Open Fan Would Be Suboptimal On Narrowbody, RTX Chief Scientist Says

PARIS—An open-fan engine such as the RISE demonstrator CFM International has been building would be less efficient than a more conventional ducted turbofan on a next-generation narrowbody aircraft, RTX Chief Scientist Michael Winter said June 13.

His comments come three months after Airbus CTO Sabine Klauke said she saw no major roadblock for the integration of an open-fan engine into an A320 replacement next decade. 

The debate is ongoing between the proponents of the open fan and its expected 20% fuel burn improvement versus those who contend integration challenges may wipe out the benefits of the innovative propulsion system. 

Demonstration projects, including some launched under the EU's Clean Aviation research and technology program, will help conclude the discussion by the end of this decade.

An open-fan engine features a larger-diameter fan for better efficiency. As the accompanying nacelle would make the engine too heavy, the fan is unducted, thus resembling a multi-blade propeller with a stator just downstream. CFM partners GE Aerospace and Safran Aircraft Engines are planning to start RISE ground tests next year.

Design engineers should carefully balance efficiency and the secondary effects of the technology they choose. “The overall efficiency of the engine is the propulsive efficiency multiplied by the thermal efficiency,” said Winter, who was speaking at the Paris Air Forum on the eve of the Paris Air Show. “The propulsive efficiency of a fan gets higher when you slow it down to generate less pressure.”

As thrust is the pressure multiplied by the area, a slower fan such as an open fan needs a larger diameter. At the scale of a narrowbody, it does not integrate well with the overall aircraft, Winter asserted, because “the airframe needs to be modified ... and made heavier to protect the fuselage.” There Winter was referring to certification requirements: with the open fan architecture, fuselage reinforcements might be needed to mitigate a blade-out event.

Moreover, the next generation of aircraft may receive thinner wings designed for a laminar airflow, which is another way to improve efficiency. “If I put a large open fan in front of that, it won't preserve a nice smooth flow over the wing,” Winter said.

“For a higher propulsive efficiency, Pratt & Whitney introduced the geared turbofan (GTF) to slow down the fan just enough, while not compromising the installation on the aircraft,” he continued, alluding to the in-service PW1000G engine family. “At the scale of a single-aisle, a GTF is a much better solution,” Winter said. On the next generation of narrowbodies, Winter expects a Pratt GTF to go just above the current gear ratio of 3:1, for better propulsive efficiency.

RTX engineers are also working on thermal efficiency. “Since the dawn of the jet age, we've come only half way to the theoretical limit, so there is a tremendous amount of runway,” Winter said.

To overcome the challenges brought by higher temperatures, he is counting on ceramic-matrix composites. Moreover, cooling patterns inside turbine blades will become more sophisticated, he predicted. RTX intends to make the most of a new medical technology and is already investing in a new dedicated factory.