Gulfstream's long-standing bond with Rolls-Royce has generated new life for the Tay turbofan on the G350 and G450 business jets
When they first sketched out the Tay engine over dinner in December 1982, it is unlikely that even the far-sighted vision of Sir Ralph Robins, then Rolls-Royce managing director, and Allen Paulson, founder of Gulfstream, could have foreseen the latest version of this durable family starting a new life in 2004.
Not only is the engine enjoying a renaissance, but the upgraded Tay 611-8C is in the vanguard of a manufacturing streamlining change at Rolls-Royce Deutschland, where its recent transfer from the UK has been key to stabilising production in an uncertain climate. The transfer of the turbofan is also part of a company-wide strategy that sees two-shaft production focused on Dahlewitz near Berlin and triple-shaft production in the UK.
Thanks to the introduction of the Tay 611-8 and the follow-on -8C, overall German engine production is expected to reach almost 200 for 2004, virtually doubling the previous year's total. The Dahlewitz line, previously established around the BR710/715 engines, peaked at 257 units in 2001 with 146 BR710s and 113 715s, but by 2003 was down to just 107 engines, of which 85 were 710s and 22 715s. The introduction of the -8 and the transition to the -8C in the second half of 2004 adds more than 50 engines to the mix, which is scheduled to include about 122 BR715s and 25 BR710s.
German efficiencies
"As always, there are a mix of reasons for the move to Germany," says R-R Deutschland chairman Axel Arendt. "By implementing a moving line for the 710/715, we discovered we had gained pure space in our factory, so moving the Tay also increased capacity in Derby." Spey and Dart engine certificates were also transferred to Germany to balance the workload, while Derby continues to provide the fan and other components for the newer engines. As well as confirming Dahlewitz as a "centre of excellence" for the two-shaft family, it also presented "one face for the customer - Gulfstream, which is already one of our biggest with the 710, and now the Tay 611-8C", adds Arendt. This was underlined at last year's NBAA, when R-R confirmed orders for 300 611-8Cs, plus options on a further 300, to support the newly launched Gulfstream G350 and G450 upgraded versions of the GIV.
The announcement was another milestone in a 46-year relationship between the two manufacturers that now entails five major aircraft models, four engine families and several sub-variants of both.
Since Grumman launched the Dart-powered Gulfstream in 1958, Rolls-Royce has delivered more than 2,600 engines for Gulfstreams, with firm orders and options due to take this figure well above 3,000 over the next few years.
The latest Tay variant is a natural successor to the lineage established around the Gulfstream family and the stiff noise requirements that led to the original specification of the Tay for the corporate jet specialist's GIV. Having built the GI around the Dart, and the GII/III around the Spey, Gulfstream's Paulson went straight to R-R when it came to developing a powerplant for the GIV.
The new jet was top of the agenda when he met Robins in December 1982 and, with the engineering imperative of a higher bypass ratio well understood, Paulson and Robins were able to thrash out the rest of the deal that launched the Tay in virtually no time. The basic details - price, quantity and payment terms - were written and agreed on a napkin in about 10min between Paulson, Robins and the then R-R commercial director Dennis Jackson at a company Christmas dinner at New York's Waldorf Astoria.
The engine that was finally launched, along with the GIV the following March, was essentially a high-bypass development of the rugged RB183 Spey Mk555 that provided the basic core and gearbox. The revised design borrowed from R-R's technological "box of bits", in particular an all-new low-pressure system and a fan derived from the RB211-535E4, then in early development for the Boeing 757. The fan was a direct 0.6:1 scale of the wide chord design for the larger engine, measuring 1.11m (44in) in diameter against the 1.95m of the -535E4. For structural efficiency and weight reasons, the Tay's fan blades were solid, and not honeycomb-filled like those of the larger engine. The fan provided a bypass ratio of 2.99:1, which kept jet exit velocity low enough to satisfy Stage 3 noise limits, and yet small enough to suit the rear fuselage applications being considered.
As well as the GIV, these also included the Fokker 70/100, re-engined BAE and Rombac One-Elevens and even, potentially, Boeing 727s.
To a greater or lesser degree, all would come to pass apart from the One-Eleven application (except as a one-off testbed/demonstrator). The baseline Tay 611-8, rated at 13,850lb thrust (61.6kN), entered service on the GIV in 1987. Following the delivery of the 500th GIV/IV-SP in December 2002, the engine continued in production to support the newly redesignated G300 and G400.
