The CFM RISE Open Fan, the Unducted Rotor Architecture That Could Cut Narrowbody Fuel Burn by Twenty Percent, and the Certification Problem That Killed This Idea Before

CFM International’s RISE program is pursuing an engine architecture that removes the nacelle entirely, exposing the fan blades directly to the airstream. The goal is a fuel burn and CO2 reduction of more than 20% compared to today’s LEAP engines. Entry into service for a commercial aircraft powered by RISE is targeted at mid-2035.

What Is CFM International and Why Does This Program Matter?

CFM International is a 50/50 joint venture between GE Aerospace (United States) and Safran Aircraft Engines (France), formed in 1974. Their current LEAP engine family powers the Airbus A320neo (LEAP-1A), the Boeing 737 MAX (LEAP-1B), and the COMAC C919 (LEAP-1C). If you’ve flown commercially on a narrowbody in the past decade, there is a better than even chance CFM hardware was providing the thrust.

CFM announced the RISE program - Revolutionary Innovation for Sustainable Engines - in 2021. In an industry where a two or three percent efficiency gain on a single engine program is considered a meaningful result, a 20% improvement is not incremental. It is generational.

Why the Nacelle Is the Limiting Factor

The efficiency of a jet engine is closely tied to bypass ratio: the ratio of air flowing around the core to air flowing through it. On the LEAP, that ratio is approximately 9-to-11-to-1 depending on variant. Moving a large mass of air gently generates thrust more efficiently than blasting a smaller mass at high velocity through a hot core.

Higher bypass ratios are better - up to a point. The larger the fan, the larger the nacelle, and a larger nacelle adds aerodynamic drag and structural weight. Around 11-to-12-to-1, the nacelle penalty starts canceling out the gains. The efficiency curve flattens.

An open fan removes that constraint entirely. With no nacelle, bypass ratios of 35-to-1 or higher become achievable. The propulsive work is spread across a vastly larger mass of air. The thermodynamic gains are substantial.

The 1980s Propfan: Why This Failed Before

The 1973 oil crisis made open rotor technology - called the “propfan” at the time - a genuine engineering priority at both GE and Pratt & Whitney throughout the late 1970s and into the 1980s.

GE built the UDF (UnDucted Fan), also designated the GE36. By the mid-1980s it was mounted on a modified McDonnell Douglas MD-80 test aircraft and flying. The fuel burn numbers came in close to predictions. The aerodynamics worked. On paper and in the air, the propfan concept demonstrated exactly what the engineering said it should.

Two problems emerged that the tools of the era could not fully solve.

Noise was the first. Without a nacelle to attenuate sound, open rotors are inherently louder. The early propfan designs used counter-rotating blade rows that produced a distinctive tonal beat frequency - uncomfortable inside the cabin and analytically difficult to predict.

Economics killed it faster. Oil prices collapsed in the mid-to-late 1980s. The business case for a complex, expensive, unproven engine with a noise problem simply evaporated. The programs were shelved, and the industry spent the next three decades refining conventional high-bypass turbofans - which, to be fair, it got very good at.

What Has Genuinely Changed Since the GE36

RISE is CFM’s argument that the reasons the propfan failed then are not the reasons it would fail today. The argument has merit.

Computational aeroacoustics. In 1987, predicting the acoustic behavior of complex rotating blades was an analytical problem with no precise solution. Today, computational fluid dynamics combined with aeroacoustics software can model blade noise with accuracy that simply didn’t exist before. The RISE blades are swept, scimitar-shaped composite structures whose geometries were optimized by software evaluating millions of geometric variations against acoustic targets simultaneously. That capability did not exist when the GE36 was being designed.

Variable pitch. The RISE open fan incorporates continuous variable pitch - blade angle adjusts across all flight phases. On takeoff, where noise certification is most critical, pitch can be managed to reduce tip speed and alter acoustic output. In cruise, it optimizes for efficiency. This is central to the architecture, not an add-on.

Advanced composites. Carbon fiber reinforced composites allow blade shapes - thinner trailing edges, more complex camber profiles - that would be structurally impossible in metal. Both GE Aerospace and Safran have deep manufacturing experience with composite fan blades from the LEAP program. Safran has run its own open rotor demonstrator tests in France, feeding acoustic and aerodynamic data directly into RISE development.

Structural emissions pressure. The 1980s programs died when fuel got cheap. Today’s pressure on commercial aviation - the EU’s sustainable aviation fuel mandates, ICAO’s CORSIA program, and net-zero commitments from major carriers - is structural. It does not move with crude oil prices. The business case for 20% better fuel burn is more durable now than it was forty years ago.

