Development is progressing on schedule for GE’s Passport 20 engine, which is scheduled for certification in 2015 and is expected to enter service in 2016 on Bombardier’s Global 7000 and 8000 ultra-long-range twinjets.
While GE has earned renown for its military and commercial engines, until recently its experience in the business aviation arena had been limited to the CF34, which has powered Bombardier’s large-cabin Challengers for the past 30 years (as well as the airframer’s CRJs and Embraer’s ERJ regional jets).
The Passport program, which will cost GE and its joint-venture partners Japan’s IHI and Safran subsidiary Techspace Aero, more than $1 billion in development costs, represents a bit of a departure for Cincinnati-based GE Aviation (Booth No. N5500). “We’ve had derivative engines in that market space for a long time,” said Judd Tressler, Passport engine program manager, during a pre-show media tour at the company’s Cincinnati manufacturing plant. “This is the first time we actually went after business- and general-aviation in a large-cabin bizjet independent of whether we’re going to do something on the commercial side or not.”
Derived from GE’s E-core
The new engine (formerly known as the Tech-X until its rebranding two years ago) is, like its larger commercial Leap sibling, derived from GE’s E-core program, which was started nearly a decade ago. “Those are really the basis for both the Leap and the Passport,” said Tressler, who also serves as the powerplant maker’s director of Bombardier programs and small commercial engines. “The Passport is scaled down a little bit and the Leap is scaled up a little bit and the E-Core demonstrators that we have are in the middle.”
Both engines share an overall high-pressure ratio of more than 40, which GE says will give the Passport 20 an 8-percent better specific fuel consumption (SFC) than its closest competitor, the Rolls-Royce BR725, which powers the Gulfstream G650.
Company engineers predicted the exact date of the first engine to test (Fett) more than a year in advance (in fact it was ready a day earlier). That first test at the end of June was a success, running for more than 130 hours, including 220 starts without a problem, and accomplishing all of its objectives by the time the initial test ended on August 12. While Bombardier has specified an installed thrust of 16,500 pounds (slightly less than the BR725’s 16,900), a maximum net thrust of 19,200 pounds was achieved during testing.
Lack of Vibration
One immediate result of the test was a notable lack of vibration, attributable largely to the use of a single piece 51.9-inch fan blisk rather than a traditional separate blade-and-hub system. “This is a new technology,” noted Tressler, who recalled addressing initial customer concerns about the possibility of foreign object damage to the blisk. “It’s line replaceable, so you are going to be able to replace it on-wing if you have to,” he said, adding that the engine maker installed the blisk on the Fett using the same tool that would be used in that rare situation.
The titanium blisk starts out as a separate hub with stubs to which the blades are friction-welded, resulting in a bond stronger than the actual metal itself, minimizing the threat of blade-out situations. At the size of the Passport, Tressler said, the all-metal unit will still be lighter than if constructed using composites and it will retain the aerodynamic efficiencies from the use of a metal blade as well.
The company makes a compelling argument over the use of traditional fan blades, which dovetail into slots on the hub. “No matter what you do, if you put two pieces together, you are going to have a gap, and that gap is going to cause air leakage, which causes performance loss,” explained Tressler. He added that another benefit was realized by removing the dovetail architecture and associated weight and size of the hub. “On a blisk, I can pull the diameter of my hub in so I can get more airflow through the same annulus area,” he said. Without separate blades, the process of lubricating the friction joints between the blades and the hub is eliminated on the engine.
As a new engine, the Passport, not surprisingly, is host to a variety of recently developed processes and technology. The airfoils in its compressor are the first to receive an ultra-smooth proprietary finish developed by GE. Whereas normal airfoils might have a 20-micron finish and appear smooth to the eye, on the Passport they receive a surface finish on the 4- to 5-micron range, after being placed in media and vibrated at a certain frequency. As a result, “the little inconsistencies on the surface are so small that they are below the normal laminar boundary layer of the part and therefore the part looks smooth to the air,” Tressler said. In addition to improving airflow, with a smoother part there are fewer microscopic ridges for grime to collect in, which helps maintain the efficiency of the compressor.
