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

SUMMARY: CFM’s RISE open fan ditches the engine cowling to chase a 20% fuel cut, with entry into service targeted around 2035.

CFM International’s RISE program is developing an “open fan” jet engine that removes the cowling wrapped around a conventional turbofan, allowing a far larger fan and a dramatic efficiency gain. The goal is at least 20% better fuel burn and 20% lower carbon dioxide emissions compared to today’s LEAP engines, with entry into service targeted around 2035. If it works, it could power the next generation of single-aisle jets that replace the Airbus A320 and Boeing 737.

Why do jet engines have a cowling around the fan?

Every large jet engine you’ve flown behind is a turbofan. Air enters the front, a large fan grabs a huge slug of it, and most of that air flows around the hot core rather than through it. That air is called bypass air.

Engineers figured out decades ago that moving a lot of air slowly is more efficient than moving a little air fast. That’s why fans have grown steadily larger over the years. Compare the skinny engines on a 1960s Boeing 737 to a modern one, where the fan is so wide the bottom of the cowling had to be flattened and the accessories moved to the side to keep it off the runway.

The ratio of bypass air to core air is called the bypass ratio. Old engines ran around 1:1. The modern CFM LEAP, which powers most of today’s 737s and A320s, runs about 11:1 - eleven times as much air goes around the core as through it.

Why can’t turbofans just keep getting bigger?

The cowling is the wall. Every time you enlarge the fan to move more air, you must enlarge the cowling wrapped around it. A bigger cowling is heavier and produces more drag, and eventually that penalty eats the gain from the larger fan.

At that point you’re paying more for the wrapper than you’re getting from what’s inside. That’s the wall of diminishing returns that ducted engines have essentially reached.

So engineers asked the obvious question: what if you removed the tube entirely? Take the cowling away and there’s no wall. The fan can grow as large as needed, chasing bypass ratios a ducted engine could never reach. The CFM open fan targets a bypass ratio of around 70:1 - not eleven, seventy.

What is the CFM RISE program?

RISE stands for Revolutionary Innovation for Sustainable Engines, and it’s run by CFM International, a 50/50 joint venture between GE Aerospace in the United States and Safran Aircraft Engines in France. Together they build the engines on the majority of single-aisle jets flying today.

CFM launched RISE in 2021. The headline target is the number to remember: at least 20% better fuel burn and 20% lower CO2 versus the LEAP - and LEAP was already a major step over what preceded it.

In the engine business, gains are measured in single percentage points. Companies fight for one percent and build marketing campaigns around three. Twenty percent is a generational jump, which is why the entire industry is watching.

Hasn’t the open fan been tried before?

Yes - and understanding why it failed the first time explains why it might succeed now. In the 1980s, after the oil price shocks of the 1970s, the industry went hunting for radical fuel savings. GE built the UDF (unducted fan), also called the propfan or open rotor, and flew it on a Boeing 727 and a McDonnell Douglas MD-80. It worked, delivering fuel savings in that same 20%-plus range.

Then it died, for two reasons. First, oil got cheap again in the late 1980s, erasing the financial case. Second, the design was loud - those early open rotors used two counter-rotating rows of blades that produced a penetrating buzz nobody believed could pass modern noise rules or satisfy passengers.

The idea wasn’t wrong; it was early. It was waiting on tools that didn’t yet exist.

What changed to make the open fan viable now?

Three things have shifted since the 1980s:

1. Computing. Engineers can now simulate airflow over every square millimeter of a blade, model the exact noise it produces, and refine the shape before cutting metal. The noise problem that was a brick wall in 1986 is now a design variable to optimize.

2. Materials. Modern blades are woven carbon fiber composites rather than solid metal. GE and Safran have built composite fan blades for years, enabling blades that are large, light, strong, and shaped in ways metal never allowed.

3. A smarter design. The old open rotor used two spinning rows fighting each other. The new CFM open fan uses one row of large rotating blades followed by a row of stationary vanes that don’t spin. Those stationary vanes can change their pitch to straighten the airflow and recover energy. One spinning row, one still row - quieter by design and mechanically simpler than the counter-rotating engines of the 1980s.

What are the biggest obstacles to the open fan?

There are real ones, and anyone calling this a done deal is overselling it.

Ground clearance. The open fan is much larger across than a ducted engine of the same thrust. On a low-wing airliner like the A320 or 737, there simply isn’t enough room between the wing and the runway. The whole aircraft may need to change - taller landing gear, or engines mounted at the rear of the fuselage. The engine and airframe must be designed together from a blank sheet. You cannot bolt this onto existing jets.

Blade containment. On a normal engine, if a fan blade fails and breaks off, the steel cowling is built to catch and hold it - a certification requirement called containment. Remove the cowling and there’s nothing to catch a liberated blade, which becomes a free projectile potentially heading toward the fuselage. The safety case shifts entirely: instead of catching a broken blade, engineers must prove a blade will essentially never break, and protect the cabin from the direction a failure would travel. This is one of the biggest questions the Federal Aviation Administration and its European counterpart will spend years working through.

Noise, still. The new design is far quieter than the 1980s version, but quieter than a disaster isn’t the same as quiet. It must meet noise rules that have grown dramatically stricter - both for communities under the approach path and for passengers seated a few feet from the blades. CFM says its modeling shows it can meet the standards, but modeling is a promise, not a certified fact.

When will the open fan actually fly?

CFM isn’t treating this as a science project. In 2024, CFM signed a partnership with Airbus to flight test the open fan, with the plan to mount it on an Airbus A380 - the giant four-engine double-decker - so an experimental engine can be strapped to one wing while three proven engines carry the airplane. Full-engine ground testing is set to ramp up in the middle of this decade, with the flight test to follow.

The realistic timeline for entry into service is around 2035, roughly a decade out. That aligns with when Airbus and Boeing are both expected to launch their next single-aisle aircraft - the replacements for the A320 and 737 families. Whoever builds the next narrow-body will sell it for the next 30 to 40 years, and the engine is the heart of that airplane.

Why this matters for pilots and the industry

Ten years out is real, but it’s also where many aviation dreams quietly die. The open fan, however, is not vaporware: it’s backed by the two companies that build most of the world’s jet engines, it has a genuine flight test partner in Airbus, hardware is being built, and the core physics was proven decades ago. That’s a far stronger position than most “future of flight” concepts.

The two biggest risks aren’t the engineering - they’re certifying an uncontained fan and whether airlines will accept an airplane redesigned from the wheels up to carry it. Technology risk and adoption risk, both real.

Even if the open fan itself doesn’t reach the finish line, RISE is an umbrella program. Beneath it, CFM is also developing a more compact, hotter, more efficient engine core, hybrid-electric systems, and compatibility with sustainable aviation fuel and even hydrogen. Much of that technology flows into the next generation of engines regardless of what the fan on the outside ends up looking like. Unlike the 1980s, this research won’t go back in a drawer.

Key Takeaways

• CFM’s RISE open fan removes the engine cowling, allowing a much larger fan and a target bypass ratio of about 70:1 versus 11:1 for today’s LEAP.
• The program targets at least 20% lower fuel burn and CO2 compared to the LEAP engine, a generational leap in an industry that fights for single percentage points.
• CFM is a 50/50 GE Aerospace–Safran joint venture; RISE launched in 2021, with an Airbus flight test partnership signed in 2024 and an A380 testbed planned.
• The hardest challenges are blade containment certification (no cowling to catch a failed blade) and ground clearance, which may force a redesigned airframe.
• Entry into service is targeted around 2035, timed to the next-generation single-aisle jets replacing the A320 and 737.