The CFM RISE open fan engine and the unducted propulsion revolution coming to airliners by twenty thirty-five

The CFM RISE (Revolutionary Innovation for Sustainable Engines) program represents the most significant change in airliner propulsion architecture since the turbofan replaced the turbojet. Developed by CFM International, the joint venture between GE Aerospace and Safran, the open fan engine eliminates the traditional nacelle enclosure, allowing two counter-rotating rows of fan blades to spin in open air. The result is a projected bypass ratio of approximately 70:1 and a 20% reduction in fuel consumption and CO2 emissions compared to the current LEAP engine.

How Does an Open Fan Engine Work?

A modern high-bypass turbofan generates most of its thrust by pushing large volumes of air around the engine core. The fan sits inside a nacelle — the large cylindrical housing visible on the wing. The LEAP engine achieves a bypass ratio of about 11:1, meaning eleven parts bypass air for every one part flowing through the hot core. Higher bypass ratios mean greater efficiency.

But there is a physical limit. As the fan grows larger, the nacelle grows with it, adding drag and weight. Eventually the nacelle itself cancels out efficiency gains.

The RISE engine solves this by removing the nacelle entirely. Two counter-rotating rows of fan blades spin exposed to the airstream, driving the bypass ratio to roughly 70:1. The core section retains a partial housing, but the fan blades breathe free — producing far more thrust per unit of fuel burned.

Why Did Open Fan Engines Fail in the 1980s?

This is not the first attempt at unducted propulsion. In the 1980s, GE built the GE36 Unducted Fan, which flew on a Boeing 727 testbed. It delivered impressive fuel efficiency, but airlines rejected it for two reasons: noise and appearance. The GE36 was noticeably louder than contemporary turbofans, and its exposed scimitar blades looked unfamiliar to passengers. When oil prices dropped in the late 1980s, the economic case collapsed and the program was shelved.

What Has Changed Since Then?

Three enabling technologies have matured in the intervening four decades:

Advanced materials. The RISE fan blades use carbon fiber composites, with ceramic matrix composites in the hot section. These materials did not exist in usable form in 1986. Carbon fiber blades are lighter, stronger, and can be shaped into complex aerodynamic profiles that were impossible to manufacture in metal. The blade geometry is radically different from the GE36’s straight paddle blades — thin, swept, and precisely sculpted using modern computational fluid dynamics.

Noise reduction. The counter-rotating blade rows use unequal blade counts and optimized spacing to break up the tonal noise that doomed the original concept. GE and Safran have invested years of acoustic modeling to target compliance with current Chapter 14 noise standards, the same standards ducted turbofans must meet. Computational models suggest this is achievable, though independent acoustic engineers remain cautiously optimistic rather than fully convinced.

Under-wing integration. The GE36 could only be mounted on the tail or in unconventional pod configurations. The RISE engine is designed for conventional under-wing installation on a standard tube-and-wing airframe. This means it could power a successor to the A320 or 737 family without requiring a radically new aircraft design. CFM renders show the engine mounted under a swept wing with a partial nacelle around the core and the fan blades exposed behind it.

Where Does Testing Stand?

GE Aerospace built and ground-tested a full-scale open fan rig at its facility in Peebles, Ohio in late 2024 and into 2025. Early results were encouraging: propulsive efficiency numbers fell within predicted ranges, and noise signatures reportedly performed better than pessimistic projections.

Safran has been developing the compact core at facilities around Villaroche, France, testing individual compressor stages, combustor designs, and turbine materials. The hot section will operate at higher pressures and temperatures than any previous commercial engine, extracting more energy from less fuel.

CFM targets a flying demonstrator by approximately 2028–2029, with the engine mounted on a testbed aircraft to validate performance in actual flight conditions. If successful, a production engine could be ready for a new narrowbody airliner entering service in the mid-2030s.

What About the Competition?

The RISE engine is not developing in a vacuum:

Pratt & Whitney continues advancing its Geared Turbofan architecture, which still has growth potential, and is studying its own next-generation concepts.
Rolls-Royce has its UltraFan program, which takes a different approach — a very large ducted fan with a power gearbox that pushes bypass ratios higher while retaining a conventional nacelle. Rolls-Royce has already ground-tested the UltraFan demonstrator at its facility in Derby, England.

Why Does This Depend on a New Airplane?

Neither Boeing nor Airbus has officially launched a next-generation single-aisle aircraft, but both are studying one. Airbus has publicly evaluated concepts for an A320 successor. Boeing, following the 737 MAX crisis, arguably needs a clean-sheet design more urgently. The RISE engine is being developed for an airplane that does not formally exist yet — a calculated risk, since both manufacturers know a new narrowbody is inevitable, and CFM is positioning RISE as the obvious powerplant choice.

What Are the Remaining Risks?

Significant challenges remain before the open fan reaches commercial service:

Noise in flight may prove harder to manage than ground test results suggest.
Certification of an entirely new engine architecture through the FAA and EASA will be lengthy and expensive.
Bird ingestion testing on exposed blades presents a genuine engineering challenge, with different failure modes than ducted engines.
The business case depends on Boeing or Airbus launching a new narrowbody — a multi-billion-dollar corporate decision neither has committed to.

Why This Matters for Pilots

Airline pilots operating in the late 2030s may fly aircraft with engines that look fundamentally different from anything in current training. While thrust generation follows the same physics, operating procedures, failure modes, inspection requirements, vibration harmonics, bird strike considerations, and icing behavior will all differ from ducted turbofan experience.

General aviation pilots will not see the RISE engine directly, but the technology it drives — advanced composite manufacturing, compact high-efficiency cores, and noise reduction techniques — will filter into smaller engines over the following decade.

The Efficiency Arc of Jet Propulsion

Aviation propulsion has followed a consistent trajectory: the turbojet gave way to the low-bypass turbofan in the 1960s, the high-bypass turbofan arrived in the 1970s with the CF6 and JT9D, and the ultra-high-bypass geared turbofan debuted in the 2010s. Each generation increased bypass ratio and decreased fuel burn. The open fan is the logical next step on that curve. The concept has always been sound — the question was whether materials, acoustics, and market conditions would align. CFM is betting that alignment has arrived.

A 20% fuel burn reduction across the global narrowbody fleet would translate to billions of gallons of jet fuel saved and millions of tons of CO2 eliminated annually.

Key Takeaways

The CFM RISE open fan engine eliminates the nacelle, allowing a bypass ratio of ~70:1 and targeting a 20% fuel burn reduction over the LEAP engine.
Three technology breakthroughs — carbon fiber composites, advanced acoustic engineering, and under-wing integration — solve the problems that killed the 1980s GE36 program.
Full-scale ground testing is underway, with a flying demonstrator targeted for 2028–2029 and potential airline service in the mid-2030s.
Certification, noise compliance, and the launch of a new narrowbody aircraft remain the critical hurdles between the test rig and the flight line.
If successful, the open fan represents the largest propulsion architecture change since the turbofan replaced the turbojet, with significant implications for fuel costs, emissions, and pilot training.