CFM RISE: where does the open fan engine stand?

For decades, airliner engines have all looked the same: a large fan enclosed in a nacelle. The RISE programme (Revolutionary Innovation for Sustainable Engines) run by CFM International, the 50-50 joint company of GE Aerospace and Safran Aircraft Engines, proposes to do away with that casing. Unveiled in 2021, it is not an engine for sale but a technology demonstration programme intended to prepare the next generation.

The idea: move more air, more gently

A turbofan produces most of its thrust by accelerating a large mass of air around the engine core. The larger that mass and the more gently it is accelerated, the better the propulsive efficiency. The limit is the nacelle: enlarging the fan of a ducted engine makes the whole assembly heavier and increases drag.

The open fan gets round the problem. The fan blades turn in the open air, allowing a much larger diameter without a nacelle. Behind them, a row of fixed vanes (OGVs, outlet guide vanes) straightens the airflow swirled by the fan so as to recover energy that would otherwise be lost. The result resembles a propeller, but is designed to fly at speeds close to those of today's airliners.

The stated goal: over 20% less fuel

CFM is targeting fuel burn more than 20% better than commercial engines in service today. That gain does not rely on the fan alone. The programme is also working on:

• a compact core with high thermal efficiency, using materials that withstand higher temperatures;
• hybrid electric technologies, to assist the engine during certain phases of flight;
• compatibility with alternative fuels: CFM says it has carried out tests with 100% sustainable aviation fuel (SAF).

What has been tested

In a press release published on 18 July 2026, ahead of the Farnborough Airshow, CFM gave a progress report. The programme reports around 500 test campaigns and more than 3,000 endurance cycles. Milestones cited include:

• mechanical and materials tests on fan blades and OGVs: impact, ingestion, fatigue and icing;
• wind tunnel tests whose aeroacoustic results exceeded the maturation targets;
• more than 1,000 hours of testing on a first high-speed low-pressure turbine;
• more than 2,000 dust ingestion cycles on next-generation high-pressure turbine airfoils;
• two hybrid electric engine ground tests by GE Aerospace, and the launch by Safran of PHILEAS, a full-scale hybrid electric ground test campaign at Istres.

Preliminary design reviews have been completed for the fan, the OGVs and the compact core. Safran also plans to test a full-scale open fan front module in a new 8-metre test cell at Villaroche.

The remaining challenges

Removing the nacelle raises questions engineers have known about since the high-speed propeller trials of the 1980s:

• noise, both in the cabin and on the ground, since nothing shields the blades any more;
• integration on the aircraft: a large-diameter fan means rethinking the engine's position under the wing and its ground clearance;
• certification, in particular protecting the airframe in the event of a blade failure, a role the nacelle partly fulfils on today's engines.

The timeline

CFM says these technologies are being matured towards ground and flight tests this decade. They are meant to form the basis of the next generation of CFM engines, which could be available in the second half of the 2030s. The final choice of architecture will also depend on aircraft manufacturers and the configuration of their future single-aisle aircraft.

Key points

• RISE is a CFM (GE Aerospace and Safran) demonstration programme, not yet a product.
• The open fan removes the nacelle to enlarge the fan and improve propulsive efficiency.
• The target is fuel burn more than 20% lower than today's engines.
• Ground and flight tests are planned this decade, for an engine generation envisaged in the second half of the 2030s.