Large Bizjet Programs Spur Engine Development
Most activity in business jet engine research and development is taking place for business aircraft at the top end of the size range. Snecma (Booth 5515) is developing the Silvercrest for the Dassault Falcon 5X, while Pratt & Whitney Canada (Booth 3834) has readied a new variant of the PW307 for the newly revealed Falcon 8X. The Québec-based manufacturer is also running the PW800, a demonstrator in the 10,000- to 20,000-lb-thrust range. GE (Booth 5551) is working on its Passport engine for Bombardier’s Global 7000 and 8000. Only Honeywell (Booth 6622) looks relatively quiet, as its HTF7500E is closing in on certification, while Rolls-Royce (Booth 5855), which is producing engines for super-midsize to ultra-long-range jets but has no development ongoing, is thinking of the next generation.
Snecma says its Silvercrest is on time for certification next year. The 11,450-lb-thrust engine (at sea level, ISA+20C) is to power the Falcon 5X. A 11,000-lb-thrust rated version is to equip the Cessna Longitude.
As of early May, a Gulfstream II had been modified as a flying test bed for the Silvercrest but still had to make its maiden flight. The modification, performed by Texas-based Sky Aerospace Technology (in the same group as Sierra Industries), apparently has taken longer than expected, as the first flight was postponed several times last year.
Snecma intends to obtain a permit to fly the modified aircraft in the U.S., with one of its original Rolls-Royce Speys and one Silvercrest. After a first series of flights in the U.S., it will be refitted with its two Speys and ferried to France. After the transatlantic flight, the Silvercrest will again replace one of the Speys. An extensive flight test program will then take place at Snecma’s development center in Istres, starting this summer.
“We don’t need flight tests for certification but we want to reassure our airframers by operating the engine in realistic conditions,” program general manager Laurence Finet said. Finet also said the company finds such flight tests useful, too, because some differences exist with ground testing; for example, flight testing allows aircraft attitude to be factored in.
A total of seven Silvercrest engines are involved in the ground-test program. Tests have started recently in an altitude simulation chamber. Still on the to-do list are bird and ice ingestion, as well as endurance testing. Emissions and noise have not been measured yet but Finet expressed her confidence, as initial trials on the core engine had given encouraging results. Compared to exiting engines in this class, Snecma targets a 15-percent cut in fuel burn, a 50-percent margin below the CAEP6 NOx emission standard and a noise level 20 EPNdB below Stage 4.
The French engine manufacturer is also gearing up for production. The assembly line is up and running, and it is planning to deliver the first engines to Dassault in the second half of this year. Snecma is responsible for the integrated powerplant system, including the nacelle, thrust reversers and mounting systems. Final assembly takes place in Villaroche, just south of Paris, and podding will happen in Toulouse.
Rival manufacturer Pratt & Whitney Canada started running a full PW800 engine demonstrator late last year, company vice president of business aviation and military programs Michael Perodeau told AIN. He confirmed that the demonstration effort has progressed according to plan but gave little detail. The engine maker is “preparing itself for the eventuality of a program launch” in the 10,000- to 20,000-pound-thrust range, said Perodeau. Pratt & Whitney Canada would thus be able to develop an engine in “36 months, notionally, from program launch.”
The PW800 uses a core of the PW1000G geared turbofan series. “Using a common core gives us a head start, as PW1000G engines will be in service with airlines before the PW800 is certified,” he noted. This should translate into durability and reliability, thanks to the experience these flight hours will bring.
For the PW800, Pratt & Whitney Canada targets a two-digit percent improvement over in-service engines in both fuel burn and emissions. The PW800 schedule revealed last year calls for flight-tests this year.
Perodeau would not disclose the precise thrust of the demonstrator. It is “not pertinent,” he said, as the technology is scalable within the 10,000- to 20,000-pound range. Three core sizes are already available in the PW1000G family, and PW800 designers had to choose just one.
The PW800 was to power the Cessna Citation Columbus before that business jet was cancelled in July 2009. Perodeau explained that Pratt & Whitney Canada is now waiting for an opportunity to arise–either a new-generation aircraft or an upgrade program. The PW800 could suit business jets from the large-cabin to the ultra-long-range aircraft segments.
Pratt & Whitney Canada is also developing a PW307 version, the PW307D, for the Falcon 8X trijet. The PW307D provides 6,725 pounds of thrust at sea level, ISA+17C conditions. This represents a 5-percent increase over the 7X’s PW307A. Specific fuel consumption has been cut thanks to improved fan seals, impeller clearances and exhaust mixer, as well as a new Fadec.
Engine certification is expected in March next year with reproduction engines due to be delivered to Dassault shortly. The three PW307Ds that will power the 8X on its first flight have already flown on Pratt & Whitney Canada’s Boeing 747 flying testbed.
