The AE3007 turbofan marks a quiet advance in Allison Engine's product range.
SOLID ENGINEERING, a wealth of military-research work and a degree of luck have all contributed to the successful development of the AE3007, Allison Engine's first commercial turbofan and one of the quietest jet engines in the world.
The vital ingredient in the AE3007 story, however, is the rugged "common core" of the T406 turbo-shaft, around which the new turbofan and its AE2100 turboprop sibling are based. "We would classify it as a model development programme, in part because we started with a core that had 50,000h of development running on it. We were developing a turbofan at the same time as two other programmes were developing the core. It made it really quite painless for us," says Allen Novick, Allison Engines large commercial engines vice-president.
The roots of the AE3007 development go back to 1988, when Allison and Rolls-Royce began joint studies of a 33kN (7,500lb)-class engine to power the FJX, a regional jet proposed by Short Brothers of Northern Ireland. The FJX engine, dubbed the RB580, was to have combined the T406 core with a low-pressure spool developed by R-R. By late 1989, amid uncertainty over the Shorts project and the growing importance of the Trent programme, R-R pulled out and Allison elected to go it alone.
(The fleeting relationship between Allison and R-R was consummated six years later when the UK manufacturer bought the Indianapolis-based company. Ironically, 1995 was also the same year that the re-christened AE3007 engine had its first flight on a regional jet, the Embraer EMB-145.)
Allison restructured the programme, although it was still aimed at the original thrust bracket under the initial "airline standard" design criteria. It also embarked on the design of a new wide-chord, clapperless (or snubberless), fan and low-pressure turbine.
Within months, Allison secured a vital launch order from Embraer to power its EMB-145 and, six months later, in September 1990, it announced the selection of the engine for Cessna's Citation X, the world's fastest business jet. The first GMA3007 engine, as it was called during Allison's years under the ownership of General Motors, went to test in mid-1991 and, by the middle of 1992, flight tests had begun on a Citation testbed.
The AE3007 is a two-spool turbofan with a 5:1 bypass ratio and a thermodynamic thrust capability of slightly over 38kN. It is externally distinguished, by a large one piece duct, of almost constant cross section extending from the deeply recessed wide-chord fan, to the 12-lobed exhaust mixer.
The 24-blade fan is 970mm in diameter and made from solid titanium. "We looked at alternatives," says Novick, "but the weight was OK for what we needed and it met every target we had. The weight advantage of a composite fan gave us very little in the way of pay-offs for this size of design."
The fan was the biggest single development challenge for Allison. The company conducted the first rig test ten months after the programme launch. Although initial tests showed that the fan was flutter-free, a second fan was designed to improve high-speed efficiency and airflow. The new fan then sustained some damage during birdstrike testing and the blade leading edge was "sharpened" to increase tolerance to large-bird strikes. Verification of the final blade design was achieved during a fourth fan rig-test in 1993 and full-up 1.8kg bird-ingestion tests in 1994.
Another part of the fan assembly to change during development was the spinner. To save weight, Allison initially adopted a composite spinner, but changed to a more conventional aluminium design after it was damaged in a hail test. The composite unit showed potential for causing secondary damage by shedding debris into the core intake. The slope of the spinner is aligned with the lip of the core path inlet, which is spaced well behind the fan. "This was done intentionally for centrifuging water and foreign objects, though we also get some noise benefit as well," Novick says.
The tip of the spinner is made from rubber, which distorts when loaded with ice. This movement should shatter the accumulation before it can grow dangerously large. The entire fan assembly is enclosed in an aluminum fan case with an integrated Kevlar-based containment system.
The fan is driven directly by an uncooled, three-stage, low-pressure (LP) turbine with no intermediate gearbox. Through the combination of wide-chord blades and the multi-staged LP turbine, fan tip-speed is lower than for other, similar, high-bypass, ratio engines, eliminating the familiar "buzz-saw" noise usually associated with a turbofan on take-off.
