Airbus and CFM International presented the new visual identity of an Airbus A380 that will be converted into a flying laboratory for testing Open Fan propulsion technology at this year’s Farnborough International Airshow. By the end of the decade, the aircraft is expected to participate in an extensive flight-test campaign as part of the CFM RISE programme, one of the aviation industry’s most important current initiatives focused on next-generation propulsion.

The newly unveiled livery marks the programme’s gradual transition from research and ground testing towards the integration of a full-scale demonstrator engine on an aircraft. Engineering teams from Airbus and CFM have recently completed the first conceptual flight-test design review, confirming the basic technical assumptions for installing the engine, modifying the aircraft and preparing the future test campaign.

CFM International, a joint venture between GE Aerospace and Safran Aircraft Engines, launched the RISE programme in 2021 with the goal of developing technologies capable of reducing fuel consumption and carbon dioxide emissions by more than 20 percent compared with today’s most efficient engines for single-aisle aircraft.

RISE is not the development programme for a single defined production engine. Instead, it encompasses a broad range of technology demonstrators intended to prepare propulsion systems for the next generation of commercial aircraft expected to enter service during the 2030s.

The most recognisable part of the programme is the Open Fan architecture. Unlike a conventional turbofan engine, in which the fan blades are enclosed within a large nacelle, the Open Fan uses exposed blades with no surrounding fan casing.

Removing the nacelle makes it possible to use a significantly larger fan and move a greater mass of air around the engine core. A larger share of the thrust is therefore generated by accelerating a large quantity of air by a relatively small amount, rather than accelerating a smaller quantity of air to a much higher velocity.

This method is more energy-efficient and allows the engine to achieve a substantially higher bypass ratio than conventional turbofan designs. CFM says the single-stage Open Fan could have a bypass ratio more than five times higher than that achievable by even the most advanced ducted engines.

The fan blades are expected to be manufactured from carbon-fibre composites to reduce mass while providing the strength and fatigue resistance required for long-term commercial operations.

Although its appearance resembles that of a turboprop engine, the Open Fan is intended for commercial aircraft operating at cruising speeds comparable with those of today’s jet-powered airliners. Preserving high cruising speeds while improving fuel efficiency is one of the architecture’s principal advantages.

At the same time, removing the protective fan casing creates several engineering challenges. The exposed blades must meet strict safety requirements covering bird strikes, foreign-object damage, vibration, fatigue and possible blade failure.

Safran has already conducted extensive mechanical testing of the blades, including foreign-object impact tests and prolonged vibration trials. Hundreds of hours of aerodynamic testing have also been carried out in wind tunnels.

Across the broader RISE programme, thousands of endurance cycles and hundreds of individual test campaigns have examined not only the Open Fan but also a compact engine core, high-temperature materials, advanced combustion systems, electrical components and technologies associated with hybrid-electric propulsion.

Before the demonstrator takes to the air, it will undergo an extensive series of ground tests. These will evaluate its operation at different thrust settings, blade behaviour at high rotational speeds, vibrations, temperatures, gearbox reliability and the performance of the engine control system.

Aerodynamic and acoustic testing will be particularly important. The Open Fan must demonstrate not only lower fuel consumption but also acceptable noise levels inside the cabin and around airports.

Engineers are therefore analysing the shape, curvature and spacing of the blades, their rotational speed and the interaction between the fan and the airflow generated by the aircraft’s wing and fuselage.

Testing has already included reduced-scale demonstrators in wind tunnels at speeds representative of commercial aircraft cruising at approximately Mach 0.8. Tests are also examining an underwing engine installation to determine how the wing, engine mounting structure and Open Fan influence one another aerodynamically.

The Airbus A380 will enable the programme to move from controlled laboratory conditions into the real flight environment. The demonstrator Open Fan will be installed on a specially designed mounting structure positioned on the side of the fuselage, behind and above the wing.

The four existing engines will continue to power the aircraft, while the experimental engine will operate independently as a test article.

This arrangement will allow the new propulsion system to be evaluated safely while enabling precise measurements of its effects on airflow around the fuselage and wing. The A380 is particularly suitable for this role because of its size, payload capacity and the large amount of internal space available for instrumentation, data-processing systems and engineering workstations.

The joint Airbus and CFM test campaign will measure actual propulsive efficiency, fuel consumption, thrust and aerodynamic drag, as well as structural loads transferred from the engine mounting structure to the aircraft.