Since the last EBACE, GE ran the first full Passport, which is the 16,500-pound-thrust engine for Bombardier’s Global 7000 and Global 8000. The first ground test took place on June 24, 2013, and the first campaign finished last August, completing all test objectives, according to Judd Tressler, director of Bombardier programs for GE. “Also in 2013, we conducted engine crosswind tests and two fan blade-out rig tests, successfully demonstrating fan blade-integrated disk [blisk] capability and composite fan-case containment characteristics,” he said. Other completed fan rig tests include bird ingestion and a fan efficiency demonstration.
Ground testing in an altitude chamber was completed in February. It demonstrated engine performance and operability from sea level up to 51,000 feet. “We’re finishing engine icing tests at our Winnipeg icing facility–those tests began in March,” Tressler said.
GE has built four full Passport engines and has another four, plus a dedicated core module, currently in the assembly process. For the development program, GE plans to run a total of nine engines with multiple builds of each.
By entry into service, they will have run more than 4,000 hours and 8,000 cycles. “To date, we’ve completed multiple tests totaling approximately 400 hours, 100 cycles and 200 starts,” Tressler added. Based on the test results, the engine is said to be matching or exceeding pretest expectations. Therefore, Tressler and his team expect certification to take place on time, in 2015.
This summer, flight-testing is scheduled to begin aboard GE’s new 747 flying test bed in Victorville, California. That activity will include performance, air start and above-idle transient testing. Other certification tests such as hailstorm and water ingestion will begin later this year. GE is targeting an 8-percent reduction in specific fuel consumption.
A smaller engine in GE’s range, the 2,095-pound GE Honda HF120 (the joint-venture engine company is exhibiting at EBACE in Booth 2218) was certified in December last year. “We are now in full production mode at GE’s Lynn facility and significant planning activity is under way to enable smooth transitioning to the Honda Aero Inc. facility in Burlington, North Carolina,” said Terry Sharp, president of GE Honda Aero Engines. The HF120 powers the HondaJet very light jet.
Asked about in-development engines, a Honeywell spokesperson answered that the company, “…does not have anything new or in the pipeline or significant updates it can share at this time.” Its latest business-jet engine development is the 6,000-pound-thrust HTF7500E for the new Embraer Legacy 450 mid-light and Legacy 500 midsize jets, for which it is expecting certification in June.
Meanwhile, Rolls-Royce is preparing technologies for the next generation of business jet turbofans, and the design engineers’ motto seems to be “smaller, faster, leaner.” As one of the main drivers is reducing fuel burn, the engine maker’s specialists in Dahlewitz, Germany, are endeavoring to make the core engine more compact. A smaller core greatly aids design of a lighter engine. At the heart of the next generation of engines will be a new high-pressure (HP) spool nearly half the size of those in today’s engines. Nevertheless, it will feature a pressure ratio of about 23:1, much greater than the 16:1 currently on a Rolls-Royce business-jet engine (and thus increasing efficiency). The compressor will have 10 stages and the turbine will have two.
However, smaller blades create a challenge–the need to prevent air leaks. If blades are inserted into disk slots, there are still gaps, and as there already are gaps between each blade, during each run they can slightly reposition themselves and, therefore, tip clearance can slightly vary. The smaller the blades, the more difficult it is to have a good command of clearances. As a solution, Rolls-Royce is developing blisks for all compressor stages, rather than for just a few.
Rotors in future HP systems, being smaller for a given airflow, could spin at more than 26,000 rpm. This is far in excess of the rotation speeds found in large civil engines. So designing the rotors for the lowest vibrations possible becomes increasingly important. Again, blisks are a solution.
In the low-pressure spool, more compression is also needed. Just as GE is doing on the Passport engine, Rolls-Royce is pursuing a blisk fan concept. Manufacturing the blades and disk as one unit will allow a greater airflow and higher pressure ratios.
Meanwhile, in combustion, the UK company is targeting lean burn for the long term. Lean burn combustion means burning the fuel in an excess of air to significantly reduce the formation of NOx. The operation of the system is based on premixing the fuel and air inside the fuel injector prior to entering the combustor. The injectors use fuel staging, with a pilot and a main fuel injector. The pilot maintains combustion stability at low power conditions, while the main injector reduces NOx emissions at high power conditions. However, design engineers see major challenges in the complexity of a fuel-staged system and the integration of controls.
However, all this progress has to be achieved within strict cost limits. Corporate jets are flown less intensively than airliners and, for the owner, the balance between acquisition and maintenance costs is different. In other words, the pressure on reducing the purchasing price is even more intense with business jet engines. Rolls-Royce is thus looking for cheaper materials and, for example, is developing a new, lower-cost alloy for HP turbine disks–a high-strength nickel alloy called “Allvac 718 plus.”
With no replacement in sight for the venerable gas turbine, continuous improvement remains the name of the game.