The CFM RISE Open Fan Engine, the Return of the Propfan, and the Forty-Year-Old Idea That Could Reshape Every Narrowbody in the Sky

The CFM RISE program - a joint effort between GE Aerospace and Safran Aircraft Engines - is developing an open fan engine architecture targeting more than 20 percent improvement in fuel efficiency over today’s LEAP engine. If it succeeds, RISE could power the next generation of narrowbody aircraft and mark the most significant shift in commercial propulsion in decades. The underlying technology is forty years old. What changed is the ability to actually build it.

What Is the CFM RISE Engine?

RISE stands for Revolutionary Innovation for Sustainable Engines. Announced in 2021 by CFM International - the 50/50 joint venture between GE Aerospace and Safran - it targets over 20 percent better fuel burn than the LEAP engine, compatibility with 100 percent sustainable aviation fuel, and an eventual capability to run on hydrogen.

The defining feature is an open fan architecture. Instead of enclosing fan blades inside a cylindrical nacelle, RISE exposes two counter-rotating rows of large composite blades directly to the airstream. No fan case. No cowling. Just blades and air.

Why Open Fans Are More Efficient

The physics of fan propulsion favor diameter. A larger fan moves more air mass per revolution and produces more thrust per unit of fuel. That’s the entire sixty-year story of turbofan development - the CFM LEAP powering the Boeing 737 MAX has a fan diameter of roughly 68 inches; the GE9X on the Boeing 777X reaches 134 inches.

But nacelle diameter has limits. Ground clearance, runway constraints, and aerodynamic drag all impose a ceiling on how large a ducted fan can grow. Removing the nacelle removes that ceiling entirely, allowing a step-change in fan size that no ducted turbofan can match.

The Propfan That Almost Worked in 1987

This isn’t a new idea. In 1987, GE flew an Unducted Fan (UDF) demonstrator on a modified MD-80 test aircraft in Victorville, California and also on a Boeing 727 testbed. The fuel consumption numbers those aircraft achieved were roughly 30 percent better than the turbofan engines of that era - numbers that modern turbofans are only now approaching after three more decades of incremental improvement.

It never entered service. Two reasons killed it.

First, noise. The counter-rotating blade rows created acoustic interference patterns at frequencies passengers found genuinely unpleasant. The blade-passing tones weren’t just loud - they were the kind of loud that made airline focus groups reject the concept outright.

Second, timing. Oil prices collapsed in the late 1980s, wiping out the economic case for accepting a noise penalty in exchange for fuel savings. The program was shelved.

The noise problem wasn’t solved. It was abandoned.

How Modern Technology Changed the Equation

RISE is not the 1987 UDF with updated paperwork. The differences are fundamental.

Variable-pitch blades are the most significant advance. RISE fan blades can change their angle of attack in flight to optimize for different operating conditions - something the original UDF couldn’t do effectively. The blades are manufactured from carbon fiber composites using techniques refined in part by the wind energy industry, which has spent decades engineering enormous rotating structures built for continuous loading over decades.

The blade geometry was optimized using AI-assisted aerodynamic modeling to break up the wake interference patterns that caused the original noise problem. Instead of letting the downstream blade row collide predictably with the wake shed by the upstream row, the new geometry disrupts those wake structures before they can fully form.

What the RISE Architecture Actually Looks Like

RISE is not an open fan bolted onto an existing engine core. CFM is developing an entirely new architecture.

The open fan replaces the conventional low-pressure turbine and fan system of a standard turbofan. Behind the fan, a high-pressure compressor, combustor, and high-pressure turbine perform standard gas turbine functions. The low-pressure section is replaced by a gear-driven, variable-pitch open fan with two counter-rotating blade rows.

The architecture also incorporates hybrid electric capability. The system is designed to recover energy during descent and inject it back during takeoff - analogous to regenerative braking in a car, scaled to a narrowbody jet. This isn’t an electric engine; it’s a way to flatten fuel burn spikes at the most demanding phase of flight.

Ceramic matrix composite (CMC) materials extend deeper into the high-pressure turbine hot section than in any previous CFM engine. CMCs tolerate temperatures that would destroy metal alloys, enabling a higher pressure ratio core that contributes meaningfully to the overall efficiency target.

