The CFM RISE Open Fan, the Engine With No Cowling, and the Bet That the Ducted Turbofan Has Hit Its Limit

The CFM RISE program is developing an “open fan” jet engine that removes the cowling entirely, leaving huge fan blades exposed to the airflow. The goal is at least 20% lower fuel burn than today’s best narrowbody engines - a leap the industry believes is impossible to reach with the traditional ducted turbofan. CFM International, the GE Aerospace–Safran joint venture that dominates the single-aisle engine market, is spending billions on the bet that the enclosed fan has run out of room to improve.

What Is the CFM RISE Open Fan Engine?

RISE stands for Revolutionary Innovation for Sustainable Engines. It is run by CFM International, a company owned fifty-fifty by GE Aerospace (United States) and Safran Aircraft Engines (France).

That partnership is not a newcomer. CFM builds the LEAP engine and, before it, the CFM56 - together the best-selling jet engines in aviation history. They power the Boeing 737 and Airbus A320. If you’ve flown a narrowbody in the last forty years, a CFM engine was almost certainly doing the work.

The headline design change is radical: there is no cowling. The fan blades - each roughly as long as a person is tall - spin in the open air rather than inside a duct. It is a real engine under real test, not a concept sketch.

Why Engineers Want to Remove the Engine Cowling

Most of the thrust on a modern airliner does not come from the hot exhaust. Around 80% of the push comes from the big fan up front, which acts like a giant ducted propeller. The core mostly exists to spin that fan.

That single fact explains the entire history of jet engine progress: make the fan bigger. Move more air, move it more slowly, and you get more thrust for less fuel. Engineers measure this with the bypass ratio - the amount of air going around the core versus through it.

Early jets had a bypass ratio near 1:1. Today’s LEAP sits around 11:1. Every increase bought real fuel savings, which is why the cowlings on new aircraft keep getting fatter - they wrap around ever-larger fans.

But there’s a wall. You cannot keep growing the fan inside a duct. The cowling is heavy, and it creates drag. The bigger the fan, the bigger, heavier, and draggier that housing becomes - until the weight and drag of the box cancel out the efficiency gains from the fan inside it. Bypass-ratio improvements have grown smaller, harder, and more expensive with each new engine. The ducted turbofan is scraping the bottom of the barrel.

How Much Fuel Does the Open Fan Save?

Removing the duct lets the fan grow enormous - a diameter of about 13 feet. For scale, the fuselage of a Boeing 737 is a little under 13 feet wide, so the fan is roughly as tall as the airplane’s body is round.

That geometry produces an effective bypass ratio north of 70:1 - not a small step past 11, but a different category of machine. CFM’s target is at least 20% lower fuel burn than the LEAP.

In an industry that celebrates gains of two or three percent, 20% is the entire reason the program exists. Less fuel means proportionally less carbon, and on a single-aisle jet flying six or eight legs a day, it adds up to enormous savings over the life of the airframe.

Haven’t Open Rotor Engines Been Tried Before?

Yes - and the history matters. During the fuel crisis of the 1980s, both GE and Pratt & Whitney flew propfans, also called unducted fans. GE’s UDF flew on a Boeing 727 and an MD-80 testbed. It worked, and it was genuinely fuel efficient. Then oil got cheap again and the idea was shelved for a generation.

Beyond the price of kerosene, two real technical problems kept open rotors on the ground - and both are exactly what RISE must solve:

Noise. An open fan isn’t wrapped in a sound-absorbing duct. The old propfans were loud and had an ugly, buzzy tone caused by two rows of blades chopping through each other’s wakes. In an era of tightening airport noise rules, a loud engine can’t sell no matter how little fuel it uses.
Blade containment. In a ducted engine, if a fan blade fails, the cowling and containment ring are built to catch it before it strikes the fuselage, wing, or passengers. That containment is a hard certification requirement. Remove the duct, and there is nothing there to catch a departing blade.

How RISE Solves the Noise and Safety Problems

The design directly attacks the old weaknesses. RISE uses one row of large rotating blades up front, followed by a row of non-rotating stators (guide vanes). The older propfans used two counter-rotating rows, which is a major source of their buzz. Going to a single spinning row plus a fixed row cleans up much of the noise and simplifies the machine.

What makes this possible now, and wasn’t in the 1980s, is computing and materials. Engineers can now model the airflow, swirl, wakes, and acoustic tones over open blades in ways that were pure fantasy forty years ago. They can shape blades from carbon-fiber composites into curves a metal blade could never hold. CFM claims the engine will meet next-generation noise standards - as quiet as or quieter than today’s ducted engines. That claim still has to be proven, but the tools to get there are real and new.

On blade containment, the answer is part material and part architecture: the composite blades are designed to be extraordinarily robust, there are fewer of them, and they spin much more slowly than a ducted fan. Even so, this is the single hardest obstacle between RISE and passenger service. Convincing the FAA and its European counterpart that an open blade is safe enough to fly over people will be a long, grinding certification fight.

Why the Open Fan Requires a New Airplane

A 13-foot fan is a packaging problem. It will not fit under the low wing of a 737 as it sits on the ramp - there isn’t enough ground clearance. The open fan effectively forces a new aircraft, or at least a substantially new wing set higher off the ground, possibly with a different engine mounting.

That’s why RISE is always discussed alongside the next single-aisle airliner - the eventual replacement for the 737 and A320. The engine and airframe have to be designed together. You cannot simply bolt this onto the fleet flying today.

When Will the Open Fan Enter Service?

CFM has run RISE since 2021, completing extensive ground testing on the open fan rig, the compact core, and hybrid systems. The next major milestone is flight test: the plan is to fly the open fan demonstrator on a modified Airbus A380, swapping one of the jumbo’s four engines for the new unit to gather data in real air. Airbus and CFM have that partnership in place.

Even so, entry into service isn’t targeted until the mid-2030s - and only once Boeing or Airbus actually launches a new single-aisle program to carry it. As of 2026, neither manufacturer has committed to that airplane, so the engine’s schedule is chained to an airframe decision that hasn’t been made.

The honest takeaway: this is not vaporware - there is real metal, real composite, and the two most credible engine companies on earth behind it. But it is also not around the corner. If you fly commercially in the 2030s, you might ride behind one. If you’re a passenger next year, you will not.

Why This Matters for Pilots

The open fan represents the biggest rethink of jet engine architecture in half a century. For pilots, it signals a coming generation of narrowbodies with higher wings, greater ground clearance, and different engine placement - changes that will affect ramp handling, ground operations, and type training. It also marks the industry openly conceding that the familiar ducted turbofan has reached its practical limit. The physics of open rotors never changed; the computing, composites, and manufacturing finally caught up to an idea that waited forty years for its moment.

Key Takeaways

CFM’s RISE program is testing an “open fan” engine with no cowling, targeting at least 20% lower fuel burn than the LEAP.
The open fan reaches an effective bypass ratio above 70:1 with a ~13-foot fan, versus about 11:1 for today’s ducted engines.
Open rotors were flown in the 1980s (GE’s UDF on a 727 and MD-80) but shelved over noise, safety, and cheap oil.
The toughest hurdle is certifying blade containment without a duct; noise is being addressed with a single rotating row plus fixed stators and composite blades.
Flight testing is planned on a modified A380, with entry into service in the mid-2030s - dependent on Boeing or Airbus launching a new single-aisle jet, which neither has done as of 2026.