GE9X For Boeing 777X Delivered For Final Flying Testbed Certification
Providing graphic proof of aviation’s never-ending drive for higher bypass ratios and greater propulsive efficiency, General Electric’s gargantuan GE9X, the largest turbofan ever to fly, is poised to return to the skies in November for a final evaluation campaign on the company’s Boeing 747-400 flying testbed.
Ready for flight test in its definitive production configuration, the 105,000-lb.-thrust GE9X is in development for Boeing’s new 777X twinjet flagship, about to enter the closing stages of an intense and broad-ranging process that began with core tests in late 2015. The effort will clear it for the start of flight tests of the 777-9, the initial 777X-family variant, in March 2019, followed shortly by certification of the engine itself.
The flight-test engine, which is due to be attached to the 747-400 at GE’s Victorville, California, flight-test operations facility in the second half of October, is one of eight GE9X development units in the baseline program. A further batch of eight compliance engines, plus two spares, are also under assembly, with the first expected to arrive at Boeing’s Everett, Washington, site in November for completion with buildup units and accessories prior to installation on the first 777-9.
Rebuilt for the flight-test program, Engine 004 is the same unit that was used for the initial flight phase between March and May. The rebuild adds stronger variable stator vane (VSV) actuator lever arms, which in the original design failed during runs of the second ground-test engine as it was being used to demonstrate extreme performance conditions required to pass the official FAA 150-hr. block test. During this intensive test, the engine was run at triple red-line conditions (maximum fan speed, core speed and exhaust gas temperature) to evaluate the engine at its operational limits.
The flaw in the lever arm, which moves the VSVs to modulate flow through the GE9X’s 11-stage high-pressure compressor section, led to a pause in the program and contributed to a roughly three-month delay to the start of flight tests. Despite this, and the resulting schedule impact, Boeing and GE say the knock-on effect has been minimal and the initial 777-9 remains on track to complete the bulk of tests in 2019 and enter service in 2020. The follow-on 777-8 variant, also powered exclusively by the GE9X, is set to debut in 2022.
“We are building up on the testing. We are nominally about 50% of the way through certification, with lots coming up. We’re going to be very busy over the next 6-9 months,” says GE9X program general manager Ted Ingling. Although acknowledging the lever arm issue led to a more limited initial flight-test program than usual because of the nonproduction-representative configuration of the test engine, “the performance at altitude was just what we were looking for,” Ingling says. “It was a great day when we got the data back from it, which confirmed what we were expecting.”
Designed for long haul at high altitude, the GE9X has a compressor pressure ratio of 27:1, the highest of any commercial engine developed to date, and an overall pressure ratio of more than 60:1. The core steps up the use of ceramic matrix composites (CMC) beyond the pioneering use of the material for turbine shrouds in the CFM Leap 1 and additionally incorporates an advanced high-pressure (HP) turbine blade design and a next-generation twin annular preswirl combustor, among other new technologies.
The core is vital to the overall performance improvement of the GE9X compared to the GE90, with the HP compressor providing a fuel-burn benefit versus previous designs of up to 2%. “We added another stage to get that extra boost, and now we are getting the operability right and making it stall free. That’s where the magic is, plus the efficiency,” says Ingling. The bulk of the fuel-burn benefit comes from the boost in propulsion efficiency generated by the higher-bypass-ratio 134-in.-dia. fan. Comprising just 16 composite blades, compared to 22 on the GE90-94/115 versions and 18 on the GEnx that powers the 747-8 and 787, there is plenty of daylight visible through the GE9X fan.
While the engine worked well in ground tests, it was the flight test that really told the story, says Ingling. “We’ve been fooled by ground engines telling us one answer and altitude engines telling us something different. So, we had a few reservations until we got the engine up to 40,000 ft., which is the main mission for it. Those included all the starting work we had to do where we had logic adjustments for the software to be able to do relights and windmilling and so forth. We will finish up some of those elements when we bring it back [to Victorville],” he adds.
“We were somewhat limited on what we could accomplish in the first configuration but did get an early look at the performance numbers to allow the software engineers to start working on that. This really helped with our schedule,” says GE Flight Test Operations chief test pilot Jon Ohman. “The results we saw were very encouraging, particularly in the ‘up and away’ heart of the envelope testing where the engine will spend most of its time.”
For the upcoming test effort, which is due to run from November through possibly as late as March, “we are predicting a really high operational tempo,” Ohman says. “It’s going to be a busy campaign conducted at a relatively high pace. The first campaign was kind of a snapshot, and this will cover more of the critical milestones for certification. We will explore more of the low-altitude and other edges of the envelope in this test campaign.”
The initial test phase, which encompassed more than 105 hr. of flight time over 18 sorties, “was really clean,” says Ingling. “Nothing jumped out of the campaign which really told us something was an issue,” he adds, reflecting that the main target was optimizing clearances and control schedules. “We’ve had some learnings in the development program, but they’ve all been minor in nature, and once we understood them we got them corrected. The lever arm is the most significant item we came across, and we’ve had lever arm failures in the past on other engines. We were able to pick it up on instrumentation in this case.”
Engines are meanwhile continuing, or are being prepared for, a final phase of certification work at GE’s Peebles, Ohio, ground-test and production facility. Engine 003, having completed crosswind testing, is running cyclic and loads testing of the thrust reverser cascade assembly for Boeing, which is responsible for the overall propulsion system. Engine 002 is being readied for the blade-out test, an arduous graduation exercise for all commercial engine certification programs in which a fan blade will be deliberately released at its root while the engine is running at takeoff power.
