The CFM RISE Open Fan, the Unducted Engine With Blades You Can See, and the Bet That the Next Big Jump in Fuel Burn Means Taking the Cowling Off

The CFM RISE program aims to deliver the biggest change to the jet engine in roughly 40 years by removing the cowling around the fan entirely. This “open fan” or unducted design targets at least 20% lower fuel burn than today’s LEAP engines by making the fan far larger than any duct could contain. Flight testing on a modified Airbus A380 is planned for around the 2027-2028 timeframe, with entry into service unlikely before the mid-2030s.

What Is the CFM RISE Open Fan Engine?

RISE stands for Revolutionary Innovation for Sustainable Engines. It comes from CFM International, the partnership between GE Aerospace in the United States and Safran Aircraft Engines in France.

CFM is not an outsider making a wild claim. It builds the LEAP engines on most narrowbodies flying today, and before that the CFM56, likely the most-produced jet engine in history. This is the establishment making a radical bet.

The bet is simple: if the cowling is the problem, take the cowling off. An open fan puts the big fan blades out in the open air, with no ring around them.

Why Did Airliner Engines Get So Fat?

Most of the thrust on a modern airliner does not come from the hot jet exhaust. It comes from the big fan up front, which acts like a many-bladed propeller in a duct, shoving a huge column of cold air around the engine core. The hot core mostly exists to drive that fan.

Engineers track this with the bypass ratio - the amount of air going around the core divided by the amount going through it. On the first Boeing 707, that ratio was less than 1:1. Those engines were loud, thirsty, and inefficient.

The physics favors moving a huge amount of air backward gently over throwing a small amount backward fast. A big, slow column of air beats a small, fast jet for fuel burn every time - the same reason a helicopter with a giant rotor hovers more efficiently than one with a tiny rotor.

So for four decades the industry chased higher bypass ratios. A new Airbus A320neo moves roughly 11 to 12 times as much air around the core as through it. That is why nacelles have grown enormous.

Why Can’t Engines Just Keep Getting Bigger?

The industry has hit two walls.

Ground clearance is the first. The engine hangs under the wing, and the wing is only so high off the ground. You cannot keep enlarging the fan without the engine scraping the pavement or forcing tall, heavy landing gear. Airbus and Boeing are both wrestling with this today.

The cowling itself is the second. That duct shapes airflow, contains a broken blade, and quiets the engine - but it also adds weight and drag. On a very high-bypass engine, the nacelle accounts for a large fraction of the engine’s drag and weight. Past a certain point, the ducted fan eats its own gains.

Taking the cowling off lets the fan grow far larger, without the weight and drag of the duct, and lets designers reposition it to solve ground clearance. The result is a huge effective bypass ratio - numbers like 60:1 - without a huge cowling.

Haven’t We Seen Open Rotor Engines Before?

Yes, and the honest history matters. In the 1980s, during the oil shocks, both GE and Pratt & Whitney built open rotor engines. GE’s was the UDF (unducted fan). GE flew it on a Boeing 727 and an MD-80, and it worked, delivering fuel savings of around 30%.

Then fuel got cheap again, and the idea went in a drawer for 35 years. Nobody wanted a weird-looking, loud engine to save fuel that was suddenly inexpensive.

What’s Different This Time?

Three things have changed since 1986.

Blade design caught up. The entire concept lives or dies on blade shape. Where 1980s engineers relied on wind tunnels, slide rules, and early computers, CFM now uses enormous physics simulations that model airflow over every millimeter of a blade. The blades are 3D-woven carbon fiber composite, twisted and swept into shapes you could never machine from metal. The blade is the technology.

The noise problem got tamed. Old open rotors used two counter-rotating rows of blades, and the rear row chopping through the front row’s wake created a distinctive buzzsaw sound. RISE keeps only one spinning row. The rear row is a set of stationary vanes - a stator - that straightens the swirl before it becomes noise. CFM says the design will meet the noise rules airports enforce today.

Fuel and carbon now cost real money and carry regulation in a way they did not in the 1990s. The airlines finally have a reason to want this.

How Much Fuel Does RISE Actually Save?

The headline is at least 20% lower fuel burn compared to today’s LEAP engines. That number deserves perspective.

