The CFM RISE Open Fan Engine, the Unducted Rotor GE Abandoned in the Nineteen Eighties, and the Engineering Bet That Could Rewrite Commercial Aviation Efficiency in the Twenty Thirties

CFM International’s RISE program - a joint initiative between GE Aerospace and Safran Aircraft Engines - is developing an open fan engine architecture that targets 20% better fuel burn than the current LEAP engine family. If it reaches service, it would represent the most significant shift in commercial engine design since the ducted turbofan became the industry standard in the 1960s. The technology has real historical precedent, was abandoned for real reasons in the 1980s, and now has serious technical arguments for why those reasons no longer apply.

What Is the CFM RISE Program?

RISE stands for Revolutionary Innovation for Sustainable Engines. CFM International, the joint venture behind the engines powering the Boeing 737 MAX and Airbus A320neo, is the organization behind it. The LEAP engine family - RISE’s predecessor - commands roughly half of all commercial narrowbody engine deliveries worldwide.

When that organization publicly commits to a fundamentally different engine architecture, it is not a research curiosity. It is the commercial aviation industry adjusting its trajectory.

How an Open Fan Engine Works

A conventional turbofan encloses its fan blades inside a nacelle. An open fan - also called an open rotor or unducted fan - does not. The blades are exposed, large, and variable-pitch. They look closer to a highly advanced counter-rotating propeller than a conventional jet engine fan.

Engine efficiency in turbofans is measured largely by bypass ratio: the ratio of air flowing around the combustion core to air flowing through it. Higher bypass ratios mean more air is accelerated to lower velocity, which wastes less energy as heat and turbulent exhaust. The CFM LEAP engine runs a bypass ratio of approximately 11:1. Pushing that higher with a conventional ducted design eventually collides with ground clearance limits, structural weight, and nacelle drag.

An open fan removes that ceiling. Without a nacelle constraining blade diameter, an open rotor can achieve bypass ratios in the range of 50:1 or higher, moving an enormous volume of air at very low jet velocity. That is the physical basis for the claimed 20% fuel burn improvement over LEAP - not an incremental gain, but a step change.

The GE36 Precedent: Why the Open Rotor Was Abandoned in the 1980s

The open rotor concept is not new. Following the oil shocks of the 1970s, NASA and GE developed the GE36 Unducted Fan (UDF), a counter-rotating open rotor engine with two rows of large variable-pitch blades spinning in opposite directions. The second row recovered swirl energy shed by the first.

Flight testing on a Boeing 747 testbed and a modified MD-80 confirmed extraordinary efficiency numbers. Engineers could see the potential clearly.

Then oil prices fell. The competitive pressure to improve fuel burn disappeared, and airlines had no appetite for the cost of certifying an entirely new engine architecture. The program also faced a serious noise problem: blade-vortex interaction, where the rotor wake from the front blade row gets ingested by the rear, produces a tonally distinct sound that communities around airports tend to remember. The GE36 program went into storage in the mid-1980s and stayed there for nearly three decades.

What Has Actually Changed Since 1985

Several things are genuinely different now, and they matter.

Composite blade manufacturing. In 1985, open rotor blades were metallic, heavy, and limited in their aerodynamic geometry by what machining could produce. The RISE program uses advanced composite structures with swept and scimitar blade shapes that were essentially impossible to manufacture forty years ago. Computational fluid dynamics tools that did not exist in the 1980s now allow precise optimization of three-dimensional blade geometry - including geometry specifically targeting acoustic performance.

CFM has stated a target of Stage 5 noise certification. That certification basis for a commercial open fan does not yet exist, but the theoretical basis for achieving low-noise performance with modern blade designs is real in a way it was not in 1985.

Control system capability. Managing the pitch angles of two counter-rotating blade rows in real time - across the full flight envelope, during transients, restarts, and failure modes - is a significant control problem. Full-authority digital engine control (FADEC) was barely viable for production engines during the GE36 era. The computational power required for real-time open rotor control optimization is not a constraint today.

The regulatory and commercial environment. ICAO member-state emissions commitments carry compliance timelines. Airlines are factoring carbon costs into aircraft acquisition decisions now. Sustainable aviation fuel is real but expensive and supply-limited, and it does not improve the underlying fuel burn of the engine consuming it. An architecture delivering 20% better fuel burn with no change to the fuel supply chain addresses exactly what the industry needs. The business case for RISE at current energy prices and current regulatory trajectories is structurally different from the business case facing the GE36 program in 1986.

