Initial performance and noise studies conducted in the wind tunnel show that an open rotor engine with counter-rotating propellers in pusher configuration will have margin for Stage 4 operations and will likely meet and exceed the more stringent Stage 5 levels to be set by ICAO next year and implemented by 2020, General Electric (GE) says.
The wind tunnel testing took place at NASA's Glenn Research Center in Ohio using a subscale rig with 76cm (30in) fan diameter with a variety of blade designs in a low-speed wind tunnel for acoustic results and a high-speed wind tunnel for cruise performance.
Different composite fan blades were tested, including some made by Snecma, the joint venture partner with GE in the CFM engines business. Snecma has developed a 3D resin transfer mould (RTM) process for building composite fan blades that is being used for the Leap series engines. Snecma says it will build RTM blades for a full-scale open rotor pusher engine prototype set for ground testing in 2015 as part of the European Clean Sky programme.
"Overall the results were outstanding," says Ron Klapproth, GE Aviation's director of commercial product strategy, of GE's wind tunnel tests. "In the 1980s, designers of the first unducted fan architectures had to compromise performance to achieve an acceptable acoustic signature. With what we tested in wind tunnels, we're confident we can design high performance blades [with acceptable] acoustic signatures."
GE envisions such an engine as a potential CFM offering for the next round of clean sheet narrowbody aircraft, designs that are to be 25% more fuel efficient, quieter and more environmentally friendly than today's narrowbodies. The next-generation aircraft are not expected to come to market until the 2030 timeframe due to the decisions by Boeing and Airbus to launch re-engining programmes for the Airbus A320 and Boeing 737.
In January, NASA had discussed a trade study of open rotor propulsors versus geared ducted fan engines, independent of the wind tunnel testing. Results showed that the open rotor was 10-13dB quieter than Stage 4 noise thresholds and 9% more fuel efficient than the ducted fan; the ducted fan's noise was 10-12dB lower than the open rotor noise.
Airlines have a very pragmatic approach to noise requirements however. "What you'll find when you talk to customer base, almost without exception, is that they want you to design to required certification levels for noise, then 'give me every ounce of fuel burn savings that you can.'," Klapproth says.
Klapproth says Stage 4 requirements are 10dB lower than Stage 3, and he anticipates that Stage 5 will likely be 6-8dB quieter than Stage 4. If true, the initial open rotor results would appear to show ample margin to Stage 5 levels.
"We think we can cover Stage 5 without performance penalties to get there," says Klapproth. "We continue to work fundamental technologies required to make the open rotor work, but the timeframe has clearly moved to the right."
GE's next research task with the engine is to study airframe integration of various open rotor designs to determine performance and noise in a realistic configuration. "We're working with various airframers to look at installation," says Klapproth. "Do we keep great performance? That's the next tollgate."
He says "tall poles" in the technology development path include blade-out impacts to the fuselage and icing.
Regulators are also gearing up for the possibility that open rotors will eventually come to market. Klapproth says both the FAA and EASA are "taking a fresh look at certification for this type of architecture".