Using three-dimensional viscous computational-fluid-dynamics analysis, the blade shape has been designed to minimise the strength of fan-rotor-generated shock waves and rotor/stator wake interaction. In addition, the spacing between the fan and bypass vanes has been based on sub-scale modeling designed to reduce the generation of tonal noise at harmonics of the fan-blade passing frequency.
The entire engine cycle is controlled, by corrected fan speed rather, than the more conventional exhaust pressure-ratio. "We looked at both with the RB580," says Novick, who adds that fan speed was selected because it could be more closely optimised to control the operation of the lower-thrust engine. The engine's Lucas full-authority digital engine-control (FADEC) controls the fan speed to a customer-defined schedule as a function of altitude, ambient temperature, throttle setting and Mach number.
Fan or core speed can be synchronised on command by the dual-redundant FADECs, which on both the Citation X and EMB-145 are mounted in the fuselage and away from the potentially, hostile engine environment. The control units interface with the aircraft via ARINC 429 databuses. The engine-control system also provides, for automatic take-off thrust control and auto-relight.
The T406 heritage of the AE3007 begins immediately aft of the fan with the high-pressure (HP) compressor. The 14-stage axial-flow compressor, is enclosed in a chemically milled titanium (Ti6-2-4-2) case for weight saving, but uses steel blades and vanes for durability. The first eight stages of the HP compressor are made from 17-4PH steel, as used in the T406, while stages nine to 14 are made from the higher-heat-resistant Inconel 718.
"We thought it was important to use steel for its higher resistance to [foreign-object damage], corrosion and erosion. We wanted to carry over our operational experience of the T406, which has to operate everywhere, from a carrier deck to a beach front. The use of steel is an enhancement for operational reliability reasons. Like everything, it's a bit of a compromise," says Novick.
The integrated diffuser/combustor assembly was also expected to be a direct follow-on from the T406, but Allison was forced to change because of the AE3007's higher overall pressure ratio of 24:1, compared to 16:6 in both the T406 and AE2100. As a result, the lining of the annular combustor is effusion cooled, rather than convection/film-cooled as originally planned. The combustor is fitted with 16 "piloted" directional air-blast fuel nozzles and is dotted with thousands of laser-drilled holes arranged in a specific pattern and orientation tailored to combustion patterns observed in tests and simulations.
The two-stage HP turbine differs only in fine detail from that of the T406/AE2100. All aerofoil castings are identical, but cooling-control orifices in the air-cooled blades are adjusted to suit the higher operating temperature of the turbofan. The first three aerofoil rows (first-stage blade, and first- and second-stage vanes) are air-cooled. The second-stage HP-turbine blade, which is made from CMSX-4 single-crystal alloy, like the first stage, is not cooled.
The LP turbine differs from that of the T406 in having a third stage. The three-stage design was required to give the lower rotational speeds dictated by fan tip-speed requirements. The first-stage LP turbine vane incorporates a thermocouple, which measures turbine inlet temperature. Both first- and second-stage blades are made from Inconel 738, with the third-stage blades made from Inconel 713.
Exhaust gases meet bypass air in an elaborately designed forced-air (lobed) mixer made from titanium. Allison originally considered an axisymmetric mixer, but after performance testing opted for the current 12-lobed design. The company says that the design produced improvements in specific fuel consumption, as well as a 6dB reduction in noise made by the mixing of the jet. This equates to a 2dB cut in environmentally perceived noise as measured at a sideline station during take-off.
The bypass duct channeling fan air to the mixer is a large, but relatively simple, component. It doubles as a structural load-bearing part of the engine as well as a noise suppressor. By using the bypass duct as a structural member, any bending and moment loads, which may result from heavy landings or turbulence, are transferred through the casing rather than the core of the engine.
"It was designed for performance retention and to stop clearances opening up through heavy landings," says Novick. The number and locations of the struts supporting the duct, which is made from composites and honeycomb aluminum, were also carefully optimised to cut noise generation. NASA-developed analysis methods were used to select the depths of honeycomb material and the porosity of the acoustically treated face sheet. Further work by nacelle specialist Rohr at its acoustic test site in Brown Field, San Diego, refined the design.
