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Business Aviation Engines: Year of the “725”

Engines with similar numbering but different manufacturers stand at the ready to support two Gulfstream clean-sheet aircraft market entries this year: the super-midsize G280 and the large-cabin, ultra-long-range G650.

In both cases, the engines will lift the bar for performance and environmental friendliness a notch higher than their predecessors, setting challenging targets for the engines that follow.

Due to enter service in the second half of this year, the $24 million Gulfstream G280 is powered by two Honeywell HTF7250G engines, direct descendants of the popular HTF7000 engine used on the Bombardier Challenger 300.

Gulfstream G650 production, Guulfstream

 © Gulfstream

Gulfstream's G650 will be powered by Rolls-Royce's BR725

As of February, there were 690 HTF7000s in service. These had accumulated more than 1.2 million hours of flight time and just under 740,000 flight cycles, says Jim Kroeger, Honeywell Aerospace's director of engineering for propulsion. The fleet high-time engine recently reached 7,000h, predefined as a point at which to bring the engine in and examine wear. "We're happy with the results," he says.

Certificated in May 2011, the HTF7250G packs a take-off thrust of 7,445lb-thrust (33.1kN), up 7.5% from the HTF7000, and has a 3% lower specific fuel consumption (SFC) and correspondingly lower carbon emissions. The engine is propelled by a 22-blade, 87cm (34in) fan, four-stage axial flow compressor and single-stage centrifugal compressor, annular combustor, two-stage high-pressure turbine (HPT) and three-stage low-pressure turbine (LPT).


Customers can choose either on-condition or traditional clock-based maintenance programmes for all the HTF7000-family engines, though most choose on-condition, says Kroger. Those with the traditional programme must inspect the hot section at 3,500h and the cold section at 7,000h, although Honeywell expects to increase those numbers to 4,000h and 8,000h, respectively, in the next couple of years given positive findings from the fleet-leader engines.

Ramp-up is likely to be slower than Honeywell would like, with Gulfstream reporting earlier this year that it would deliver only 10-15 midsize aircraft models this year, although they will mostly be G280s.

Keeping pace with the ramp-up expected for the new $65 million Gulfstream G650 over the next two years will be a bit tougher for Rolls-Royce, the manufacturer of the BR725 engines that power the large-cabin, ultra-long-range business jet. The BR725 produces 16,100lb take-off thrust, up by 9% and 5% compared with the earlier-generation BR710 engines on the Bombardier Global Express and Gulfstream G500 and G550, respectively. While the overall diameter of the BR725 with its new composite nacelle is the same as the BR710, the BR725 is 4% more fuel efficient. The engine obtained European and US certification in 2009 and has accumulated a total of 8,700 operating hours on the nascent G650 fleet.

"We're extremely pleased with the [BR725] performance, based on results from the testbed and flight testing," says Holger Seidemann, the BR725 programme lead for Rolls-Royce. "The engines meet specifications with a healthy margin and we exceed customer expectations."

Rolls-Royce BR725, Rolls-Royce

 © Rolls-Royce

Rolls-Royce will ramp up BR725 production to three engines per week next year

The engine's two-spool design features a solid nose cone ahead of a 127cm fan with 24 titanium swept blades encased in a solid titanium fan case. By comparison, the BR710 uses a 122cm fan.

Behind the BR725 fan is a new composite fan outlet guide, followed by a 10-stage high-pressure compressor (HPC) that uses elliptical leading edges developed through 3-D aerodynamic optimisation. A two-stage shrouded HPT with active tip clearance control behind the combustor improves cruise fuel burn, while a three-stage LPT downstream of the HPT drives the fan. Behind the LPT is a 16-lobe mixer, which cuts engine noise. The engine produces 20% less NOx than the BR710, produces no visible smoke and is 33% quieter, says Rolls-Royce.


To date, Rolls-Royce has delivered 10 engines for the Gulfstream test fleet and at least 24 production engines for the 12 "green" aircraft delivered in 2011. Gulfstream received US Federal Aviation Administration provisional certification for the G650 in November 2011 and is expecting final certification mid-year, with entry into service shortly thereafter. The airframer plans to build 24 green aircraft this year, part of a 200-aircraft backlog as of earlier this year.

Production plans for the BR725 engines at Rolls-Royce hint at pent-up demand in the market. The manufacturer will be ramping its production rate up from one engine per week now (26 aircraft per year) to two engines per week in the third quarter, further accelerating to three engines per week in 2013.

Rolls-Royce does not have a formal performance-improvement plan for the engine post-certification, but Seidemann says there are "a number of opportunities that can be taken off of our technology shelf to be introduced when and if it is necessary". He adds: "It is not necessary now."

Three years from now it could be a different story, given GE Aviation's plans for certification of its Passport 20 turbofan for the Bombardier Global 7000 and Global 8000 large-cabin, ultra-long-range aircraft, competitors to the G650. Bombardier plans to have the Global 7000 in service in 2016 and the 8,000 in service in 2017. When combined with its Nexcelle-provided nacelle, GE says, the integrated propulsion system will burn 8% less fuel than competing engines in the class, which by default means the BR725. "We're holding to those numbers," says Brad Mottier, vice president and general manager of GE Aviation's business and general aviation group.

With a maximum 16,500lb-thrust for take-off, the Passport 20 will feature a 132cm diameter, 18-blade titanium fan blisk, a three-stage booster, a 10-stage HPC with blisks for the first four stages, an annular combustion chamber, a two-stage HPT and a four-stage LPT.

GE was planning to freeze its design by the end of April, pending a successful "tollgate 6", or critical design review, and put its first engine to test in the second quarter of 2013. Mottier says there will be eight engines in the certification programme, the first of which will begin assembly late this year.

The promised performance boost comes in part from the bigger fan and the integrated propulsion system, and from the compressor, which itself is a scaled-down version of the "eCore" being developed by GE and CFM, the company's 50:50 joint venture with Snecma, for the Leap X turbofan. A follow-on to CFM's highly successful CFM56 series engines, the Leap X has been selected to power the Airbus A320neo, Boeing 737 Max and Comac C919. The first in-service Leap engines will be on the A320neo in 2015, approximately the time when GE plans to certify the Passport 20.

Mottier notes that two eCores have already been run and a third and final core, larger than the Passport core but smaller than the Leap core, is set to run later this year or early in 2013. Rig testing on an LPT will begin later this year, as will blade-out, icing and aeromechanical testing of the fan blisk. Mottier says two fan-blisk validation trials already completed had "very positive results". Having a fan blisk, he says, will reduce the vibration environment for the cabin compared to individual fan blades, which can take on a different "set" every time the engine is started.

"When you look at a bladed disk, as the joint between the blade and the dovetail wears over time, or the lubricant is not maintained, the vibration signature increases," says Mottier.

Honeywell is also seeking the benefits of a fan blisk for its HFT7000-sized engines and for its next-generation 10,000lb-class turbofans (alias the HTF10000). Kroger says a forward-swept blisk design the manufacturer developed with NASA will cut noise in addition to the other benefits that come from the architecture.

The company tested a HTF7000-sized fan blisk - meeting vibration and strain goals - and is preparing to test an HTF10000 fan blisk. "This launches us on a technology route to be ready for what the market might need a couple of years from now," says Kroger.