The CFM RISE Open Fan, the Exposed Blade Architecture That GE Aerospace and Safran Are Betting the Next Generation On, and What It Means for Every Narrow-Body That Comes After the MAX and the Neo

CFM International’s RISE program is a serious engineering commitment to bring exposed-blade, open fan propulsion into commercial aviation for the first time - targeting more than 20 percent better fuel efficiency than the current LEAP engines powering the Boeing 737 MAX and Airbus A320neo family. Announced at the 2021 Paris Air Show, RISE is a joint program between GE Aerospace and Safran Aircraft Engines, with an entry-into-service target in the mid-2030s.

What the CFM RISE Open Fan Actually Is

A conventional turbofan encloses a large fan stage inside a nacelle. The nacelle ducts air, reduces noise, and provides structural mounting. RISE removes that nacelle entirely. Two counter-rotating rows of variable-pitch composite blades spin in open air, with no cowling, no fan case, and no bypass duct.

The propulsion physics behind this choice are straightforward: a larger fan moving more air at lower velocity is more efficient than a smaller fan accelerating less air to higher velocity. Modern high-bypass turbofans achieve bypass ratios of 12 to 15 to 1 - meaning 12 to 15 pounds of air travel around the core for every pound going through it. The nacelle makes those ratios possible, but it also sets the ceiling. Beyond a certain diameter, the nacelle becomes heavier, generates more drag, and runs into ground clearance limits under low-wing aircraft. The open fan breaks through that ceiling.

Why This Architecture Has Been Tried Before

The open fan concept is not new. The oil crises of 1973 and 1979 prompted NASA to launch the Advanced Turboprop Project, and the major engine manufacturers followed with what they called propfans or unducted fans. GE built a demonstrator called the GE Unducted Fan (GE UDF) and flew it on a Boeing 727 testbed in the mid-1980s. Hamilton Standard, Pratt and Whitney, and the Soviet Union - with the Ivchenko Progress D-27, still flying on the Antonov An-70 today - all produced competing designs.

The efficiency results were real. In some configurations, propfans demonstrated 30 to 40 percent better fuel burn than the turbofans of that era. The technology worked.

Then oil prices crashed. By 1988, crude prices had fallen far enough that the efficiency advantage no longer justified the risk of unconventional propulsion. Airlines passed on engines with exposed blades and unresolved noise characteristics when cheap fuel made conventional turbofans good enough. The propfan went into the archive.

What Changed to Make RISE Viable Now

The economic and regulatory environment that killed the propfan in 1988 has reversed. Fuel costs are structurally higher and volatile. The European Union Emissions Trading System and global carbon mechanisms are adding real costs to emissions. Sustainable aviation fuel carries a cost premium that only pays off when less of it is burned. And industry-wide emissions reduction targets cannot be met through incremental turbofan improvements alone.

The engineering toolkit has also changed completely. CFM’s RISE program uses computational fluid dynamics tools that can simulate the aerodynamic interaction between two counter-rotating blade rows in conditions that would have required months of wind tunnel time in 1985. Advanced composite manufacturing can produce blade profiles with tolerances that were impossible to fabricate then. Fifty years of turbomachinery acoustics research means the noise problem the propfan never fully solved is at least better understood.

The Three Genuine Technical Challenges

Blade aerodynamics at cruise speed is the first obstacle. A narrow-body in cruise operates at roughly Mach 0.80. Depending on blade length and angular velocity, the blade tips can approach transonic conditions relative to the passing air, generating shock waves, vibration, and noise. CFM has described deeply swept, thin-section composite blades designed through computational fluid dynamics optimization across the full flight envelope, with variable pitch capability to optimize blade angle for each phase of flight.

Noise certification is where the 1980s propfan faced its steepest regulatory resistance. Counter-rotating open fan blades are inherently noisier than enclosed turbofans at certain conditions, particularly during takeoff when blade loading is highest. The wake interaction between the front and rear blade rows generates tonal noise at frequencies determined by blade count and rotational speed. CFM states publicly that RISE will meet current noise certification standards, with variable pitch and blade geometry as tuning variables. Meeting that standard on a demonstrator and meeting it in sustained service over communities are different benchmarks.

