CFM RISE, the Open Fan Engine With No Cowling, and the Twenty Percent Fuel Burn Bet on the Airplane That Replaces the Single-Aisle Jet

SUMMARY: CFM’s RISE open fan engine ditches the cowling to target a 20% fuel burn cut on the next 737 and A320 replacement.

CFM RISE is an open fan engine - one with no cowling wrapped around the fan - that GE Aerospace and Safran are developing to power the next generation of single-aisle airliners. By running a single row of large fan blades in the open air instead of inside a duct, the program targets a 20% reduction in fuel burn compared to today’s best Leap engines. If it works, it could replace the ducted turbofan that has defined jet propulsion for the last fifty years.

What Is the CFM RISE Open Fan Engine?

RISE stands for Revolutionary Innovation for Sustainable Engines. Its defining feature is what it leaves off: the round nacelle, or cowling, that wraps around the fan of every airliner engine flying today.

Instead of enclosing the fan in a duct, RISE lets the blades spin exposed to the open air. This is why it’s called an open fan engine. The architecture is a deliberate break from the smooth, cowled shape that has become one of the most familiar silhouettes in aviation.

The companies behind it are betting billions of dollars that the airplane replacing the Boeing 737 and the Airbus A320 will fly behind an engine with no cowling at all.

Why Do Jet Engines Have a Cowling in the First Place?

Every modern airliner uses a turbofan. Air enters the front, a large fan pushes most of it around the outside of the engine core as bypass air, and that bypass air produces most of the thrust.

For fifty years, the entire story of jet engine efficiency has been one idea repeated over and over: make the fan bigger. Move more air, more slowly, and more efficiently. A bigger fan means a higher bypass ratio.

The first turbofans moved roughly one pound of bypass air for every pound going through the hot core. Today’s best engines - the Leap and the geared turbofan on the newest narrow-bodies - move around eleven to twelve pounds of bypass air per pound through the core. That’s why the engines on a new A320 look enormous next to the skinny tubes hanging off a 1970s jet.

But a fan needs its cowling. The duct shapes the airflow, contains the fan, and keeps everything behaving predictably. The problem is a trap: the bigger the fan, the bigger and heavier the cowling has to be. More cowling means more drag, more weight, and eventually an engine so large it won’t fit under the wing without dragging on the runway. Success starts fighting itself.

How Does Removing the Cowling Improve Efficiency?

Engineers have known for decades that eliminating the duct would let the fan run as a truly enormous effective fan, pushing bypass ratios to thirty, forty, or even higher. That represents a large jump in efficiency.

The physics has never been in doubt. The real question has always been whether you can build an open fan and make it safe, quiet, and certifiable.

Has an Open Fan Engine Been Tried Before?

Yes. In the 1980s, after the oil shocks gutted the airlines and made fuel the enemy, General Electric built the Unducted Fan (UDF), also known as the GE36. GE bolted it to the back of a Boeing 727 and later an MD-80 and flew it.

The design used two rows of open, counter-rotating propeller blades at the back of the engine. It worked, and it burned dramatically less fuel. Then the price of oil collapsed, the airlines lost interest, and the concept sat on the shelf for roughly thirty years.

RISE is not a wild new idea. It’s a decades-old idea that engineers never stopped believing in, revived now that fuel prices and carbon targets are both pushing hard.

Who Is Building the RISE Engine?

CFM International is a joint venture between GE Aerospace of the United States and Safran Aircraft Engines of France. Together, CFM built the most successful jet engine program in aviation history.

The CFM56 and its successor, the Leap, power the vast majority of narrow-body jets flying today. When you fly on a 737 or most A320s, there’s a strong chance you’re flying behind a CFM engine.

This matters because RISE isn’t a startup with a slide deck. It’s the reigning champion of narrow-body propulsion signaling that it’s willing to walk away from the ducted turbofan that made it dominant.

How Does the RISE Architecture Work?

RISE uses a single row of open fan blades out front - not the two counter-rotating rows of the 1980s design.

Behind that single spinning row of about a dozen large blades sits a row of stationary blades called stators, which can pivot. Their job is to straighten out the swirl in the airflow and recover efficiency.

