The CFM RISE Program, the Open Fan Architecture, and the Twenty Percent Fuel Burn Promise That Could Define the Next Generation of Narrowbody Aviation

CFM International’s RISE program - Revolutionary Innovation for Sustainable Engines - is the most ambitious engine development effort in commercial aviation since the high-bypass turbofan went into service in the late 1960s. The core proposition is straightforward: remove the nacelle, expose the fan blades to open air, and use the resulting freedom to build a rotor that is dramatically larger, slower, and more efficient than anything a conventional turbofan can achieve. The target is more than 20% reduction in fuel consumption and CO2 emissions compared to the current LEAP engine - on a platform that is already among the most efficient turbofans ever produced.

What Is the CFM RISE Program and Who Is Behind It

CFM International is a 50/50 joint venture between GE Aerospace and Safran Aircraft Engines. They produce the CFM56, which has powered more commercial aircraft than any other engine in history, and the LEAP, the current-generation turbofan on the Boeing 737 MAX and the Airbus A320neo family. Between those two engine lines, CFM has produced tens of thousands of engines. When CFM announces a technology direction, the entire narrowbody market pays attention.

RISE was announced in June 2021. The program’s stated goal is a fuel burn reduction exceeding 20% versus the LEAP - a figure that translates, on a narrowbody flying 800 cycles per year, into a meaningful reduction in airline operating costs and a substantial contribution to the emissions targets commercial aviation has committed to regulators.

The technology to reach that number is the open fan architecture.

Why the Nacelle Is the Efficiency Ceiling

Propulsive efficiency increases as fan diameter increases. Larger, slower blades move more air more gently, producing thrust more efficiently than smaller, faster blades. The problem is the nacelle that surrounds them. Larger nacelles create more drag and more weight, creating a practical ceiling on how big the fan can grow before the enclosure itself becomes a liability.

Remove the nacelle and that constraint disappears. An open fan can be significantly larger. It can operate at much lower rotational speeds. It can achieve bypass ratios - the ratio of cold fan air to hot core exhaust - that a conventional high-bypass turbofan cannot approach. The bypass ratios CFM is targeting for RISE would make today’s most efficient engines look, by comparison, like the turbojets of the 1950s.

This Is Not a New Idea - and That Is the Point

The open fan concept was rigorously developed in the 1980s under the name “unducted fan” or UDF. GE Aviation flew its version, the GE36, on a modified Boeing 727 testbed and on an MD-80 in 1987 and 1988. The fuel burn numbers were impressive. The technology worked. Then oil prices collapsed, the economic urgency evaporated, and the programs were shelved.

The technology was shelved, not disproven. That distinction matters when assessing whether a revival has merit.

What Is Different in 2026 Versus 1988

The reasons the open fan was abandoned are no longer the situation the industry is in. Fuel costs have fundamentally restructured airline economics. Carbon regulation is moving from policy discussion to binding law across multiple jurisdictions. And the narrowbody market specifically - the replacement aircraft that must be designed in the next decade - needs a propulsion breakthrough that incremental turbofan improvement cannot deliver on its own.

The engineering tools are also different. Advanced computational fluid dynamics now allows blade shapes to be designed with a precision in managing acoustic interactions that was simply not achievable in the 1980s because the computing capability did not exist.

The Four Engineering Challenges CFM Has to Solve

Noise was the first major problem in the 1980s, and the certification standards have gotten considerably stricter since then. An open fan generates a complex acoustic signature: blades spinning in freestream air interact with each other, with the pylon, and with wake from the airframe structure. CFM’s approach combines counter-rotating blade rows - two sets of blades spinning in opposite directions to manage aerodynamic wake - with variable pitch blades that continuously adjust angle of attack throughout the flight envelope, similar to a constant-speed propeller but with jet-speed precision.

Blade containment is the second challenge. A conventional turbofan has a titanium fan case; if a blade fails, the case contains the debris. An open rotor does not have that. The certification path for demonstrating safe blade departure is a genuinely hard engineering problem. The FAA and EASA are working through that framework with manufacturers, but the regulatory pathway does not fully exist yet.

Ground clearance is the third. Large exposed blades on an under-wing engine installation require significant clearance between rotor tips and pavement - affecting fuselage height, wing geometry, landing gear length, and gate compatibility at existing airports. CFM has been working with Airbus on what an airframe optimized for open fan installation would require. This is not a retrofit question. It is a clean-sheet airplane question.

