The CFM RISE Program, the Counter-Rotating Open Fan, and the Thermodynamic Bet That Could Redefine Every Narrowbody Flying in the Twenty Thirties

The CFM RISE program is developing a counter-rotating open fan engine that could deliver greater than 20 percent improvement in fuel consumption compared to today’s LEAP-powered narrowbodies. If it achieves certification and wins an airframe commitment from Boeing or Airbus, it would represent the largest single-generation efficiency leap in commercial turbofan history. The launch window for that airframe decision is the late 2020s.

What the CFM RISE Program Actually Is

CFM International announced the RISE program at the Paris Airshow in June 2021. RISE stands for Revolutionary Innovation for Sustainable Engines. The program is a joint effort between GE Aerospace and Safran, the two companies that form CFM International.

The centerpiece technology is a counter-rotating open fan - an engine with no nacelle, no cowling, just two stages of swept composite blades spinning in opposite directions. This architecture is not new. A version of it flew on a modified Boeing 727 at NASA’s Lewis Research Center in Cleveland, Ohio in 1987. Oil prices collapsed that same year, the economic rationale evaporated, and the program was shelved.

The Thermodynamic Case for Open Fan Propulsion

The efficiency argument behind RISE comes from basic propulsion physics: moving a large mass of air at a small velocity increase is more efficient than moving a small mass of air at a large velocity increase.

When generating thrust, you accelerate air rearward. Kinetic energy scales with the square of velocity - doubling exit velocity to maintain the same momentum quadruples the energy wasted as exhaust heat. That wasted energy is fuel burned for no useful work. The engineering implication: maximize mass flow, minimize velocity increment. Build the biggest fan you can physically manage.

This is why bypass ratios have climbed for five decades. The CFM56 engine, which powered the original 737 family and A320 for decades, runs around 5-to-1 bypass ratio. The LEAP engine - powering the 737 MAX and A320neo today - reaches 9-to-11-to-1. Higher bypass means bigger fan, lower exhaust velocity, better efficiency.

The problem is the nacelle. A bigger fan requires a bigger nacelle. That nacelle adds weight and aerodynamic drag. At some point, the structural and aerodynamic penalty of the housing begins consuming the gains from the higher bypass ratio. Current turbofan architecture is approaching that ceiling.

The open fan eliminates the ceiling entirely. No nacelle means no duct limiting airflow, making effective bypass ratio practically unlimited. CFM projects greater than 20 percent fuel burn improvement over current LEAP engines - compared to the 5 to 15 percent delivered by each previous turbofan generation. This is a step change, not an incremental gain.

Why Two Counter-Rotating Stages

A single fan stage doesn’t just accelerate air rearward - it imparts a rotational swirl to the airflow. That swirl represents kinetic energy that produces no useful thrust. On a ducted engine, stator vanes downstream of the fan straighten this swirling air and recover some of the energy. An open fan has no structure to mount stators.

The solution is a second fan stage spinning in the opposite direction. The contra-rotating stage intercepts the swirl from the first and straightens it, recovering energy that would otherwise exit as waste. The result is higher net thrust per unit of fuel than either stage alone could produce.

The same principle appeared decades ago on aircraft like the Bristol Brabazon and the Avro Shackleton using counter-rotating propellers. RISE applies it to a modern open fan architecture with composite blades and 21st-century manufacturing tolerances.

Why the 1980s Propfan Failed - and What’s Different Now

The concept worked aerodynamically in the 1980s. Two technical problems blocked certification.

First, blade geometries achievable with the manufacturing technology of that era produced noise signatures that would not have met evolving airport community standards. Open fans generate acoustics from blade tip vortices and from the aerodynamic interaction between the two counter-rotating stages.

Second, oil prices collapsed in 1986 and 1987, falling below $15 a barrel. The economic case for developing an unusual, harder-to-certify engine disappeared. Programs were shelved.

What has changed since then is not the physics - those were sound. What has changed is manufacturing precision, computational tools, and materials.

Modern carbon fiber composite fan blades can be built to tolerances and three-dimensional profiles that were unachievable 40 years ago. The highly swept, compound-curved geometries that acoustic engineers need to shift noise to less problematic frequencies can now be manufactured consistently at production scale. Computational fluid dynamics tools allow CFM to model acoustic and aerodynamic behavior in simulation before cutting metal, compressing years of physical testing into a much shorter development cycle.

The Certification Unknowns

The noise certification question is the central technical uncertainty. Whether improved blade geometry translates into actual certification compliance around real airport communities, under real atmospheric conditions, is something ground testing can approximate but only flight testing will confirm. The FAA and EASA are still finalizing the standards against which RISE hardware will be evaluated.

Foreign object damage certification is the second major open question. Every turbofan nacelle is a containment structure - if a blade fails, the nacelle captures the debris. An open fan has no such containment. The FAA and EASA are writing the requirements RISE will have to meet, but those requirements are not yet fully written. CFM is engineering a solution whose finish line is still being defined.