The blades are large, built from lightweight carbon fiber composite, and they can change pitch - much like the variable-pitch propeller on many general aviation airplanes. One rotating row, one stationary row, and no cowling.

The target is a 20% reduction in fuel burn versus today’s best Leap engines. In an industry where a strong new engine generation fights for a 10 to 12% gain, 20% is a genuine leap. Because you burn 20% less fuel, you also emit roughly 20% less carbon. RISE is additionally being designed to run on 100% sustainable aviation fuel (SAF) and to be compatible with hydrogen combustion in the future.

What Are the Biggest Challenges for the Open Fan?

There are three honest, serious problems.

Noise. A cowling contains and muffles a great deal of engine noise. Remove the duct and the sound radiates straight out. The 1980s unducted fan was loud, and modern airports enforce strict noise limits with less patient communities around them. The single-row design - rather than two counter-rotating rows - is partly a noise decision: fewer blade interactions mean less of that characteristic buzz, and the pivoting stators help too. But until these engines fly on real airplanes at real airports, noise remains the number one question mark.

Blade-out safety. This is the reason airliner engines carry a heavy containment cowling. If a fan blade fails and breaks off, the duct is designed to catch and contain it, keeping several pounds of carbon fiber traveling at hundreds of miles per hour from striking the fuselage or wing. Take the cowling away and there’s nothing to catch a departing blade. The entire safety case shifts: the blade has to be so well designed that it essentially never comes off, and the airframe has to be positioned and armored so that even a released blade can’t cause a catastrophe. Neither the FAA nor its European counterpart has ever certified a large open fan for passenger service. There is no rulebook yet - it’s being written as the program goes.

Integration. A big open fan is physically large. Hang it under the wing the traditional way and the blades may not clear the ground. Mount it at the back of the fuselage and you change the airplane’s whole balance and structure. This is why RISE isn’t just an engine program - it forces a conversation about what the whole airplane looks like. Airbus has been openly flight-testing and studying open fan integration for its next single-aisle aircraft. The engine and airframe must be designed together as one system; you can’t simply unbolt a Leap and bolt on a RISE.

When Will the RISE Engine Actually Fly?

CFM has been running the RISE technology demonstrator program for several years, working toward ground tests and then flight tests.

GE Aerospace has been preparing a modified Boeing 747 flying testbed, and Airbus has committed to flying an open fan demonstrator on a modified A380 later this decade to gather real acoustic and performance data in flight. Those flight tests are the milestones to watch - everything before them is bench work and wind tunnels.

Even in the best case, an open fan engine wouldn’t enter airline service on a new narrow-body until sometime in the mid-2030s. That assumes the noise works out, the certification path gets written, and the airframers commit to an all-new airplane - a $15 to $20 billion decision that Boeing and Airbus don’t make lightly. There’s a real scenario in which the next narrow-body is more conventional and the open fan waits for the generation after that. This is promising, not guaranteed.

Why the Open Fan Matters for the Future of Flight

This isn’t really a story about one engine. It’s a story about a ceiling.

For fifty years, the industry made the fan bigger and wrapped it in a bigger duct, squeezing the turbofan for every last point of efficiency. We are now genuinely close to the practical limit of that shape. When an industry hits the ceiling of one architecture, it has to change the architecture.

That’s what the open fan represents - not a tweak, but a different answer to the same old question: how do you move a lot of air efficiently? The fact that the most successful engine maker in history is willing to walk away from the ducted fan that made it tells you which way the smart money thinks the wind is blowing.

Key Takeaways

• CFM RISE is an open fan engine with no cowling, developed by GE Aerospace and Safran under CFM International, targeting the next 737 and A320 replacement.
• The program aims for a 20% cut in fuel burn versus today’s Leap engines, with roughly 20% less carbon and compatibility with 100% SAF and future hydrogen combustion.
• It uses one rotating row of about a dozen carbon fiber composite blades plus pivoting stators - no duct - enabling far higher effective bypass ratios.
• The three hardest challenges are noise, blade-out safety and certification, and airframe integration; no large open fan has ever been certified for passengers.
• Airline service is unlikely before the mid-2030s, and depends on an all-new airplane costing an estimated $15–20 billion to develop.