The Engineering Challenges That Remain

The noise question is not fully settled. ICAO Chapter 14 noise standards govern new aircraft certifications, and they are strict. CFM’s computational models support compliance. But ground testing only began in 2023, and flight testing on a demonstrator is planned for the late 2020s. Computational models are not flight tests. Acoustic data from real conditions - at altitude, in real atmospheric variation - hasn’t been collected yet.

Foreign object ingestion is a genuine concern without a nacelle providing protection. Bird strikes, hail, and runway debris all require engineered solutions: tougher blade materials, geometry that deflects ingested objects before they reach the core, and protection screens behind the fan stage. This is solvable, but certification requires years of testing.

Icing certification is its own challenge. An exposed open fan in icing conditions doesn’t map cleanly onto existing frameworks for turboprops or turbofans. The rotational speed, blade geometry, and regulatory precedent are all different. CFM and regulators are actively working through this now.

Why the Aircraft Matters as Much as the Engine

The most underappreciated constraint on RISE: the aircraft it would power doesn’t exist yet.

RISE targets the 150-to-220-seat narrowbody segment - the space currently dominated by the Airbus A320 family and the Boeing 737 MAX. Both manufacturers have next-generation narrowbody programs loosely targeted at the mid-2030s. Neither has committed to a configuration or confirmed an engine selection.

CFM is developing RISE on the reasonable bet that at least one major airframer - possibly both - will want an engine in this efficiency class. But RISE’s commercial future depends on programs still in early planning stages at two manufacturers currently managing significant operational and financial pressures.

Pratt & Whitney is also developing next-generation propulsion for this segment. Their approach stays within the ducted turbofan concept, pushing efficiency through a larger fan, higher pressure ratio core, and improved thermal management. The competition between open fan and advanced ducted fan for the next narrowbody hasn’t been decided. That competitive pressure is forcing both teams to deliver on aggressive targets.

Where the Program Stands in 2026

Ground testing is underway at Safran’s facility in Villaroche, France and at GE Aerospace’s facility in Evendale, Ohio. The core demonstrator has run. Individual open fan system components have been tested. The full engine architecture has been operated on test stands. Program teams describe results as tracking with predictions - a genuinely good sign at this stage.

The flight demonstrator, which will mount RISE hardware on a testbed aircraft to gather real aerodynamic and acoustic data in flight, is planned for the late 2020s. That is the critical milestone. Until it flies, noise propagation and handling characteristics in real atmospheric conditions at altitude remain unknown.

A credible planning scenario, assuming testing proceeds without major setbacks and an airframer commits to a new narrowbody program: RISE-powered aircraft entering revenue service between 2035 and 2040.

Why This Matters for Pilots

The narrowbody is where commercial aviation’s volume lives. The 737 family and A320 family together account for the vast majority of commercial flights worldwide - more aggregate fuel burned and more aggregate CO₂ produced than any other aircraft category. The widebody draws attention because of long-haul operations. But the narrowbody is where the scale is.

A 20 percent efficiency improvement across a narrowbody production run spanning thousands of aircraft over 30 to 40 years represents a genuinely meaningful shift in aviation’s environmental footprint - not through any single dramatic event, but through compounding across enormous volume.

The pattern is consistent with how aviation technology actually advances. The turbofan was theorized long before metallurgy could build one reliably. Fly-by-wire was demonstrated decades before widespread service entry. GPS was available years before avionics manufacturers trusted it in certified cockpit systems. RISE is the same pattern: an idea that was physically correct in 1987 but couldn’t be built reliably at commercially viable cost, returning now that blade materials, computational design tools, manufacturing precision, and variable pitch mechanisms at this scale have finally caught up.

Key Takeaways

• CFM RISE targets over 20% better fuel efficiency than the current LEAP engine; it was announced in 2021 by GE Aerospace and Safran through their joint venture CFM International.
• The open fan concept was first flight-tested in 1987 on a modified MD-80 in Victorville, California; it was abandoned due to passenger noise complaints and falling oil prices - not because the physics failed.
• Modern advances in carbon fiber composite blades, variable pitch systems, and AI-assisted aerodynamic design directly address the acoustic and engineering problems that killed the concept forty years ago.
• Key unresolved challenges include acoustic certification under ICAO Chapter 14, foreign object ingestion, and icing - all manageable engineering problems, but each requiring significant flight test data.
• A realistic service entry window is 2035–2040, contingent on flight testing results in the late 2020s and a new narrowbody airframe commitment from Airbus, Boeing, or both.