The CFM RISE Open Fan, the Unducted Engine With No Cowl, and the Twenty Percent Fuel Burn Bet That Could Reshape the Single-Aisle Jet

The CFM RISE program is developing an “open fan” engine - one with no cowl around the fan blades - that targets 20% lower fuel burn than today’s best narrow-body engines. Built by CFM International, the joint venture behind the world’s most common airliner engines, it represents the first serious attempt in decades to change the fundamental shape of a jet engine rather than just its size. If it clears certification, it could power the single-aisle jets that replace the Boeing 737 and Airbus A320 sometime in the second half of the 2030s.

What Is the CFM RISE Open Fan Engine?

An open fan (also called an unducted fan or open rotor) is a jet engine with no fan cowl. Instead of a large fan enclosed in a smooth cylinder, a single row of long, curved blades spins out in the open air. Behind that spinning row sits a set of stationary blades called guide vanes, which straighten the swirl coming off the rotating blades so the energy pushes the aircraft forward instead of twisting the air into a corkscrew.

The result is an enormous effective fan diameter - roughly 13 feet across, wider than a single-aisle cabin is tall - with no heavy cowl and no duct drag to carry.

RISE stands for Revolutionary Innovation for Sustainable Engines. The program has been running since 2021.

Who Is Building It?

CFM International is a 50/50 joint venture between GE Aerospace on the American side and Safran Aircraft Engines on the French side. This is not a startup with a slide deck - it’s the most experienced narrow-body engine maker alive.

Most pilots have flown behind CFM’s work without knowing it. The CFM56 is one of the best-selling jet engines in history, hanging under most older Boeing 737s and a huge share of Airbus A320s. Its successor, the LEAP engine, powers the 737 MAX and the A320neo flying today.

Why Take the Cowl Off? The Physics of Fuel Burn

Every jet engine is, at heart, a big fan. On a modern airliner, the vast majority of thrust comes not from the hot core exhaust but from that big fan moving an enormous column of air around the outside of the engine.

Engineers measure this with the bypass ratio - the amount of air going around the core versus through it. The iron rule of engine efficiency is simple: to burn less fuel, move more air, more slowly. A bigger fan, a gentler push, a higher bypass ratio.

That’s why engines have grown fatter for decades:

CFM56: bypass ratio around 5–6 to 1
LEAP: roughly 11 to 1
Open fan (target): about 70 to 1

Every step up that number is a step down in fuel burn. CFM’s target for the full RISE package is 20% lower fuel burn than the LEAP - and the LEAP was already about 15% better than the engine before it.

Why Can’t Ducted Engines Just Keep Getting Bigger?

The traditional approach is hitting a wall. A higher bypass ratio needs a bigger fan, which needs a bigger cowl. That cowl is heavy and draggy, and eventually the fan grows so large the engine won’t fit under the wing without stretching the landing gear into stilts.

The Boeing 737 already shows this strain. It sits low to the ground, and fitting a modern high-bypass engine underneath has required flattening the bottom of the cowl and getting creative. The ducted fan is approaching the limit of how efficient it can get before it becomes too big and heavy to bolt onto an airplane.

Removing the cowl sidesteps that limit entirely.

If Open Fans Are So Efficient, Why Don’t They Already Exist?

The open rotor is not a new idea, and the reasons it never reached an airliner are exactly what makes it hard.

In the 1980s, after fuel prices spiked, GE built and flew the UDF (unducted fan). They mounted it on a Boeing 727 and an early McDonnell Douglas MD-80 and showed it off at the Farnborough and Paris air shows. The fuel savings were real and large. Then fuel got cheap again, the decade ended, and the idea was shelved.

But economics weren’t the only killer. Two engineering problems went unsolved:

1. Noise. The duct is the very thing that contains the sound. A ducted engine lines its cowl with acoustic material that soaks up noise like studio foam. An open fan has no wall - fast-moving blade tips and air slamming from the front row into the stationary row create a distinctive buzzing drone. Modern noise rules, the standard regulators call Chapter 14, are strict, and a cowl-less engine must meet them anyway.

2. Blade containment. On a normal engine, if a fan blade fails, the cowl is built to catch it - a containment case, and certifying it is one of the hardest parts of building any engine. Take the shield away, and a departing blade has nothing to stop it from heading toward the fuselage. This remains the single scariest safety question in the entire design.

