Achieving excellence

Citation

Ford, T. (2006), "Achieving excellence", Aircraft Engineering and Aerospace Technology, Vol. 78 No. 5. https://doi.org/10.1108/aeat.2006.12778eaf.001

Achieving excellence

Achieving excellence

Keywords: Automotive industry, Aircraft engines

When Sir Ralph Robins the Chairman of Rolls-Royce in the late 1980s decided to develop the Trent turbofan the eventual results would be would be far-reaching for the aerospace industry. It was an astute move since although Rolls-Royce had built up a 15 to 30 per cent market share with the RB 211, more powerful engines would be needed for the large widebody aircraft envisaged. The other major manufacturers General Electric and Pratt & Whitney were racing ahead with their own designs.

The Trent series builds on the traditional and long-term experience gained by Rolls- Royce's three-shaft layout with its inherent advantages which include the ability whereby each compressor can run at its optimum speed, i.e. 3.000-4,000rpm for the front fan, around 7,000rpm for the intermediate pressure (IP) compressor, and over 10,000rpm for the high pressure (HP) compressor. Another benefit is that such a design permits each engine module to be scaled up or down and matched to each performance and thrust requirement while maintaining the proven combustion temperatures during take-off and climb. These features have been demonstrated over many years of reliable operations on a variety of aircraft.

A key attribute is the incorporation of the wide chord fan which since the 1980s has been an integral part of Rolls-Royce design efforts. Fan efficiency is very important for unproved fuel economy as the bypass air contributes some 80 per cent of the total thrust of a high bypass ratio turbofan. Whereas old technology blades required snubbers at mid-span to control vibration, the improved blades have a chord some 40 per cent greater and do not require snubbers. Also, the larger blades enables the number of blades to be reduced, resulting in a lighter fan assembly.

Developments since that time have further improved efficiency, incorporating more advanced designs. In the 1980s the fan blades were constructed of hollow titanium honeycomb, to be succeeded in the next decade by superplastic formed hollow titanium blades. Of great importance is the reduction in noise as turbojets have been replaced by turbofans. A large reduction in jet velocity transferred the emphasis from the noise generated by the turbulent mixing of the jet with the atmosphere, to the internal whining of the fan and other rotating stages. This proved to be far more controllable man the jet mixing process and led to lower noise levels, which, together with effective acoustic liners, has resulted in more than 75 per cent reduction in relative annoyance. In addition, the reduction of CO2 and NOx has been accomplished with Rolls-Royce turbofans below the successively decreasingly levels set by Stages 1,2,3, and eventually, to Stage 4, of the Committee on Aviation Environmental Protection (CAEP).

Of crucial importance are the HP turbine blades which are fundamental to reliability. Turbine entry temperatures have risen by over 650&#186C since the design of the first turbojet in the search for greater thermodynamic efficiency. Early use of wrought steel was soon superseded by the “lost wax” investment casting technique and directional solidification developed later in which a limited number of grains are formed along the major stress axis of the blade. Although this method of manufacture achieved major advances in creep life, producing blades as a single crystal would result in even greater advantages by eliminating all grain boundaries. It was realised that advanced inspection would be required and Rolls-Royce designed and patented the fully automated Single Crystal Orientation Rapid Processing and Interpretation Operation instrument known as SCORPIO which uses a back-reflection X-ray diffraction technique that quickly reveals if each blade has grown as a single-crystal grain with the correct orientation. Rigorous testing and extensive in-service experience over the last ten years or so has confirmed the toughness and reliability of such blades operating in the Trent at the highest temperatures yet achieved.

Improvements in the original RB 211-22B were followed by the launch of the – 535C on the Boeing 757 in 1982 which was succeeded hi production by the – 535E4 hi 1984 with the wide-chord fan (Figure 1) and reduced fuel burn and increased thrust of 43,100Ib. This model has a bypass ratio of 4.2 and a pressure ratio of 27. It was the first high bypass ratio turbofan to have a configuration in which the hot exhaust gases are mixed with the bypass airstream inside the final exhaust nozzle. It is notable that the developed -535E4 combustor is derived from the Trent design which produces the lowest emissions in its class. Significantly, the R 211 series has built up a reputation for durability hi the most difficult conditions and the -535E4 achieved the longest tune “on wing” hi the 1990s of more than 29,000h, leading to the lowest maintenance cost. Numerous - 535E4 engines achieve an on-wing life of over 20,000h.

Figure 1 RB 211-535E4 wide-chord fan

The more powerful RB 211-524 for long range aircraft also has technological benefits and the -524G/ H variant (58,000-60,600Ib thrust) has the low weight HP compressor as well as the low emissions combustor, plain shroud turbine blades and nozzle guide vanes from the Trent 700 and the turbine location bearing and stubshaft from the Trent 800. It has consistently shown the best mature fuel burn hi the Boeing 747 (Plate 1) and gained rapid clearance for 120min and 180min ETOPS (extended range twin engine operations) hi the Boeing 767. In the late 1990s this engine variant was certificated with increased temperature margins, unproved performance retention, and longer life on wing.