Hybrid electric integration is the fourth. RISE incorporates electric motors to assist the core during high-demand phases - particularly takeoff and climb. That means high-power electrical systems, energy storage, and thermal management challenges layered on top of an already ambitious open rotor development program.

Where the Program Stands Today

CFM began ground testing of the RISE open fan demonstrator in 2023 at Safran’s facility in Villaroche, France. Those tests are validating aerodynamic models, blade designs, and noise predictions against years of computational work. Flight testing of the demonstrator is expected before the end of this decade. Entry into service for a production engine is currently targeted for the late 2030s - a realistic first delivery figure for an airline is approximately 2037 or 2038.

That timeline is not arbitrary. Airbus has indicated its successor to the A320 family will likely enter service in the mid-to-late 2030s. The engine that powers it must be selected well before then. The two development cycles have to be synchronized - you cannot build an airplane around an engine that does not exist, and you cannot certify an engine with no airplane program to fly it.

Pratt & Whitney Is Betting on a Different Path

Pratt & Whitney’s strategy has been to continue improving the geared turbofan (GTF) family, which uses an internal gearbox to allow the fan and low-pressure compressor to each run at their individual optimum speeds. The GTF Advantage, an upgraded version of the current PW1100G series, targets meaningful fuel burn improvements over the standard GTF. It is not 20%, but it involves far less development risk than an open rotor architecture.

The industry is effectively splitting into two schools of thought. One holds that continued improvement of the conventional turbofan - through geared fans, ceramic matrix composite components, and thermal efficiency gains - is the right risk-adjusted path to the 2030s. The other holds that the next major efficiency leap requires a fundamental rethink of the fan architecture, and development must begin now before the next narrowbody generation locks in its configuration. Both positions are held by serious engineering teams with serious track records. This is not hype versus engineering reality. It is two legitimate approaches racing toward the same hard deadline.

Why This Matters for Pilots

In the near term, nothing changes in the cockpit. The 737 MAX and A320neo family are not going anywhere for decades, and the aircraft flying today will continue flying. The RISE program is about the generation of narrowbodies that will begin replacing those platforms in the 2030s and 2040s.

In the longer view, this is the decision that will define what narrowbody flying looks and sounds like for the second half of this century. The open fan has a different acoustic character than a conventional turbofan - some frequency ranges are louder on the ground, others quieter. The hybrid electric assist during takeoff may change the power management cues pilots learn to read. Aircraft designed around this engine will likely carry different configurations: altered wing geometry, different pylon integration, possibly a higher engine position to achieve the required ground clearance.

For pilots in the regional and general aviation space, the CFM program targets mainline commercial aircraft specifically - but the underlying physics argument is the same one driving propeller-driven hybrid electric aircraft emerging in the regional segment. The engineering lineage connects across categories.

The program is real. The investment is serious. The physics are sound. The GE36 flights in 1987 and 1988 demonstrated this technology works in the air. The remaining questions are about certification frameworks that must be built new, noise compliance at increasingly strict standards, airframe integration requiring a partner willing to design a clean-sheet airplane around an unproven engine, and whether the development schedule holds against the commercial pressure of fleet replacement timelines.

The easy gains from incremental turbofan improvement are nearly exhausted. The regulatory environment, long-term fuel economics, and sustainable aviation fuel pathway - which still requires significantly more efficient airframes to meet real emissions targets - are all pointing toward the same conclusion. CFM is betting the next 20% comes from taking the nacelle off. They have test data, serious industry partners, and 40 years of foundational work from the GE36 program to build on.

Key Takeaways

CFM RISE targets more than 20% fuel burn reduction versus the LEAP engine by using an open fan architecture - blades spinning in freestream air with no surrounding nacelle
The concept was proven in flight by GE’s GE36 unducted fan in 1987–1988 but was shelved when oil prices collapsed; the technology was never disproven
Ground testing began in 2023 in Villaroche, France; flight testing of the demonstrator is expected before 2030; production engine entry into service is targeted around 2037–2038
Four unsolved challenges remain: noise at current certification standards, blade containment without a nacelle, ground clearance on conventional airframes, and hybrid electric integration
Pratt & Whitney is pursuing an incremental geared turbofan path (GTF Advantage) with lower development risk, creating a genuine split in industry strategy for the next narrowbody generation