The Hybrid Electric Component

The open fan gets most of the attention, but RISE also incorporates a hybrid electric architecture. This is not battery-powered flight. It is a mild hybrid approach: motor generators coupled to the engine shafts recover energy during certain flight phases and contribute electrical power during others.

The efficiency contribution from the hybrid component is estimated at a few percentage points beyond the open fan gains. More significantly, it enables a flexible onboard power architecture - electrified accessories, electrified environmental control systems, reduced bleed air extraction from the core. RISE is designed from the beginning with electrical generation integrated into the propulsion system rather than added as an afterthought.

This is the realistic near-term template for commercial propulsion improvement. Full electric for narrowbody-class aircraft with meaningful range is not achievable within any planning timeline that matters to current aviation. Hybrid-electric integration at the engine level - using electrification to improve efficiency and flexibility within a gas turbine core - works within current energy density constraints.

Where the RISE Program Stands

CFM began open fan demonstrator ground testing in 2023 at their facility in Villaroche, France, running blade sets through aerodynamic and acoustic test matrices. The program is structured in phases, with a flight demonstration campaign targeted for the late 2020s using a modified test aircraft to validate acoustic and performance models in real conditions.

Entry into service for a production RISE engine is projected for the mid-2030s, with first deliveries on aircraft arriving roughly in the 2037–2040 timeframe. That aligns with when successor platforms to the current A320neo and 737 MAX families would be entering service. The timing is deliberate - CFM is positioning RISE as the powerplant for the next narrowbody generation.

Key Uncertainties That Remain

Noise certification is the central open question. The FAA and EASA have never certified an open fan architecture for commercial air transport. CFM and the regulators will develop that certification basis together, and that process carries schedule risk that no engineering model can fully account for. Models and flight test data are different things.

Installation geometry is unresolved. Aft-fuselage mounting addresses some ground clearance and foreign object ingestion concerns, but most narrowbody aircraft use under-wing engines. The aerodynamic interaction between an open rotor’s pressure field and a wing structure is complex. CFM has not publicly committed to a final installation configuration, and that choice will determine which aircraft platforms can accommodate the engine.

The competitive landscape includes lower-risk alternatives. Pratt & Whitney’s GTF Advantage and Rolls-Royce’s UltraFan program are both pursuing efficiency gains on more technically conservative paths. More conservative means lower development risk and potentially earlier service entry. If RISE noise certification encounters serious problems, those programs remain.

Why This Matters Even If the Timeline Slips

Even a program that encounters delays or problems generates data. NASA’s open rotor research from the 1980s informed two decades of turbofan design work even after the GE36 program was shelved. A serious, well-funded flight test campaign using modern composites and digital control systems produces results that do not disappear if the specific program timeline shifts. Any future open rotor development effort will start from a meaningfully better position because of what RISE learns.

What Pilots Should Know

An open rotor creates a sound environment on the flight deck that will differ from current turbofan aircraft. The variable-pitch control logic in the reverse and beta range adds complexity to the powerplant system. Some of the acoustic cues experienced pilots use to monitor engine health will not transfer directly from current aircraft.

Initial type ratings on RISE-powered aircraft will require substantive time on the propulsion architecture. For pilots entering training programs in the early 2030s, familiarity with variable-pitch open rotor logic and its failure modes will be part of the certification path for the next narrowbody generation.

Key Takeaways

• CFM’s RISE program targets a 20% fuel burn improvement over LEAP using an open fan architecture with bypass ratios potentially exceeding 50:1 - a step change, not an incremental gain.
• The technology has direct historical precedent in the GE36 Unducted Fan of the 1980s, which was abandoned when oil prices fell and noise problems proved difficult to solve without sacrificing efficiency.
• Modern composite blade manufacturing, computational design tools, advanced digital control systems, and current regulatory pressure on emissions all substantively change the feasibility picture.
• Ground testing began in 2023 at Villaroche, France; flight demonstration is targeted for the late 2020s; entry into service is projected for the mid-2030s, aligning with the next narrowbody generation.
• Noise certification and installation geometry are the two largest unresolved technical and regulatory uncertainties, and the FAA/EASA certification basis for this architecture does not yet exist.