The Open Fan will be evaluated during take-off, climb, cruise, thrust changes, descent and approach. Engineers will also examine its operation under different atmospheric conditions and at various altitudes and airspeeds.

Sensors installed throughout the engine, mounting structure, wing and fuselage will measure pressures, temperatures, vibrations, structural deformation and local airflow velocities. The results will then be compared with computer simulations and wind-tunnel data to validate the engineering models used to design future aircraft.

One of the key questions will be the effect of the engine on aircraft handling. Because the Open Fan has a significantly larger diameter than the fans used on current commercial engines, engineers must carefully examine aircraft behaviour during thrust changes, crosswind operations and abnormal situations, including a sudden shutdown of the demonstrator engine.

The flight-test campaign will also include measurements of both internal and external noise. Microphones installed inside the aircraft and on the ground will record the propulsion system’s acoustic signature during different phases of flight.

The results will be used to further optimise blade design, rotational speed and the position of the engine relative to the aircraft. Propulsive efficiency, aerodynamic integration, structural loads and noise are therefore equally important elements of the programme.

Another major challenge is the integration of this propulsion architecture into future single-aisle aircraft. The large fan diameter may require taller landing gear, a different engine position or a completely redesigned wing.

Airbus is consequently participating not only as the operator of the flying laboratory but also as an aircraft manufacturer assessing how the technology could be incorporated into the next generation of commercial airliners.

CFM is simultaneously developing a smaller and more thermally efficient engine core. The compact core is intended to operate at higher pressures and temperatures while using advanced metallic alloys, ceramic matrix composites and additively manufactured components.

The programme also includes combustion systems designed to operate with 100 percent sustainable aviation fuel. Some technologies are being developed with the possibility of later integration into hybrid-electric propulsion systems.

The European part of the programme is additionally supported through Clean Aviation initiatives. These projects cover demonstrator assembly, aircraft integration, flight approval, the test campaign itself and the analysis of the collected data.

The objective is to bring the demonstrator to a level of technological maturity sufficiently advanced to support decisions on a possible future production engine.

In addition to technical integration, the aviation industry is already examining how Open Fan aircraft could be accommodated in everyday airport operations.

These studies include safety procedures, aircraft movement on the apron, ground handling, maintenance, airport infrastructure and the regulatory framework that would be required for the commercial operation of engines with exposed blades.

The Open Fan concept itself is not new. Similar unducted fan and propfan architectures were intensively developed during the 1980s and early 1990s, when high fuel prices encouraged manufacturers to search for more efficient propulsion systems.

Among the best-known demonstrators were GE’s GE36 unducted fan and Pratt & Whitney and Allison’s 578-DX propfan. The GE36 was flight-tested on a Boeing 727 and later on a modified McDonnell Douglas MD-80, while the 578-DX was also installed and tested on an MD-80.

Those demonstrators achieved promising fuel-consumption results, but concerns over noise, mechanical complexity, passenger acceptance and economic viability limited further development. Falling fuel prices also reduced the commercial pressure to introduce such a radically different propulsion architecture.

Today’s Open Fan programme therefore does not represent the invention of an entirely new engine concept. Instead, it is a modern interpretation of an architecture that was already demonstrated in flight several decades ago.

Its return has been enabled by advances in composite materials, computational fluid dynamics, digital engine controls, manufacturing methods and acoustic modelling. Modern engineers can now address many of the limitations that prevented earlier propfan concepts from reaching commercial service.

The first test flights of the A380 demonstrator are expected by the end of the decade and will be conducted as part of Airbus’ flight-test activities in Toulouse.

A successful campaign would not automatically mean that the Open Fan will immediately become a production engine. The results will first allow aircraft manufacturers, engine producers and regulators to determine whether the technology can satisfy commercial requirements for safety, reliability, noise, maintainability and operating economics.

Should the results confirm current expectations, Open Fan technology could become one of the propulsion options for the next generation of single-aisle aircraft expected to follow today’s Airbus A320neo and Boeing 737 MAX families during the 2030s.

The presentation of the A380’s new test livery at Farnborough is therefore more than a promotional exercise. It is a visible indication that a propulsion concept previously demonstrated on aircraft such as the MD-80 is returning in a technologically far more mature form and is once again approaching its most important test: operation in real flight conditions.