In July, AlliedSignal engineers will start a new engine for the first time. This is a critical milestone for any engine maker, but more particularly for AlliedSignal, which plans to make the AS900 turbofan a pivotal powerplant in its strategic plan for the 21st century.
The AS900 is a next-generation engine aimed at both the regional and corporate markets. It has won business as the AS907, on Bombardier's Continental super mid-size business jet, and in its AS977 form has been selected for the yet-to-be-launched Avro RJ-X. British Aerospace plans to make a formal go-ahead decision on the re-engined Avro RJ family by the end of September, but Dominique Hedon, AlliedSignal Engines and Systems' commercial propulsion vice-president, says, "the technology is fully ready" for service as early as 2001.
"We will have 10,000h on test and 50,000 cycles before it enters service," says Hedon. The full-scale development programme for the initial AS907 variant involves 11 test engines, three of which will be dedicated to accelerated mission testing and some substantial airborne tests on the company's Boeing 720 flying testbed. The first development nacelle and thrust reverser is to arrive at the company's main site in Phoenix, Arizona, from GKN Westland in the UK next February. Initial test flights are to begin later that quarter with US Federal Aviation Administration certification due a year later.
The AS900 represents a new approach for AlliedSignal, which is taking a clean-sheet design and developing it for a raft of potential applications across different markets. In the past it has followed a more traditional methodology and developed derivatives for use in discrete market sectors. The recently developed TFE731-20/40/60 series, for example, is tailored to the business jet market, while the LF507 - an engine acquired with the 1994 Textron Lycoming takeover, is used only on the Avro RJ application. The long-term plan is for the AS900 to be a common solution for as many requirements as possible, although AlliedSignal remains busy with its existing products. It is making close to 200 TFE731-20/40/60s a year, for example.
To this end, the engine is an intriguing mix of advanced but rugged design with conventional technology. AlliedSignal has deliberately "dumbed down" the AS900 in specific areas, such as turbine materials, to reduce cost. At the same time, it has introduced a new wide chord fan and blisks in the compressor. The result is expected to be a highly efficient, low-cost engine that will provide adequate performance without necessarily incurring the higher level of operating and maintenance expenses sometimes seen with current business jet engines. The spin-off is a dependable, high-performance engine equally well suited for the rigours of the regional market.
"The engine is a balance of performance and operating economics," says Hedon. "We looked at total fuel burn and weight as a threshold and kept them at the same level as today, and the surplus of technology we have left over, we have put into the production of the engine. It will therefore be cheaper to make and run. We also use ordinary materials in the engine and do not have things like single crystal turbines."
The wide-chord fan, provided by AlliedSignal's International Turbine Engine partner AIDC of Taiwan, is made up of 22 inserted blades. In the AS977 this will be direct drive, and not gear-driven as in the current LF502/507. Aft of the fan is a four-stage axial compressor, two variable geometry stators and a single-stage, centrifugal compressor. The design also incorporates an effusion-cooled, straight-flow combustor, which Hedon says will "reduce cost by 30%". Aft of the combustor is a two-stage high pressure (HP) turbine and a three-stage low-pressure (LP) turbine. International partners for the LP turbine are still being sought as part of the company's overall goal to see global participation at 35% for the AS900. "We are also talking with several potential partners on the gearbox for the RJ-X," adds Hedon.
Dual-channel electronic control is standard to both AS907 and AS977 and is part of the drive to reduce operating and maintenance costs, as well as to improve performance. Both variants are designed with on-wing, hot-section maintenance in mind. The fan, LP turbine, accessory gearbox and even main shaft bearings and seals are also designed to be replaceable on-wing. The company plans that operators will be able to swap out line-replaceable units in as little as 15min on the AS977 and within 20min on the 907. The maintenance burden is also expected to be generally lower, thanks to several design initiatives. These include the reduced part count in such elements as the blisk axial compressors, and the reduced number of lubrication sumps - of which there are only two. These sumps are also located in cool zones. Spinner anti-icing is also eliminated from the design, removing another area of headaches traditionally found on the older engines. The discs in the AS977 are also designed for a long life of 25,000 cycles while those on the business jet engine are planned for 15,000.
Take-off rating for the AS977 is 7,000lb (30kN) with a planned 10% growth margin in the same installation, and up to 25% growth overall for later derivatives. Cruise rating is expected to be around 1,480lb at 35,000ft (10,600m) and Mach 0.7. The slightly more powerful AS907, which will have a take-off rating of 7,595lb, is expected to have a cruise rating of 1,108lb at 45,000ft and M0.8.
The engine series is initially likely to grow upwards rather than downward. "It is easier than shrinking, which is bigger investment," says Hedon.
Source: Flight International