Engine 004 is the flying testbed engine, while 005 is running endurance cycle tests in a deliberately unbalanced configuration to evaluate vibration levels. “We make the engine run with more than the usual operating levels of vibration, and we check out the entire engine against field limits,” says Ingling, who adds the engine will also be used for ETOPS certification.
Engine 006 is in final assembly at GE’s Evendale, Ohio, facility, where it is being prepared for a “whole host” of ingestion tests that will take place at Peebles later this year. Following a series of low-pressure turbine “over-temperature” tests, Engine 007 is being refurbished for a second icing certification campaign with what Ingling describes as “lighter and simpler” external systems at the company’s ice test facility in Winnipeg, Manitoba. The eighth test engine is the endurance unit intended for the FAA 150-hr. block test, or triple redline evaluation work that was undertaken using Engine 002 in the first test round in 2017. The engine was due to be installed at Peebles for these tests by mid-October.
The very first complete GE9X engine to run, 001—better known as FETT (first engine to test)—is not expected to contribute further to the certification effort and will be placed in storage. FETT kicked off the full-scale GE9X test program when it fired into life for the first time in March 2016. “We also have a core engine which does aeromechanics work and that is in its second build. We will be pushing all of these out to the test stand through this period and will have a full complement of engines running for the next year. Testing will continue after we have passed certification, to make sure we cover preparations for field readiness and leading indicators,” says Ingling.
The flight test, or “compliant,” engines are being built at GE’s Durham, North Carolina, site. “The first trials of the production process are going to be on the compliant engine. They are wrapping up now and will go out to Boeing around the middle of the fourth quarter,” Ingling says. Parts for long-lead items are also being sourced, but assembly of initial production engines is not due to start until 2019.
A big element of GE’s buildup for production readiness on the GE9X is related to the increased use of CMCs. In all, five components are made from ceramics including the first-stage HP turbine shroud, the first- and second-stage HP turbine nozzles and the inner and outer linings of the combustor. “So, we went from one part on the Leap to five parts on the GE9X, and we developed technology like this using a three-step process; TRL, MRL and SRL [technology, manufacturing and service readiness level],” Ingling says.
“SRL is an extension we use at GE Aviation to cover life-cycle development of our product,” he says. “We lifted the TRL from NASA, MRL from the military, and we have generated our own process for SRL. TRL is, ‘Do you know how to design and is the technology ready?’ MRL is, ‘Do you know how to make it to the design requirement and is the manufacturing process ready?’ SRL is, ‘Do you know how to deal with it once it is in service? Can you repair it and maintain that product?’”
Based on its Leap experience, GE tested the additional CMC parts in a GEnx-1 test engine to evaluate performance and endurance at engine level. With positive results to support a sufficiently high TRL number, the next focus was on production capability and MRL. “We just built our 25,000th turbine shroud [for Leap 1] at our Asheville, North Carolina, facility, so we know we can produce CMCs in large numbers,” he adds. The combination of the technology and production progress led to the decision to commit to more CMCs in the hot section—further improving efficiency by reducing the overall amount of compressed air that needs to be bled off for turbine cooling. Although CMCs require some cooling, they operate at temperatures 500F hotter than traditional nickel alloys.
Not everything planned to be made from ceramics made its way onto the engine, however. All the CMC parts in the GE9X are static components, but GE closely studied using the material for the second-stage HP turbine blade in what would have been the first application of ceramics in a rotating part in a commercial aircraft engine. “It flunked the requirement check when we launched the product. We were not ready from a TRL and MRL perspective, even though it had good performance, durability and lots of value,” says Ingling.
To ramp up CMC production for the GE9X, GE laid up the first combustor liner manufacturing line in Asheville in August and is in the process of laying down the line for the first-stage nozzles. “By the end of the year we will have fully transitioned to all the GE9X parts,” says GE composites and ceramics general manager Mike Kauffman. An individual shipset will comprise 18 shroud segments—just like a Leap 1—as well as 42 nozzles per stage and two combustor liners, one outer and one inner.
While acknowledging CMC technology is no longer a closely kept industrial secret, Kauffman says after 20 years of research and development and $1.5 billion in investment, GE has established itself as a world leader in mass-producing ceramic engine parts. The company is close to finalizing the first vertically integrated CMC supply chain in the U.S. and by 2020 aims to be producing up to 20,000 kg (44,000 lb.) per year of CMC prepreg and 10,000 kg per year of silicon carbide fiber.
“What’s unique about what we do is that we are able to do coatings and infiltration technology at scale and at rate,” says Kaufmann. “With the fiber and matrix, we have solved the ability to coat the fiber in large quantities and to put on chemical vapor deposition coatings in a way that allows us to do it at scale. We can do it in a fiber format that can be made into a tape, which can then be laid out. That’s one of the huge technical hurdles we have overcome.”
Another key breakthrough is the ability to infiltrate or cast parts with silicon with very high densities. “One of the challenges is that many are below 90% density, which in a cyclic fatigue environment is a killer,” he adds. “Our densities are much higher and represent a substantial amount of work that has been done in thermal processing,”
Two years ahead of entry into service the focus is also shifting to SRL, which extends beyond CMC and other components to include everything needed to support the engine, ranging from preparation of maintenance manuals to the location of overhaul shops and development of repair technologies. “Right now we are putting this process in place,” says Ashley Bartowitz, GE9X product support engineering and digital director.
The GE9X is the first all-GE commercial engine to be designed from the beginning within an all-digital strategy that starts in manufacturing and overhaul and lives throughout the life cycle of the engine. “The main goal is utilization, and digital plays a big piece in that. The goal of digitalization is to make sure we are helping the engine reach entitlement,” says Bartowitz, referring to delivery of a fully mature product to the operators.