Looking 15 years beyond the Leap-1, CFM partner Snecma is heading back into the wind tunnel as it moves toward the ground demonstration of a fuel-efficient, open-rotor engine in 2015 under Europe's Clean Sky civil-aviation research program.

The high-speed aerodynamic test of the 1/5th-scale model in French research agency Onera's large S1 tunnel at Modane will focus on demonstrating the potential fuel-burn reduction with the counter-rotating, open-rotor engine. “Our objective on the open rotor is to reduce fuel burn by 25-30%, compared with the CFM56—and we are very confident we can reach this goal,” says Marc Doussinault, propeller module manager.

Low-speed tests in mid-July focused on aeroacoustic performance and were the third in a series involving the HERA (“helice rapide” or “high-speed propeller”) test rig. Results were promising, Snecma says. The test rig has two compressed-air turbines, one driving each propeller. The first test series used a baseline pair of propellers. The second involved the configuration selected for the demonstrator planned under Clean Sky's Sustainable And Green Engines (SAGE) program.

The new configuration allows the front and rear rotors to rotate at different speeds, giving the engine greater operating freedom and enhancing both aeroacoustic and aerodynamic performance, says Doussinault. “We used a number of different settings, giving us a complete profile of the propeller pair's acoustic and aerodynamic performance,” he adds.

The July tests were conducted at Mach 0.18-0.3—the next tests will cover Mach 0.4-0.8. The data gathered so far has been used to validate design codes and better understand the physical phenomena involved in reducing propeller noise—especially reducing vortices around the blade tips and the wake intensity. “Noise has always been the major problem with the open rotor. But we have made a lot of progress in this area, and we demonstrated that we can meet the new noise standards for certification,” Doussinault notes.

Full-scale propellers are scheduled for 2015 test-stand trials in Istres, located in southern France. The geared open-rotor demonstrator will be based on a gas-generator core from Snecma's M88 engine powering the Dassault Rafale fighter. Blade design for the demonstrator will be frozen soon. “We will start manufacturing the molds in early 2014, then we will need nearly a year to produce the first blades,” he says.