Engine integration is the challenge that changes what aircraft look like. A conventional turbofan hangs under the wing on a pylon. An open fan engine cannot go there - the blade diameter extends well beyond any nacelle diameter, putting blades at ground level during takeoff rotation. The two viable options are over-wing mounting, which keeps blades clear of the ground and may generate beneficial aerodynamic interaction with the upper wing surface, and rear fuselage mounting, which preserves wing aerodynamics but shifts the center-of-gravity envelope and adds aft structural weight. Both have precedents: rear fuselage mounting on the McDonnell Douglas DC-9, MD-80, and Boeing 717; over-wing mounting on the Fokker VFW 614.

Neither option can be retrofitted to an existing airframe. A RISE-powered aircraft requires a clean-sheet design.

How RISE Compares to Pratt and Whitney’s Approach

Pratt and Whitney has not announced an open fan program. Their bet is on continuing to advance the geared turbofan (GTF) architecture, which uses a planetary gearbox between the fan and the low-pressure turbine to allow both to run at optimal speeds. The GTF powers the A320neo through the CFM LEAP-1A’s competitor and has delivered real efficiency improvements within a conventional nacelle.

The two positions represent a genuine technical disagreement. Pratt and Whitney believes meaningful efficiency improvement remains within the enclosed turbofan architecture. CFM believes that architecture eventually hits a hard ceiling and the only path through is the open fan. Both are defensible with current data. The industry may end up with both architectures in service on different aircraft - the same way turboprops and turbofans coexist today.

The Hybrid-Electric Component

RISE includes provision for hybrid-electric capability that tends to get overlooked in discussions of blade geometry. This is not full electric propulsion - jet fuel stores roughly 43 times the energy by weight of the best lithium-ion batteries available today, and batteries cannot substitute for kerosene on a commercial aircraft in any foreseeable timeframe.

The hybrid system would allow the engine to drive a generator, store a modest electrical reserve, and deploy it for specific phases: electric taxiing without running the core, peak power assist during takeoff to reduce thermal stress on the core, and potential energy recovery during descent. The efficiency gains from each element are modest in isolation. Across thousands of flight cycles, and combined with the core efficiency gains from the open fan architecture itself, the cumulative effect justifies the complexity.

What This Means for Pilots in Service

Variable-pitch open fan blades respond differently than conventional turbofan spools. Engine response characteristics during rapid power changes would be different, and asymmetric thrust scenarios would carry different yawing moment profiles depending on engine mounting location. These are real differences that would need to be reflected in type rating training, not insurmountable ones.

The cockpit noise environment would change at certain frequencies. Cabin acoustic design would likely compensate for passengers, but it remains a design variable. Ground operations would require heightened FOD awareness and equipment clearance discipline given the blade diameter extending significantly from the engine centerline.

The Timeline and What to Watch

Airbus has an internally confirmed next-generation single-aisle program and has acknowledged working with propulsion partners including CFM on future architectures. Boeing, currently focused on recovering production rates on existing programs, is also evaluating next-generation propulsion. Neither manufacturer has announced a launch customer or formal program entry for a RISE-powered aircraft.

The mid-2030s entry-into-service target is achievable but contingent on demonstrator test campaigns delivering the projected results. CFM’s demonstrator testing is the first gate. Engine certification, clean-sheet airframe design, aircraft certification, and production ramp-up all follow in sequence.

Watch the demonstrator test campaigns. That is where the data will confirm whether the efficiency claims hold at cruise conditions, and whether the noise certification path is as manageable as the program currently projects.

Key Takeaways

CFM RISE targets 20%+ fuel efficiency improvement over the already-efficient LEAP-1A and LEAP-1B engines, by removing the nacelle entirely and running two counter-rotating open composite blade rows
The open fan concept was proven in the mid-1980s but shelved when oil prices collapsed in 1988; the economic and regulatory environment that killed it has now reversed
The three core challenges are blade aerodynamics at Mach 0.80 cruise, noise certification over communities, and airframe integration - none of which can be solved by retrofitting an existing aircraft
A RISE-powered aircraft requires a clean-sheet airframe design, with over-wing or rear fuselage engine mounting replacing the conventional under-wing pylon
The mid-2030s service entry target is contingent on demonstrator testing results; Airbus and Boeing are both evaluating next-generation propulsion but neither has formally launched a RISE-powered program