The core of the engine is also enclosed in a composite inner-duct lining from the inlet lip of the core to the diffuser/combustor section. Allison originally intended to enclose the entire core as far aft as the LP turbine, but dropped the idea to give easier access for maintenance personnel to areas such as the fuel nozzles and thermocouples. "It cost us about 0.2% efficiency in performance to take it out, but we get some weight advantage and the maintenance benefits make it worthwhile," comments Novick.
These access considerations also prompted Allison to change the size and, in one case, orientation of six large access panels, located in the bypass duct.
The engine is mounted to the airframe at the front frame and rear mount ring. Four mount pads on the front frame are designed to accommodate either fuselage or underwing mounting, an important consideration given the early changes in the EMB-145 design. The rear titanium mount ring is designed to accommodate a heavier, four-point, universal mounting system for regional-jet applications and a slightly lighter two-point rotatable system for business aircraft. In either case, the system is designed to retain the engine in the event of a mount failure.
The accessory gearbox is mounted on the front frame below the engine and takes power from the HP spool via a bevel-gear/tower-shaft arrangement to drive engine and airframe systems. These include oil and fuel pumps on the engine, as well as the permanent-magnet alternators, which power the FADECs. Airframe accessories powered by the gearbox include two generators, an air-turbine starter and a hydraulic pump. The AE3007 accessory gearbox differs from the turboprop unit, which is divided into two and which takes power for aircraft systems from the propeller gearbox.
The engine uses a self-contained lubrication system, which consists of an oil tank, pump, 3µ-filter, and both fuel-cooled and air-cooled oil coolers. The oil pump both scavenges and pressurises the system.
FLIGHT TEST AND THE FUTURE
The AE3007C version of the engine, rated at 28kN, was certificated by the US Federal Aviation Administration for the Citation X in February 1995. Full FAA type certification, is expected by the end of April, followed by European Joint Aviation Authorities (JAA) certification around the middle of 1997.
The 39kN AE3007A is expected to be certified for the EMB-145 in mid-1996. The thrust rating of the engine on the Brazilian airliner was evaluated at 33kN and new FADEC software is now being written.
Allison received a boost in 1995, when the AE3007H version of its engine was selected for the US Defense Department's Tier II Plus unmanned-aerial-vehicle programme led by Teledyne Ryan Aeronautical. The requirement calls for long-endurance flying at altitudes of up to 70,000ft (21,300m). An engine underwent testing at these rarefied altitudes in February 1996 at the Arnold Engineering Development Center at Tullahoma, Tennessee. The first AE3007H will be delivered in May.
Changes made to enable high-altitude operation include a slight trimming of the trailing edges of the HP-turbine vanes, to suit changes in flow characteristics expected in the thinner atmosphere, and a "tune-up" of the lubrication system. Allison is confident of good high-altitude performance having tested the unmodified AE3007C version at the high-altitude test site in Trenton, New Jersey, in 1992 and 1994. The tests included steady-state performance at 51,000ft, snap accelerations and decelerations, inlet distortion, control stability, wind-milling and air-starting.
With a finger in each of the pies of commercial, business and military aviation and with the industrial weight of R-R behind it, Allison is looking to spread its market penetration even further. "With the A and C versions, we are on two programmes that are winners and we've got some other letters of the alphabet that we're studying," says Novick. Indeed marketing campaigns for the B, D, E, F and G variants are under way, involving "both regional jets and business jets", says large-engine marketing director Ron Riffel.
Growth variants continue to be studied and technology developed to support the initiative. The successful run of the 44kN AE301X demonstrator, in September 1994, confirmed the basic engine's ability to run at higher ratings, with little change other than an advanced technology cast-cooled HP turbine. Further growth to provide a power plant with a 42-75kN range is also envisaged with the use of the advanced HP turbine and a larger, 1.12m-diameter fan.
Negotiations are under way with the parent company to define the upper levels of thrust growth for AE3007 derivatives and avoid conflict with the rest of the family. In the meantime, 1996 promises to be the busiest year ever for the AE3007 family with entry into service on the Citation X and Embraer EMB-145 and flight-testing on the Tier II Plus.