A Note on the “Hybrid-Electric” Label

RISE program descriptions include the term hybrid-electric, which requires context. This is not a battery-plus-motor aircraft program. The RISE engine still burns jet fuel through a conventional gas turbine core. The hybrid-electric element refers to an integrated power extraction and generation architecture that optimizes the engine cycle across different flight phases. The 20% fuel burn improvement comes primarily from the open fan aerodynamics, not from stored electrical energy.

The Problems That Are Not Yet Solved

Blade containment is the hardest item on the list. FAA Federal Aviation Regulation Part 33 requires that if a fan blade separates from the disk, the engine structure must contain it. On a conventional turbofan, the nacelle acts as a structural containment shell. Without a nacelle, a separated open fan blade has nothing to stop it from becoming a ballistic projectile.

CFM, the FAA, and the European Union Aviation Safety Agency (EASA) are working through several approaches simultaneously: composite blade failure modes that fragment predictably, blade retention systems that constrain a separated blade, and potentially revised certification standards that account for how composite blades fail differently than titanium blades. This work is ongoing and the outcome is not predetermined.

Noise certification. ICAO Chapter 14 noise standards, updated in 2017, are significantly more stringent than what the GE36 had to meet. CFM has stated they are targeting compliance. Full-scale acoustic data at operating speeds and fan diameter has not yet come back from flight testing. Those results will determine whether the computational models match physical reality at the scale that matters.

Aircraft Integration: The Most Underestimated Challenge

A large-diameter open fan has serious ground clearance problems mounted under the wing on a conventional low-wing aircraft. Foreign object ingestion from runway debris and ramp equipment alone would be a certification and maintenance issue. Most credible RISE aircraft concepts show aft-fuselage mounted engines - similar to the MD-80 layout used for the GE36 flight tests.

That is not a cosmetic change. Aft-mounted engines change the center of gravity envelope, structural load paths through the aft fuselage, and handling characteristics. An asymmetric thrust event feels different on the rudder than on a conventional wing-mounted configuration. Rejected takeoff profiles change. Climb-out procedures change.

The aircraft that first enters service with RISE will be a clean-sheet design. For pilots currently flying glass cockpit narrowbodies, the operational procedures for RISE-powered aircraft will be written from scratch. Modern full authority digital engine control handles much of the complexity, but the muscle memory from the current tube-and-wing narrowbody fleet will not map cleanly onto whatever the next-generation aircraft looks like. Transition training programs will need to account for that.

The Scale Argument for Open Fan Over Electric

Narrowbody airliners - the 737 family, the A320 family, and their eventual clean-sheet successors - fly more routes, carry more passengers, and burn more aggregate fuel than any other category of aircraft. A 20% reduction in fuel burn applied across that fleet, at the scale of thousands of aircraft flying hundreds of thousands of revenue cycles annually, represents a reduction in carbon and fuel consumption that exceeds what any current battery-electric aviation program could achieve in the same timeframe by orders of magnitude.

Battery energy density is a physical constraint improving slowly. The gains from open fan thermodynamics are not fighting that same wall. The most consequential path to reducing commercial aviation’s environmental impact over the next twenty years very likely runs through propulsion efficiency at the narrowbody scale.

Where the Program Stands Now

CFM is currently running ground tests on technology demonstrator hardware. Flight testing on a demonstrator aircraft is planned for the mid-2020s. Entry into service for a commercial aircraft powered by RISE is targeted at mid-2035 - a decade away, with real test milestones between now and then and real uncertainty about what those tests will show.

The engineering problem that killed open fan in 1987 was economics. The problems remaining today are more purely technical, and in some ways harder. The tools available to solve them are incomparably better than what existed when the GE36 got shelved.

Key Takeaways

CFM RISE targets 20%+ fuel burn reduction versus the current LEAP by eliminating the nacelle and using an open fan with bypass ratios of 35-to-1 or higher
The concept was flight-tested as the GE36 UnDucted Fan in the mid-1980s but was abandoned when oil prices collapsed; today’s emissions pressure makes the business case more durable
Three key advances make the program viable now: computational aeroacoustics, variable pitch blades, and advanced composite materials
The hardest unsolved problem is blade containment - without a nacelle, a separated blade has no structural barrier; FAA and EASA certification pathways are still being worked through
Aircraft integration requires aft-fuselage engine mounting, making RISE aircraft clean-sheet designs with new operational procedures - not a drop-in upgrade to existing airframes
Commercial entry into service is targeted for mid-2035, with demonstrator flight tests planned for the mid-2020s