What’s Changed Since the 1980s?

The airframe hasn’t changed much. What’s changed is everything you can’t see.

The blades. In the 1980s the blades were metal, in two rows spinning in opposite directions - mechanically brutal and a doubled noise source. The RISE design uses a single spinning row of carbon fiber composite blades, woven and shaped with a precision that simply wasn’t manufacturable 40 years ago. Fewer blades, one row, carefully swept to cut noise.

The math. In the 1980s, to learn how a blade would sound or how air would swirl off it, you built it, ran it, and listened. Today that problem lives inside a computer first. CFM can simulate airflow and acoustics blade by blade and iterate a shape hundreds of times in software before cutting any metal - the quiet revolution behind the whole program.

The blade-out problem. The approach being chased is a combination of blades strong enough to be extraordinarily unlikely to fail, plus careful placement so a departing blade does the least harm. To be clear, this is not yet solved - it’s an open certification question, and both the FAA and its European counterpart, EASA, are involved early because everyone knows it’s the crux of the program.

RISE Is More Than the Open Fan

The program is deliberately hedged. Beyond the open fan, RISE also includes:

A compact engine core redesigned to run hotter and more efficiently
Hybrid electric elements
Full compatibility with 100% sustainable aviation fuel (SAF) and, down the road, hydrogen

That’s smart engineering and smart politics. Even if the open fan hits a certification wall, the improved core and other technologies can migrate into a more conventional ducted engine. CFM is building a toolbox, not betting the entire future on the cowl coming off.

When Will the Open Fan Actually Fly?

Here’s the honest timeline, current as of July 2026:

Since 2021: Ground testing of components - blades, cores, and the full architecture - building confidence piece by piece.
Second half of the 2020s: Flight testing. GE Aerospace plans to fly an open fan on a testbed, and a partnership with Airbus will mount one on an A380 - the four-engine jumbo has enough spare wing to carry an experimental engine and still fly home safely on the others.
Early-to-middle 2030s: Technology maturity.
Second half of the 2030s: A passenger-carrying aircraft, if the certification questions get answered.

One crucial caveat: there is no launched airplane for this engine yet. An engine doesn’t carry passengers until an airframe is built around it, and Boeing and Airbus have repeatedly delayed deciding when to replace the 737 and A320. The engine may well be ready before there’s an airplane to hang it on.

This is not vaporware - but it is not next year, and anyone promising you’ll board one soon is selling something.

Why This Matters for Pilots

The balanced scorecard, pilot to pilot:

On the promise side. A 20% fuel cut is the kind of gain that comes once every couple of engine generations - real physics, not hype. The company building it has more narrow-body experience than anyone, and the design is flexible enough to deliver value even if its boldest element doesn’t survive.

On the problem side. Noise is a genuine hurdle, and airport communities get a vote. Blade containment without a containment case is the hardest safety question in aviation propulsion right now, and it isn’t closed. Integrating an engine this large may force real airframe changes - possibly mounting engines at the rear or high on the wing - and the timeline depends on Boeing and Airbus decisions entirely outside CFM’s control.

For six decades, the jet engine story has been the same: make the fan bigger, wrap it in a cowl, chase the bypass ratio. The open fan changes the shape of the answer instead of just its size. It looks strange because it is strange - our eyes were trained on the cowl. If it works, the airplane your kids fly on could look genuinely different from yours, all traceable to one iron rule: move more air, more slowly.

Key Takeaways

The CFM RISE open fan removes the engine cowl entirely, using a single row of composite blades plus stationary guide vanes to reach an effective bypass ratio near 70:1 and a targeted 20% fuel-burn reduction versus the LEAP.
CFM International (a GE Aerospace–Safran joint venture) builds the CFM56 and LEAP engines that power most single-aisle jets today, giving RISE serious pedigree.
The two historic obstacles - noise (must meet Chapter 14 standards) and blade containment without a duct - remain the program’s toughest, still-unsolved certification questions.
Advances in carbon-fiber composite blades and computer simulation of airflow and acoustics are what make CFM believe the concept can succeed where the 1980s UDF failed.
Realistic timeline: ground and flight testing through the late 2020s (including an A380 testbed with Airbus), technology maturity in the early-to-mid 2030s, and a possible passenger aircraft in the second half of the 2030s - with no airframe yet launched to carry it.