The Open Fan and CFM's RISE Program, the Unducted Engine Betting That the Next Big Efficiency Jump Comes From Taking the Cowling Off

Two of the world’s largest engine makers are betting billions that the next big leap in aircraft efficiency comes from taking the cowling off the engine entirely. CFM International’s RISE program centers on an “open fan” - large propeller-like blades spinning bare in the slipstream, with no tube around them - targeting a 20% reduction in fuel burn versus today’s best engines. It’s not a new idea; it’s a 1980s concept that the 1980s couldn’t build, now revived because computing, materials, and design have finally caught up.

What Is an Open Fan Engine?

Nearly every airliner you’ve flown on uses a high-bypass turbofan. A big fan at the front grabs air, and most of that air bypasses the hot core where fuel burns - it flows around the core, through the cowling, and out the back. That bypass air produces most of your thrust.

The key metric is the bypass ratio. If an engine moves five times as much air around the core as through it, that’s a bypass ratio of 5:1. Early turbofans in the 1960s sat near 1:1. Modern engines on the Boeing 737 MAX and Airbus A320neo run around 11:1 or 12:1.

That single number is essentially the story of jet engine progress for the last half century. Bigger fan, more slow air, less fuel burned. Higher bypass, better efficiency.

An open fan takes this to its logical extreme by removing the tube. Replace the ducted fan with a set of large, exposed blades, and the fan can grow enormous - delivering a massive effective bypass ratio, because functionally almost all the air is now bypass air flowing around a core with no cowling wrapped around it.

Why Not Just Keep Making the Fan Bigger?

Because the tube fights back. The nacelle around the fan has weight and drag, and both grow as the fan grows. Eventually the cowling costs more than the extra fan buys you.

There’s also a physical limit: ground clearance. Look at a 737 and you’ll see the engine nacelle flattened on the bottom, almost like a hamster cheek, because engineers ran out of room to the runway years ago. There’s nowhere left to go.

Taking the tube off sidesteps both problems at once - which is exactly what CFM is trying to do.

Who Is Building the Open Fan?

CFM International is leading the charge. It’s a 50/50 joint venture between GE Aerospace (United States) and Safran Aircraft Engines (France). Between them, they build the engine on the 737 MAX and one of the two engine options on the A320neo.

By volume, CFM is the most successful jet engine partnership in history, having built tens of thousands of engines. This is the incumbent, not a startup with a rendering.

In 2021, CFM announced RISE - Revolutionary Innovation for Sustainable Engines. The open fan sits at its center, chasing that 20% fuel-burn reduction. In an industry where a new engine generation excites everyone at 15%, 20% is genuinely large. And fuel isn’t just a cost line - it’s carbon, range, and payload. That figure changes the economics of an entire fleet.

Why Didn’t We Already Have Open Fan Engines?

We almost did. Roll back to the early 1980s, when two oil-price spikes had left airlines desperate for efficiency. NASA ran a program studying advanced turboprops, and General Electric built the Unducted Fan (UDF).

GE bolted it onto the back of a McDonnell Douglas MD-80 and flew it. A demonstrator flew at the Farnborough and Paris air shows in 1988, and people who saw it never forgot it: two rows of curved blades spinning in opposite directions, out in the open, on the tail of a jet. Engineers called it a propfan, and on paper the fuel numbers were spectacular.

Then two things killed it. Oil got cheap again in the late 1980s, erasing the financial urgency overnight. And the engine was extremely loud.

Noise is the open fan’s oldest enemy. A fan inside a tube is quieter partly because the tube’s inner walls are lined with sound-absorbing acoustic liner that soaks up noise before it escapes. Take the tube away and you’ve taken away your muffler. Back then the noise went straight into the sky - and straight into the cabin, rattling passengers in the back rows.

The propfan didn’t die because the idea was wrong. It died because the timing was wrong and the tools weren’t ready.

What’s Different About the Open Fan This Time?

Three things changed since 1988.

