Open Fan CFM RISE vs Turboprop: How these Engines Differ

Airbus is collaborating with CFM International (a joint venture between GE Aerospace and Safran) on a pioneering open fan engine technology under the RISE (Revolutionary Innovation for Sustainable Engines) program. This technology is viewed as the cornerstone for the next generation of narrow-body aircraft.

Here is a Detailed Breakdown of the Technology:

The Core Architecture

“Unducted” Fan: The most striking feature of the new Airbus/CFM engine is the absence of a nacelle (the heavy outer casing or “ring” found on traditional jet engines).

Massive Bypass Ratio: By removing the casing, the engine can accommodate fan blades that are significantly larger than those on a standard engine.

This results in an ultra-high bypass ratio (the amount of air that flows around the engine core vs. through it), reaching levels as high as 70:1.

Greater Efficiency: Because larger fans move a massive volume of air at a lower velocity, they are far more efficient than smaller fans encased in ducts.

2. The RISE Program Technology

While early open-fan designs from the 1980s were criticized for extreme noise, the Airbus-CFM RISE technology includes modern innovations to solve these issues:

Single-Stage Rotating Fan: Unlike older “counter-rotating” designs (two sets of blades spinning in opposite directions), the RISE engine uses a single stage of rotating carbon-fiber blades.

Variable Pitch Stators: Behind the rotating fan is a set of stationary “stator” vanes. These vanes can change their angle (pitch) to “clean up” the airflow and reduce noise, significantly improving the engine’s acoustic profile to meet modern regulations.

Carbon Fiber Composite Blades: The blades are made from advanced 3D-woven carbon fiber composites, which are incredibly strong yet light, allowing them to withstand the high centrifugal forces of high-speed flight without the protection of a duct.

3. Sustainability and Performance Goals

The primary driver for Airbus’s adoption of this technology is environmental impact.

20% Fuel Emission Reduction: The goal is to reduce fuel consumption and CO2 emissions by more than 20% compared to today’s most efficient engines (like the CFM LEAP).

SAF and Hydrogen Compatibility: The engine is being designed to run on 100% Sustainable Aviation Fuel (SAF) and is also being researched for potential integration with liquid hydrogen combustion.

4. Integration and Flight Testing (The A380 Demonstrator)

One of the biggest challenges for Airbus is how to mount such a large engine on a traditional aircraft. Because the fan is so large, it cannot easily be placed under the wing of a standard low-wing plane like an A320 without hitting the ground.

Airbus is using its A380 (MSN0001) flight-test aircraft as a demonstrator. They have modified the aircraft to carry one Open Fan engine on a pylon located on the upper fuselage/wing junction to gather data on how the engine interacts with the aircraft’s aerodynamics.

Flight Test Timeline: Ground and wind-tunnel testing are currently underway, with flight tests on the A380 expected to occur in the mid-2020s.

5. Why is Airbus is Investing Now?

Current turbofan technology is reaching a point of diminishing returns; engines cannot get much bigger because the weight of the casing (the duct) becomes too heavy, canceling out the efficiency gains.

By opening the fan, Airbus and CFM are breaking through this weight-to-diameter limit, paving the way for a new era of ultra-efficient commercial aviation by the mid 2030s.

While an Open fan engine and a Turboprop engine may look similar because both have exposed blades, the Airbus – CFM RISE (Open Fan) technology is a generational leap over Turboprop technology.

The primary difference is that the Open Fan is designed to match the speed of a jet, whereas a turboprop is physically limited to much slower speeds.

Here are the Specific Technical Differences

1. Cruise Speed and Altitude: Turboprop is designed for short-haul, low-altitude flights. They are most efficient at speeds around Mach 0.5 to 0.6 (approximately 350 to 400 miles per hour. If they try to go faster, the straight blades suffer from compressibility drag, where shockwaves form on the blades, causing a massive loss in efficiency and high noise.

Open Fan engine technology is Specifically engineered to fly at Mach 0.7 to 0.8 which is approximately 500 to 600 miles per hour which is the standard cruise speed for modern narrow-body jets like the A320. It allows airlines to maintain their current schedules while using 20% less fuel.

2. Blade Aerodynamics (The “Scimitar” Factor): Turboprop technology typically uses relatively thick, straight blades. These are excellent for generating high thrust at takeoff but are inefficient at high speeds.

Open Fan technology Uses highly thin, swept-back “scimitar” blades made of 3D-woven carbon fiber. The sweep allows the blades to “trick” the air into behaving as if it is moving slower than it actually is, preventing the formation of efficiency-robbing shockwaves at high flight speeds.

3. The Stator System vs Propeller: Turboprops Usually consists of a single set of rotating blades. Any swirl or spiral motion in the air left behind the propeller is essentially wasted energy.

Open Fan (RISE): Features a unique architecture where a set of rotating fan blades is followed by a set of stationary, variable-pitch stator vanes.

These stators act as aerodynamic straighteners; they catch the spiraling air from the front fan and straighten it out to provide extra thrust and significantly reduce the noise that plagued older unducted designs.

4. Bypass Ratio: While Turboprop engines move a lot of air, we don’t usually measure them by bypass ratio. The engine core is relatively small and optimized for low-speed torque.

The Open Fan is essentially an ultra-high bypass turbofan with the the outer ring, known as cowling, removed. It achieves a bypass ratio of 70 to 1.

For comparison, the most advanced engines on planes today (like the CFM LEAP) have a ratio of about 11 to 1. The Open Fan moves a vastly larger volume of air, which is the secret to its extreme fuel efficiency.

5. Pitch Control and “Jet” Integration:
Turboprop Uses a constant speed governor where the pilot or computer changes the blade angle to maintain a certain RPM.

An Open Fan engine Uses a highly integrated FADEC (Full Authority Digital Engine Control) that manages both the rotating blades and the stationary stators simultaneously.

It behaves more like a modern jet engine, automatically optimizing the pitch of all surfaces for every stage of flight (climb, cruise, descent) to minimize the noise footprint.

In short, a Turboprop is a tractor designed for efficiency at low speeds, while the Airbus Open Fan is a “high-speed hybrid” designed to deliver the efficiency of a propeller with the performance and speed of a turbofan jet engine.