Tay variants
Derivatives include the 620-15, which entered service on the Fokker 70/100 family in 1988, and the higher-thrust, 15,100lb-rated Tay 650-15, which entered service in 1989 on the heavier Fokker 100 variant. In 1992, the Tay 651-54, rated at the slightly higher level of 15,400lb, entered service on United Parcel Service Boeing 727-100s - the only 727 variant to be specially modified in this way, and the following year saw the first service of the 611-8 on the GIV-SP.
Although the subsequent GV was fitted with the more-powerful BR710, Gulfstream wanted to maintain the competitiveness of the GIV versus the Dassault Falcon 900. "They needed more range and more take-off thrust," says R-R airframer marketing director Gurdip Ubhi, who adds that the company also requested "better control of the engine". The result was the birth of the -8C, which received engine certification in December 2002 and which, in production form, first powered the G450 the following April. The aircraft finally received US Federal Aviation Administration certification in August 2004, and the first G350 was expected to make its first flight a few days before the start of the 2004 NBAA convention in Las Vegas. Certification is expected before the end of the year.
The main difference between the -8C and former Tay family members is the introduction of the Goodrich-produced full-authority digital engine control (FADEC) system as well as a host of other improvements. The engine control system employs an engine-mounted dual-channel digital engine electronic controller (EEC) as well as associated engine pressure, temperature, speed and position transducers and actuators to provide full authority control. In common with most FADECs, the system provides the capability to start, shut down and crank the engine both automatically and manually, while controlling thrust in response to pilot inputs.
Using engine pressure ratio (EPR) as the main metric, the FADEC maintains primary thrust control while automatically accommodating variations in flight conditions, engine acceleration, deceleration and bleed offtake. In the event of an EPR sensor failure, the FADEC will revert to low-pressure (LP) rotor speed as the back-up thrust control measure. Additional features of the FADEC include flame-out and surge recovery, and protection against "hazardous failures" such as a rotor overspeed or an overheat close to the EEC. Being a digitally based system, the unit also provides maintenance indication and engine dispatch status, as well as displaying primary and secondary display data in the cockpit via Arinc 429 databuses. Power and primary thrust controls (throttle position and fuel switch) and oil quantity are interfaced via dedicated wiring.
"The FADEC also brings other benefits in maintenance and indication," says Ubhi, who adds that the installation required some redistribution and rerouting of engine build-up units. "It was kind of a knock-on effect. We had to make design changes to improve the maintainability of the engine, and the experience we had gained on the BR715 was crucial to this because of the regional airline uses of that powerplant [on the Boeing 717]. We really improved the accessibility to the line replaceable units."
Bigger fan
To meet Gulfstream's requirement for a 5% growth in take-off thrust, R-R increased fan diameter by less than 20mm, resulting in a 6% increase in flow. At ISA +20°C (68°F), this sees effective take-off thrust power increased from 11,540lb on the -8 to 12,135lb with the -8C. Bypass ratio also increases to 3.29:1, "and that is one of the reasons why we get an increase in range", says Ubhi.
Another requirement was a 2% cut in specific fuel consumption (SFC), "although at cruise we actually get 3%, partly because of the fan, and partly the scheduling of bleed valves in a more optimum way because of the FADEC", he adds.
Although marginally altered in overall diameter, the fan remains essentially identical to the baseline Tay and comprises 22 solid titanium wide-chord blades. Mounted to a titanium disc, the entire assembly is attached to the fan shaft by a bolted flange while each blade is retained in the compressor disc by a dovetail at the blade root and a corresponding dovetail slot in the disc itself.
The blades are held in place axially by a ring in front of the blade set at the root front face, and behind by a stepped lug. Forward of the fan is a conical spinner which, borrowing the philosophy of its larger R-R siblings, incorporates a soft rubber nose section for ice shedding. Blade containment is ensured by the use of a fan casing made of Armco steel, and an inlaid honeycomb segment allows for close blade-tip clearances.
R-R refers to the single-stage fan as the LP compressor assembly, and the three-stage axial compressor immediately aft as the intermediate compressor (IP). This therefore matches the Tay with the taxonomy of the larger triple-shaft engines in the R-R stable, even though it is a relatively conventional dual-shaft design. The IP compressor is connected to the fan disc assembly by a curvic coupling. The individual IP compressor blades are made of titanium alloy and are attached by dovetail roots to the discs that together form an integral bolted compressor drum.