First, computing. The hardest part of an open fan is shaping the blades so they’re efficient and quiet - and so any second row behaves correctly in the churning wake off the first. In 1988 you couldn’t simulate that airflow; you built a blade, flew it, and found out. Today, computational fluid dynamics lets engineers model that swirling air blade by blade and iterate thousands of times before cutting metal. The noise problem didn’t get easier - our ability to attack it did.

Second, materials. Big spinning blades want to be heavy, and heavy blades are dangerous and inefficient. CFM builds its blades from three-dimensional woven carbon fiber composite, a technology Safran has matured for years on current engine fan blades. Light, strong, and shapeable in ways aluminum never allowed.

Third - and this is the clever one - the RISE design has only one rotating row of open blades. The 1980s propfan used two counter-rotating driven rows, which was mechanically brutal. RISE puts a single row of spinning blades in front and a set of fixed, non-rotating guide vanes behind them. Those stationary vanes straighten the swirl and recover energy without the nightmare of a second powered row. It’s a quieter, simpler, smarter version of the old idea.

What Are the Risks and Open Questions?

The advantages are real, but so are the challenges.

Noise isn’t settled. It’s better, not solved. Airport noise rules have grown much stricter since 1988, not looser, and the open fan must meet standards the original propfan never faced. CFM says its design will comply - but until a real engine flies a real certification profile, that’s a claim, not a result.

Integration is a blank-sheet problem. An exposed blade with no tube changes everything about how you mount an engine. The blades are so large that ground clearance under a wing becomes a serious problem again - which is why some concepts move the engines to the rear fuselage, echoing that MD-80 demonstrator. The engine and airframe must be designed together from scratch, a far bigger commitment than hanging a new engine on an existing jet.

The uncontained-blade question is genuinely hard. Regulators, including the FAA, spend enormous effort ensuring that if a fan blade fails, the tube contains it. There is no tube. So the entire safety case has to be rebuilt around a blade that essentially cannot fail - or an airframe positioned so that if one ever did, it wouldn’t strike anything critical. That certification path is not fully paved.

When Will Open Fan Engines Enter Service?

Here’s the honest timeline. CFM has framed RISE as a technology demonstration program through the middle of this decade. The company has partnered with Airbus to flight-test an open fan on a modified A380 - the four-engine testbed - later this decade.

Note the words carefully: flight test, not entry into service. The realistic path is that if everything goes well, an open fan powers a new single-aisle airliner sometime in the second half of the 2030s - the aircraft that eventually replaces the 737 and A320. And in this business, that timeline will move. It always does.

Why This Matters for Pilots

The splashy futures of flight - electric air taxis, hydrogen aircraft, supersonic jets - get the headlines. But the single biggest cut in aviation’s fuel burn over the next fifteen years probably won’t come from any of them.

It will most likely come from a tube-and-wing airliner that looks almost exactly like the one you flew last week, with one strange difference out on the wing: the cowling gone, and the blades turning slow in the open air. Sometimes the future of flight isn’t invention - it’s patience, waiting for the tools to catch up to the concept.

Key Takeaways

• CFM International’s RISE program, announced in 2021, is developing an open fan engine targeting a 20% reduction in fuel burn versus today’s best engines.
• Removing the cowling lets the fan grow far larger, delivering a huge effective bypass ratio - modern ducted engines top out around 11–12:1 - while sidestepping the weight, drag, and ground-clearance limits of an ever-bigger nacelle.
• The concept first flew as GE’s Unducted Fan on an MD-80 in 1988, but was killed by cheap oil and severe noise.
• Three advances revived it: computational fluid dynamics, 3D-woven carbon fiber blades, and a simpler single rotating row plus fixed guide vanes (versus the 1980s twin counter-rotating design).
• CFM plans to flight-test the open fan on a modified Airbus A380 later this decade, with a potential new single-aisle airliner in the second half of the 2030s - though noise certification and uncontained-blade safety remain unresolved.