The CFM RISE Open Fan, the Unducted Engine Aviation Tried and Abandoned in the Eighties, and the Twenty Percent Fuel Bet Coming Back to the Single-Aisle Jet

CFM International’s RISE open fan is a next-generation jet engine that removes the cowling around the fan entirely, exposing large curved composite blades to open air. The design targets more than 20% lower fuel burn than today’s LEAP engine - a generational leap in a single step - and is aimed at powering the replacements for the Boeing 737 and Airbus A320 around 2035. It revives an idea the industry tested and shelved in the 1980s, brought back because the economics of fuel and carbon have decisively shifted.

What Is the CFM RISE Open Fan Engine?

An open fan strips away the smooth cylindrical duct and nacelle that wrap around a conventional jet engine’s fan. What’s left is a bare row of long, swept, scimitar-shaped blades spinning in open air, with a second row of stationary vanes mounted directly behind them to straighten the airflow and recover energy.

The program is called RISE, short for Revolutionary Innovation for Sustainable Engines, and CFM International launched it in 2021. The headline goal is more than 20% better fuel burn than the current LEAP engine, which also translates to roughly 20% less carbon emitted.

That number is the whole story. A typical new engine generation celebrates a 10 to 15% improvement. A gain above 20% is not an increment - it’s a step change, and you don’t get there by refining existing hardware. You get there by changing the shape of the engine.

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. The partnership is older than most of the pilots flying the aircraft it powers.

CFM is not a boutique experimenter. It is the best-selling engine maker in the history of jet aviation. If you’ve flown on a Boeing 737 or Airbus A320 in the last four decades, you’ve almost certainly flown behind a CFM engine - first the CFM56, then today’s LEAP. When this company redesigns the single-aisle engine from a blank sheet, the entire industry pays attention.

Why Remove the Duct? The Bypass Ratio Problem

Every large fan engine works on the same principle: a big fan pulls in a column of air, most of which bypasses the hot core and gets pushed straight out the back. That bypassed air produces most of the thrust and most of the efficiency. The ratio of cold bypass air to hot core air is the bypass ratio.

For 50 years the goal has stayed the same - move more air, move it more gently, burn less fuel. Today’s LEAP has a bypass ratio around 11:1. That’s already very good.

The problem is physical. Moving more air means a bigger fan, which needs a bigger duct and heavier nacelle. Eventually the cowling grows so large it won’t fit under a normal wing without stilt-like landing gear. The ducted fan has almost nowhere left to grow.

Removing the duct breaks that ceiling. Freed from the nacelle, an open fan can move far more air, pushing the bypass ratio up toward 70:1. That is where the 20% fuel savings comes from.

What Makes the Blades Different?

The fan blades are carbon fiber composite, large, and variable pitch - like the propeller on many general aviation aircraft, they rotate on their own axis to find the right bite for takeoff, climb, and cruise. There are roughly a dozen of them in the front row.

This isn’t a party trick. GE and Safran have built composite fan blades for the LEAP for years, quietly maturing the capability for over a decade.

We Already Tried This in the 1980s

Here’s the part most people don’t know: the industry already flew this concept 40 years ago.

In the early 1980s, oil prices had gone vertical and the industry was desperate for fuel savings. GE built the UDF (unducted fan), also called the GE36 - open blades, no cowl, the full concept. It was flown at airshows on a Boeing 727 and an MD-80 as real hardware in the sky in 1986 and 1987. Pratt & Whitney and Allison chased the same idea with their own design.

Then it died, for two reasons. First, oil got cheap again and the funding panic evaporated. Second - and this one still matters - it was loud. A duct is where you hide the acoustic liners that muffle engine noise. Take the duct away and you take away the muffler. In an era of tightening noise rules, nobody wanted to certify it or sit behind it.

What Are the Risks and Engineering Challenges?

The open fan carries three major hurdles, and they’re the reason this is genuinely difficult rather than merely expensive.

Noise. This is the ghost from the 1980s. CFM argues that modern blade-by-blade airflow simulation, the noise-canceling second row of stator vanes, and far quieter blade designs have changed the equation. They may well be right - simulation today is light-years ahead of the 1980s. But it’s unproven at scale, and airport noise rules are far stricter now than in 1986. That has to be demonstrated, not asserted.

Blade containment. On a conventional engine, if a fan blade fails, the surrounding case is built like a vault to contain it - a hard federal requirement, tested by deliberately blowing a blade off to prove the case holds. An open fan has no case. The blade itself must be proven so robust it effectively cannot fail, and the airframe must be arranged so that if the improbable happens, no one in the cabin is in the line of fire. This is the single biggest technical obstacle.

Integration. The engine is physically larger in diameter than the LEAP. It can’t simply bolt where the old engine went - it changes mounting points, landing gear sizing, and the entire airframe layout. That’s precisely why the aircraft makers are already involved.

Why This Matters for Pilots

If the open fan reaches service, it will reshape the single-aisle fleet that flies the overwhelming majority of airline routes. The physics point to roughly 20% lower fuel burn and carbon output, along with compatibility with sustainable aviation fuel and, potentially, hydrogen down the road.

For crews, that means new engine handling characteristics, a wider engine changing ground clearance and ramp awareness, and eventually new type training built around a fundamentally different powerplant. The airplane your future passengers board in the 2030s may look distinctly unfamiliar on the wing.

The Timeline: What to Watch and When

CFM has run ground and component testing through the middle of this decade. The milestone everyone is watching is a flight test on a real airborne testbed: CFM and Airbus have agreed to mount an open fan demonstrator on an Airbus flying laboratory later this decade.

The target for entry into service is around 2035 - not coincidentally, about when Boeing and Airbus are expected to launch the successors to the 737 and A320. CFM is building the engine for an airplane that hasn’t been designed yet, betting the open fan is what hangs off the next single-aisle jet.

The honest engineering read: the physics are sound, and were sound in 1986. What killed the concept then was economics and noise. The economics of fuel and carbon have flipped completely in its favor; noise and containment are now engineering and certification challenges rather than dead ends. There is metal, there are blades, there is a flight test partner and a date - this is not vaporware.

But 2035 is far off, and certification of a caseless fan has never been done on a modern airliner. The decisive moment will be the flight test. The day an open fan flies on the Airbus testbed and produces real noise numbers off a real airplane is the day this stops being a promise and becomes a program. Everything before that is simulation - good simulation, but the air always gets the final vote.

Key Takeaways

• CFM RISE targets more than 20% better fuel burn than the LEAP engine, achieved by removing the fan duct and pushing the bypass ratio from about 11:1 toward roughly 70:1.
• The engine uses variable-pitch carbon fiber composite blades - about a dozen in the front row - with a second row of stationary vanes to straighten airflow and cut noise.
• The concept was flown in 1986–1987 as GE’s UDF/GE36 on a Boeing 727 and MD-80, but was shelved due to cheap oil and excessive noise.
• The three hardest challenges are noise, blade containment (there is no case), and airframe integration due to the engine’s larger diameter.
• Entry into service is targeted around 2035, aligned with the expected 737 and A320 replacements; the key milestone is an open fan flight test on an Airbus flying testbed later this decade.