To increase fan flow, the company has created an integrally finned surface cooler system composed of metal parts that form a band circling the interior of the fan case. The fins are attached to blocks through which channels of oil flow, all of which serves to cool the air flowing through the engine. “It turns out to be a much more efficient way to do it, so you don’t get the fan pressure loss,” noted Tressler, comparing the system to traditional air-cooled oil coolers.
Perhaps the biggest introduction in the engine is the use of ceramic matrix composite (CMC) parts in key areas for the first time in a commercial engine. Since 2011, the divergent seals at the exhaust end of the F414 engine, which powers the F/A-18 Hornet, have used the material. “It’s an afterburning environment, very harsh with a lot of acoustic energy, a lot of temperature and a lot of hot-streaking,” said principal engineer Bernie Renggli, “a very challenging environment, and that’s where we really got to know this material system well.”
The Passport incorporates 15 of the oxide-oxide composite parts on three assemblies, which, combined, account for a weight savings of approximately 45 pounds per engine. “It’s very similar to typical polymeric matrix composite processing,” said Renggli. A specialized cloth is run through a slurry bath to form a pre-impregnated spool of material that requires refrigeration until use. The material has a cumulative room temperature life of seven days, therefore, its time out of the freezer is carefully clocked. Once rolled out, it is cut into patterns and applied to a form, much like papier-mâché. The entire mold is then placed into a vacuum bag and undergoes a lamination cycle in an autoclave. The part is then removed from the mold and undergoes the additional step of heating in a sintering furnace, which oxidizes it, removing all organic components. “From there you can machine it with conventional tools,” said Renggli. “Due to the way it’s manufactured, it’s a much lower cost system, and we’ve got some repair techniques as well.”
Among the CMC parts is the center body, a cone-shaped structure that protrudes from the back of the engine. Approximately 30 inches high with a more than 20-inch diameter on the front end, the part is surprisingly light, weighing around nine pounds. Surrounding it is the mixer, a highly complex pleated structure that takes three days to lay out on its form. It combines the engine’s core flow with the fan flow to minimize pressure loss.
“When we started thinking about mixers, a lot of people said, ‘You can’t make that out of CMC; you guys are nuts,’” said Renggli. “Well, we did it anyway.” The CMC material is suitable for temperatures up to 1,800 degrees F, while the maximum temperature on the Passport is 1,250 degrees F, leaving a comfortable margin.
The other CMC parts are large panels that are part of the fixed fan duct. The five-foot long panels can be easily removed, allowing access to the core beneath. The fan cowl itself is designed as a clamshell, allowing easy access to components for maintenance.
The Passport will come wrapped in a long-duct mixed-flow nacelle, which is being developed integrally with the engine by Nexcelle, a joint-venture between GEs Middle River Aircraft Systems and Safran’s Aircelle. While such architecture may be uncommon for GE’s civilian aircraft, the company is able to leverage technology from its military applications such as the F110 engine that powers the F-16. “If you look at all long-range large-cabin bizjets, not only aesthetically is it the way to go because looks are very important, it’s also a more efficient design form,” said Tressler, noting the nacelle is being designed with accessibility in mind.
Once in service, GE expects benefits from its next-generation full-authority digital engine control system (Fadec), the same system used on the commercial Leap engine. The system is pre-flagged with many of proactive maintenance actions. “On our previous engines we could detect what’s happening,” said Tressler. “This is going to [have] prognostic capabilities. Based on the number of increased sensors, the Fadec will be able to forecast what components are in danger of failing, due to the trends that it is monitoring.”
As of the beginning of the month, the company scheduled crosswind testing at its Peebles, Ohio engine test facility, followed by ingestion testing in two engines currently undergoing completion. Another blade-out test is slated for next month.
By the end of the year, GE expects to have five engines in testing. With the pylon definition already completed, flight testing is expected to commence next year, as the Passport will be the first engine tested on GE’s newest Boeing 747 flying testbed. “We are going to have over 32 tests, eight full engines with 20-plus builds,” said Tressler. “Before we ever fly with Bombardier, we will have over 4,000 hours and 8,000 cycles on the engines.”