A typical new engine generation buys maybe 15%, sometimes less, and it takes a full clean-sheet design to get there. The jump from the CFM56 to the LEAP was in that range. A 20% improvement on top of the already-excellent LEAP is effectively two generations of gain in one leap.

Twenty percent less fuel is 20% less carbon dioxide, one for one. The engine is also being designed to run on 100% sustainable aviation fuel and to be compatible with hydrogen combustion later - a further path on top of the 20%.

RISE is really a bundle of technologies. It includes a compact, higher-temperature, higher-pressure core using ceramic composite materials in the hot section that tolerate temperatures which would melt the metal parts they replace, plus hybrid-electric integration where electric motors help manage the engine across phases of flight. The open fan gets the headlines because you can see it, but the core advances matter just as much.

What Are the Real Problems With an Open Fan?

There are three, and none are small.

Integration is the big one. A conventional engine hangs in a neat pod under the wing - a problem solved for 60 years. An open fan is a huge spinning disc of blades. Hang it under the wing and you may reintroduce the ground-clearance problem. That is why much of the concept art mounts these engines near the tail, sometimes as pushers. But moving engines to the tail means designing a whole new airplane, not modifying a 737 or A320. That is expensive, slow, and risky.

The blades are not contained. On a ducted engine, the cowling is built to catch a failed fan blade - a certification requirement that has saved lives. An open fan has no cowling to catch a blade. CFM must prove to the FAA and European regulators that a blade will essentially never liberate, and that passengers are protected if the worst happens. No one has fully cleared that bar for a passenger airplane yet, and it can add years.

Cabin noise and public acceptance remain unproven in service. RISE is quieter than the old open rotors, but a ducted turbofan’s cowling also acts as a sound barrier between the blades and the cabin. Whether an open fan is as quiet for passengers over a long flight has to be demonstrated with real people in real seats.

Where Does the RISE Program Stand? (As of August 2026)

RISE is officially a technology maturation program - CFM is not selling an engine you can order next year. The company is running hundreds of individual technology tests through the 2020s to prove out the pieces.

The milestone to watch is a flight test. CFM and Airbus have agreed to fly an open fan demonstrator on a modified Airbus A380, using the four-engine jumbo as a flying testbed with the open fan on one side. That testing is planned for roughly the 2027-2028 timeframe. GE has already run open fan blade tests in wind tunnels, including at a NASA facility, with encouraging published results.

A testbed engine is a long way from certification. Realistic entry into service is the mid-2030s at the earliest - 2035 is the number people cite, and in this industry, dates that far out tend to slip.

Why This Matters for Pilots

This does not change your next type rating. If you are flying today, an open fan is not something you will see in the near term - it may be an aircraft you fly in the back half of your career, if at all.

The signals that separate a real program from pretty renderings are worth tracking: watch for the A380 flight test to actually happen and log real hours; watch what regulators say about blade containment, the gate everything else waits behind; and watch whether Airbus or Boeing commits to an actual new airplane built around the engine. An engine with no airframe to live on is just a science project.

The Bigger Picture

For 40 years, progress meant the same move repeated: make the fan bigger, wrap it in a duct, raise the bypass ratio. RISE is what happens when that straight road runs out. You do not get the next 20% by doing more of the same - you get it by discarding an assumption everyone stopped questioning, in this case that the fan must live inside a cowling.

Sometimes the future of flight is not a brand-new idea. It is an old idea that was 35 years too early, waiting for the computers, the materials, and the fuel prices to finally catch up.

Key Takeaways

CFM RISE removes the engine cowling to enable a much larger fan, targeting at least 20% lower fuel burn than today’s LEAP engines - effectively two generations of improvement at once.
The open fan achieves an effective bypass ratio near 60:1 and is designed for 100% sustainable aviation fuel and future hydrogen compatibility.
The concept revives GE’s 1980s UDF, but is enabled now by 3D-woven carbon fiber blades, physics-based design, and a single spinning row plus a stator that cuts noise.
The biggest hurdles are airframe integration (likely tail-mounted, requiring a new aircraft) and blade containment certification with the FAA and European regulators.
Flight testing on a modified Airbus A380 is planned for around 2027-2028, with entry into service unlikely before the mid-2030s (2035 or later).