GE Aviation last month began certification testing of the GE9X powerplant for the Boeing 777X widebody airliner following more than a year of ground trials with the first engine-to-test (FETT), marking culmination of a long process of analysis and refinement of the various elements of its architecture. For the 100,000-pound-thrust-class GE9X, the company fundamentally changed its approach to flight testing, waiting for more than a year after the first run of the FETT before starting testing of the certification engine.
“It has been a huge success,” said GE9X general manager Ted Ingling. “We did something different on this program that we had not done and I think will become our foundation going forward, and that was to separate that first engine from the second engine and subsequent engines by a little bit more than a year…We finally concluded that the best answer to that was to get ahead of it, have enough time in there to understand any findings you have and fix them for all the follow-on [engines] without being in a scramble.”
Most recently, GE performed the 9X’s icing trials in Peebles, Ohio, completing a process that historically proved difficult to predict for a variety of reasons, explained Ingling. In this case, the engine passed the more than 50 validation points GE had prescribed and covered certification basis and beyond, allowing the company to calibrate its tools, all a year in advance of icing certification tests. “It’s a great place to be,” said Ingling. “If we found something in this test we needed time to go react to it and go change the architecture before we got to the cert test, because the one thing that we can’t control is the winter season.”
The 9X icing regime starting in December marked the first time in almost 11 years GE had run such testing in Peebles, primarily because the company needed to modify its facility in Winnipeg, Canada, to accommodate the size of the engine for next winter’s certification tests. As for the recently completed trials, Ingling reported an exceptionally smooth program. “We walked away quite frankly with a very high confidence in our engine and a significant program risk abatement executed,” he said. “On the icing tests we demonstrated that the engine really needed no modification to go through the certification testing.”
Overall, testing on the FETT showed the need for some adjustments in rotor-stator clearances in the compressor and a change in grind dimensions in the high-pressure turbine. “We found a change in the hot to cold conversion that we needed to make on one of the two stages of the turbine,” said Ingling. “That was a finding through the use of clearanceometers and [a digital] X-ray…We run these machines at hair-width apart from the stators, and we want to dial them in as close as practical, accounting for manufacturing variation, ovalization, maneuver loads, all of those things.”
Notwithstanding the discovery of a need for minor modifications to the compressor and HP turbine, GE thinks its decision to wait for the close of FETT testing before running the second engine has resulted in the smoothest transition to the certification phase it has ever experienced. The resulting highly refined set of technologies and design elements deliver a 10-percent fuel burn advantage over the Boeing 777-300ER’s GE90-115B.
Part of the fuel-burn improvement will come from what GE advertises as the highest pressure ratios among any commercial engine in production; the 9X design calls for a 60:1 overall pressure ratio and a 27:1 pressure ratio in the high-pressure compressor. Of course, higher pressure ratio means a higher operating temperature in the back of the compressor and high-pressure turbine, hence the call for new nickel-based disc alloys and ceramic matrix composites (CMCs) for the inner and outer combustion liner, stator parts, the Stage 1 high-pressure turbine shroud and the Stage 1 and 2 high-pressure turbine nozzles.
While the company’s large-scale application of CMCs in commercial engines started with the Stage 1 high pressure turbine shroud in the CFM International Leap engines on which GE and Snecma collaborate for narrowbody airplanes, the recognition of their benefits began more than 20 years ago out of the military’s quest for light weight, high-temperature resistance and durability in the hot sections of its engine platforms, explained Ingling.
Last October GE finished the second phase of GE9X CMC component testing in a GEnx demonstrator engine, accumulating 1,800 hours while exposing it to harsh environmental conditions. The level of debris exposure equated to about 3,000 takeoff and landing cycles.
“I’m an ex-fan designer and I think of the CMCs like the PMCs (polymer matrix composites) that went into the fan blade in the GE90 and that has become our fan blade material,” said Ingling. “CMCs are the same transformation that’s going to happen in the hot section as PMCs happened in the fan and low-pressure compression system.
“As we get these engines bigger and bigger—this is the biggest engine in the industry from a fan diameter standpoint—having composite materials that can take you into those huge spaces and not pay a weight penalty is a critical design parameter.”
While increasing the size of the fan in the GE90-115B from 128 inches to 134 inches in the 9X, designers reduced the number of blades from 22 to 16, improving bypass ratio and the overall propulsive efficiency of the engine. To allow for fewer fan blades, GE developed its fourth-generation of composite material and a stainless steel, rather than, titanium leading edge. Both changes increase blade strength, allowing designers to make them wider, longer and thinner to improve airfoil performance.
Changes to the compressor include an increase in the number of stages from 10 in the base GE90 and nine in the GE90-115B to 11 in the 9X, leading to a boost in pressure ratio from 19:1 to 27:1. Subsequent compressors—namely those found in the Leap and GEnx—use a 10-stage design. Ingling explained that GE spent a lot of time in the development of the compressor rigs to validate the effects of the pressure rise. “It wasn’t so much about the pressure ratio that would come from that stage,” he noted. “We know we can get the extra pressure. It’s really about the efficiency of getting that compression and the operability of the airfoil stages, stage-by-stage-by-stage as you continue to increase the pressure.”
Now in the process of building the third engine for crosswind certification and general air mechanics and a fourth test engine for flight aboard a 747-400 flying test bed in Victorville, California, by the end of this year, GE has targeted the end of 2018/early 2019 for certification. It plans to use a total of eight engines for the testing program. Originally planning for certification in the third quarter of 2018, the company has adjusted the schedule, said Ingling, to align with Boeing’s certification demands for the 777-9X.
“It’s an integrated product development strategy between the two big pieces of it,” said Ingling. “We specifically line up to Boeing because…they’re going to be doing the compliance program and start flying real engines and real airplanes to certify the aircraft, so we have to stay completely aligned with their objectives and schedules.”