The lightweight magnesium intermediate casing supports the mounting for the accessory gearbox, as well as the engine mounting and lifting points. The accessory gearbox provides the mounting and drives for several systems, including the RPM indicator generator for the high-pressure (HP) system, the LP air starter, integrated drive generator, fuel pump and even a dedicated generator for the new FADEC.
The intermediate casing also provides bearing supports for the LP fan and IP compressor assembly but, perhaps just as importantly, also houses the fan outlet guide vanes (OGV), as well as guide vanes for both the IP compressor inlet and one stage of its outlet. It also houses two stages of IP compressor stator vanes as well as the bypass splitter that is built into the casing itself. A set of variable-incidence inlet guide vanes is also attached to the rear of the intermediate casing to help stabilise flow into the HP compressor.
Interior design
The HP compressor is housed within two semi-circular steel casings containing 11 rows of steel stator vanes, with external flanges forming an outlet for the air bleed valve, near which is a manifold that takes seventh-stage air for aircraft systems. Inside the casing, the compressor's 12 discs are mounted on a two-piece steel shaft. The rear section of the HP compressor rotor shaft bore contains the LP compressor shaft thrust bearing that transmits the thrust from the LP spool to the engine casing.
The diffuser case, immediately aft of the compressor, is made of steel and comprises 10 hollow struts that carry the HP compressor air to an integral manifold which supplies air to several systems, including the engine nose cowl anti-icing. At the rear of the casing, nestled between the struts, are 10 primary air scoops feeding the combustion chamber with co-located fuel spray ignition nozzles and ignition plugs in each.
Aft of the diffuser is the combustor, which is made up of 10 liner assemblies housed in an annular chamber and cooled through the transpiration technique in which cooler air is drawn through the material by the lower pressure within the combustion chamber.
The design of the combustor includes a split outer casing that allows the liners and the first-stage HP turbine nozzle guide vanes to be inspected and replaced. The HP turbine has two stages, connected by a central retaining nut, with interconnecting driving dowels transferring power from the second to the first stage. A turbine shaft, which drives the HP compressor by interconnecting the compressor/turbine shaft by a spline and locking arrangement, is attached to the front of the first-stage HP turbine by bolts.
Improvements in materials and casting technology since the start of the Tay programme have also been introduced on the newest version, particularly in the crucial HP turbine, where the first stage is now made up of cast blades instead of the forged blades on the -8. "We also have improved materials in the second-stage blade, so the blade is now without internal cooling," says Ubhi. The blades are attached to the discs with a fir tree root design, and are retained in position using lock plates in a similar way to those of the LP turbine.
The LP turbine, originally configured using the baseline aerodynamic design of the -535E4 engine, consists of three turbine stages and three nozzle guide vane stages. The first- and third-stage discs are attached to the second-stage disc by a central nut.
The second-stage disc, in turn, is bolted to the turbine shaft, which drives the fan and IP compressor. The LP turbine, which is supported at the rear with a roller-type bearing, also houses a set of nozzle guide vanes that are secured in place by machined grooves and clamps in the outer casing, and by bolts at the inner roots. The first-stage nozzle guide vanes are hollow and are used to house sensors that measure turbine gas temperature at nine positions.
Downstream is the multi-lobe exhaust mixer, which has been redesigned using up-to-date aero-acoustic analysis techniques, honed more recently by the company during development of the ultra-quiet BR715.
More margin
Located between the LP turbine support outer case and the bypass duct, it consists of 16 lobes instead of the 12-lobe design of the -8 engine. Another key difference is the "chute profile" that was revised as part of the noise-reduction work, which allows the -8C to meet current and forthcoming limits "with comfortable margin", says Ubhi.
Wrapped around the aft half of the engine, and terminating at the exhaust mixer, is a carbonfibre-composite annular bypass duct that has built-in "sole plates" or zones through which various services - such as oil and HP bleed air - are taken through the bypass. The duct also incorporates several removable access panels to allow access to the core engine, and has built-in attachments for the direct mounting of accessories and brackets. Not all of these are used and, like the accessory gearbox, have been designed for expansion and higher-power derivatives - a sure sign that Rolls-Royce is banking on yet further life for the durable Tay.
GUY NORRIS / LOS ANGELES CUTAWAY DRAWING / TIM HALL
Source: Flight International