Aircraft Integration: Why the Airframe Commitment Is Everything

Open fan blades are large. They cannot mount under a wing where current turbofans attach - blade tips need ground clearance through the full pitch attitude range, and both counter-rotating stages require clean, unobstructed airflow.

The leading integration concept is aft-fuselage mounting, the same configuration used on the original Boeing 727, the Douglas DC-9, and the BAC One-Eleven. Two engines on the rear fuselage provide ground clearance and leave the wing clean. Over-wing mounting is also under study, offering potential noise shielding but introducing structural complexity.

Neither Boeing nor Airbus has committed to an airframe design. Both are in deep studies on next-generation narrowbody aircraft. Both programs converge on a potential new aircraft entering service in the 2030s.

A formal manufacturer commitment likely needs to happen in the late 2020s for RISE to reach that first generation of replacement narrowbodies. Missing that window means competing for a second-generation airframe a decade further out.

How RISE Compares to the Competition

Rolls-Royce’s UltraFan takes a different approach. It remains fully ducted - the nacelle stays - but uses a geared transmission to let the fan spin at its aerodynamically optimal speed while the turbine runs faster and more efficiently. The UltraFan targets approximately 25 percent improvement over the older Trent 700 series.

Pratt and Whitney’s geared turbofan (GTF), already flying on the A220 and A320neo families, established the geared ducted concept with a substantial in-service track record. UltraFan pushes that architecture further.

The comparison is a genuine engineering debate. RISE carries higher theoretical efficiency potential and higher technical uncertainty. UltraFan has a shorter certification path and competes against an engine family with years of real-world operational data. The fundamental trade: RISE theoretically outperforms a ducted architecture but requires solving problems whose regulatory finish line is still being drawn.

What 20 Percent Fuel Burn Means at Airline Scale

A major airline operating 200 single-aisle aircraft flying seven to eight revenue hours per day consumes an enormous fuel volume annually. A 20 percent reduction is not a rounding error - it is a structural competitive advantage.

Low-cost carriers that compete on cost-per-available-seat-mile have restructured entire fleet strategies around efficiency improvements smaller than this. An operator committed to RISE-powered narrowbodies would sit in a fundamentally different cost position than competitors flying conventional turbofans.

RISE is also designed from the start for full compatibility with sustainable aviation fuel (SAF) - produced from agricultural residues, municipal solid waste, and used cooking oil, with significantly lower lifecycle carbon emissions than fossil jet-A. An open fan operating on SAF delivers both the efficiency gain and the emissions reduction in a single package.

What Flying Behind an Open Fan Would Be Like

For pilots, practical flight characteristics are largely driven by the airframe integration rather than the engine itself. Counter-rotating fan stages produce no net gyroscopic moment - the two stages cancel each other out. Engine response time should be comparable to current-generation turbofans.

Cabin acoustics will be different. Large, slow-moving open fan blades produce noise in a different frequency register than the higher-pitched sound of a ducted fan. Early 1980s propfan test aircraft had noticeably different acoustic signatures, though blades of that era were not optimized for passenger comfort. The aft-fuselage mounting configuration places the primary noise source well behind the passenger cabin.

Where the Program Stands as of 2026

GE Aerospace is testing the open fan architecture at its Evendale, Ohio facility. Safran is running parallel development work in France. The first flight demonstration on a dedicated flying testbed is scheduled for the mid-2020s. Entry into service is targeted for the mid-2030s.

CFM is investing billions. The aerodynamic efficiency claims are grounded in solid thermodynamic principles and tracking against early test results. The program is real, the investment is real, and the physics are not in dispute.

What remains unresolved: whether the noise signature meets certification standards around real communities, what FAA and EASA blade containment requirements will ultimately demand, and - most critically - which airframe manufacturer commits first to designing a new aircraft around the RISE powertrain.

The economic pressure driving RISE today is structurally different from the environment that killed its predecessor in 1987. Fuel costs push airlines toward efficiency. Carbon regulations in the European Union, and voluntary industry commitments across the global carrier community, push toward lower emissions. Neither pressure existed at its current scale when oil hit $15 a barrel and the Unducted Fan program was shelved in Cleveland.

The board-level decisions at Boeing and Airbus over the next few years will determine whether the thermodynamics finally win.

Key Takeaways

• CFM’s RISE program targets greater than 20 percent fuel burn improvement over current LEAP engines - the largest single-generation efficiency leap in turbofan history if achieved.
• The counter-rotating open fan concept flew in 1987 but was shelved when oil prices collapsed; modern composite manufacturing and computational fluid dynamics address the technical barriers that stopped it then.
• The two primary unresolved certification challenges are noise signature compliance around real airport communities and blade containment standards for an unconstrained fan - the FAA and EASA are still writing the final requirements.
• Neither Boeing nor Airbus has committed to an airframe; a launch decision in the late 2020s is likely required for RISE to power first-generation replacement narrowbodies in the 2030s.
• At airline scale, a 20 percent fuel efficiency advantage is a structural competitive weapon - larger than the incremental improvements that have historically driven entire fleet strategy shifts across the industry.