiency may have improved across the industry in the last 30-something years, but GE says the Rise engine should still give you 20% more miles than anything else currently on the market for a given amount of energy.

While noise – both in the cabin and on the ground – has been a problem with some previous propfan designs, the CFM team says the Rise engine is being "validated to meet the most stringent ... noise emission requirements."

“The industry can’t reach its net zero ambition by 2050 with status quo incremental improvements in fuel efficiency," says Mohamed Ali, vice president of engineering for GE Aerospace. "Revolutionary technologies are needed. That’s 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.”

The Rise engine can be mounted over the tail wings of an airliner, and it's just as happy under a top-mounted wing as it is on top of a low wing. The team has signed a deal with Airbus, which will lead to an open fan demonstrator of some kind that's expected to fly "in the mid-2020s."

For now, there are prototypes built, and CFM has already performed some 400 tests on the ground. Meanwhile, GE Aerospace has been doing some serious simulation work, announcing in a press release on Friday that it's engaged the world's fastest supercomputer.

The Frontier supercomputer, at the US DoE's Oak Ridge National Laboratory, can perform more than one quintillion calculations per second, and as such, it's providing exceptionally detailed computational fluid dynamics (CFD) modeling on how ambient air will interact with these big propfans at speed and altitude.

Innovation in the airliner business tends to happen at a snail's pace; we wouldn't expect to see anything like this on a commercial airliner until well into the 2030s, if it indeed happens at all.

But between this and other efforts like the Rolls-Royce Ultrafan, there could well be a highly efficient propulsion technology ready by the time companies like ZeroAvia and Universal Hydrogen get their clean aviation powertrains built, tested and certified up to full-scale international airliner size.

Check out a short video below, showing a mockup Rise engine being built, complete with variable-pitch rotor blades.

Source: CFM International

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13 comments
Nobody
Larger blades sweep a larger area and don't need to spin as fast as smaller blades for the same amount of thrust so they are more efficient. Also slower planes don't need the higher thrust of faster planes which burn more fuel.
CraigAllenCorson
I thought these new Sharrow™ toroidal props were going to be the new big thing? Quieter, more thrust, more efficiency, remember? https://newatlas.com/aircraft/toroidal-quiet-propellers/
paul314
One of the reasons for development of unducted fans has been about avoiding the mass and drag of those huge engine nacelles while still passing the right amount of air through the turbine core to combine with fuel for burning. Electric engines don't have the same requirement for air to burn (some are even liquid cooled), so the shape and size of the fans they drive may end up being quite different. But the airflow simulation models being developed now will probably be crucial for electric engines as well.
vince
Mach .84 is 767.27 x .84 = 644 mph. Thats nearly 100 mph faster than a jet plane cruising speed !!
Nelson Hyde Chick
The average commercial airliner has a twenty year life, so in twenty years each plane will be spewing 20% less, but thanks to bringing people out of poverty, population gain air traffic is suppoosed to double by midcentury, so it will still be a loss for the environment.
WB1200
@vince
Mach .84 at 30,000ft is 569mph
MarylandUSA
This article doesn't mention what was, for me, the most intriguing aspect of the 1989 design: The propellers would rotate at the same speed as the turbine, eliminating the need for a gearbox. I wonder whether the new design is gearbox-free, too.
Laszlo KRUPPA
Stator blades make it look like a pair of contra-rotating propellers (CRPs). It is possible even some aerodynamic features of them too will show in the characteristics of the new concept. Which is not bad as Wikipedia says, generally, they (CRPs) are 6-16% more efficient than normal propellers.

But CRPs are famous also about their high noise. Which could be a problem.
Deres
The nacelle is also used to contain blades in case of incident, especially when encountering birds. Those motors are different from turbomotors in that they do not use a gear box to reduce the speed of the blades. Thus the ejection of the blade can happen at very high speed in case of incident leading to major damages to the wings or fuselage.
joe46
@Nobody " Larger blades sweep a larger area and don't need to spin as fast as smaller blades for the same amount of thrust so they are more efficient " that's exactly why those electric VTOL's are a bad idea .