Plate 1 Qantas Boeing 747-400 powered by R 211-524G engines

With the advances referred to, it is convenient to return to the first models of the Trent to enter service which were aimed at thrusts up to 73,000Ib for the Airbus A330 and 84,500Ib for the Boeing 777. Evolving the Trent from the RB211 (Plate 2) used the latter's advantages including the three-shaft layout being better suited to a larger engine and the number of stages being reduced which allows compression to be created in three short compressors. This means that fewer stages of variable guide vanes are needed to smooth the airflow through the compressors with fewer parts required, easier maintenance and less weight As the IP and HP compressors are short and evenly loaded, each can be driven by a single-stage turbine, both turbines having tip shrouds to ensure efficient at the blade tips. The IP turbine blades are running at a slower speed and experience a much lower stress than equivalent blades on a two-shafl engine, and consequently operate without the provision of cooling air. The Trent 700 entry (Figure 2) into service (EIS) was on the Airbus A330 of Cathay Pacific in 1995 with a thrust capability of 68,000- 78,000Ib with a data entry plug the only change required for varying thrusts. The fan dia on this engine is 97.4 hi (2.47m). At the time of its EIS, the Trent 700 noise level had the largest cumulative margin by up to 5.9dB below the CAEP Stage 3 requirements.

Plate 2 Trent engines being prepared for testing at derby

Figure 2 Rolls-Royce Trent 700

As noted previously, on a three shaft engine each module can be scaled up or down and matched to suit performance and thrust requirements while maintaining proven turbine and combustion temperatures during take- off and climb. The higher powered Trent 800 makes use of this capability as it has a fan of 110in. (2.79m) giving a bypass ratio of 6.2 combined with larger IP and HP compressors to give it a thrust of at least 75,000Ib. Its EIS was in 1996 and development to over 110,000Ib thrust followed rapidly which made it suitable for all weights of the Boeing 777. The wide-chord fan incorporated scimitar-shaped blades which improves fan aerodynamics and greatly extends resistance to FOD (foreign object damage). The Trent 800 also incorporates reblading of the IP and HP compressors, increased flow burners, unproved combustor wall cooling, and other features. Initially slow to develop, the market for this engine received a significant boost in 1996 when Singapore Airlines ordered it to power 34 Boeing 777's. Other large carriers including British Airways, Delta, American and many others, made the same decision. This was an achievement as the customers for the Trent 800 included previously dedicated users of other companies' products.

Development of modules can also suit smaller power applications and the Trent 500 is provided by Rolls-Royce as the sole powerplant for the long- range four-engine A340- 500/600 series, with a fan dia of 97.4 hi (2.47m) and a thrust of 53,000- 56,000.1b. It entered service on a Virgin Atlantic A340-600 hi 2002 and on the even longer range.

A340-500 of Air Canada in the following year. It is notable that this version of the Trent embodying the scaled-down core, has a bypass ratio of 8.2 which gives higher fuel efficiency and low noise appropriate to the requirements of four-engine aircraft. The eight-stage IP compressor and the six-stage HP compressor are each scaled by 0.8 from the Trent 892 and the 5-stage LP turbine annulus, single-stage IP turbine annulus, and single-stage HP turbine annulus are each scaled by 0.9 from the Trent 892. Reduced SFC (specific fuel consumption) is achieved by changes which include the application of 3D aerodynamics. The Trent 500 meets emission requirements for CAEP 3 (all engines beyond 2008).

Power for the A380

The Trent 900 (Figure 3) enters service on the Airbus A380-800 of Singapore Airlines later this year. It is the first engine to complete certification through the European Aviation Safety Agency (EASA), which replaces the UK CAA (Civil Aviation Authority) for these purposes. Key dates for this turbofan include certification in 2004 at 80,000Ib thrust, which is well above requirements for EIS, and the initial flight on the A380 in April 2005. Seven development engines were used during the certification for the Trent 900 and testing has been undertaken at Derby and Hucknall, although by the end of 2007 all engine testing will have ceased at Hucknall and the majority of this work will be transferred to a site in Mississipi, USA. Details of the rigorous testing regime for the Trent 900 which follow are typical of the items included in the mandatory programme for all engines.

Figure 3 Rolls-Royce Trent 900

The Trent 900 has the largest fan of 116 in (2.94m) of any Rolls-Royce turbofan, giving a bypass ration of 8.6 and the noise tests and environmental considerations have played a considerable part in the certification process for the engine and aircraft combination. For the fan blade- off test, an aircraft pylon must be accommodated and the engine accelerated to full speed before a fan blade is released at the root and all the high energy blade fragments must be contained within the engine casing, In the developed bird ingestion test, it was the first time that a 5.5Ib bird had been ingested at take-off conditions, the engine losing only 2.7 per cent thrust The 150-hour endurance test involves running the engine at maximum power for extended periods, particularly valuable to monitor the behaviour of the engine at high speeds. The crosswind test looks at the dynamic response of the engine in conditions produced by a high power crosswind machine. The cold start test is undertaken at –40&#186C, this involving a “cold soak” for 12h on an outdoor test bed. The water test sends hi excess of 20,000 gallons of water an hour into the engine, this ensuring starting in a heavy rainstorm.

Current and future

Now in progress is a year-and-a-half testing programme on the Rolls-Royce Trent 1000 which is the lead powerplant on the Boeing 787 with flight tests to be undertaken later in 2007. Certification will follow in the same year at a thrust of 75,000Ib which will allow for further development of the aircraft first delivery will be to All Nippon Airlines in 2008 with an order for 50 aircraft. It has also been selected by many other airlines. It has a “swept” fan of 112in. (2.84m) with 20 blades giving a bypass ratio of 11. This is the first “bleedless” Rolls-Royce engine in which rather than drawing air from the FTP compressor to drive aircraft systems, the increased power needed is provided by electric generators linked directly to the intermediate pressure compressor.

Design work is under way in 2006 on the Trent 1700 which is destined for the Airbus A350. Few details are available although it will probably be of similar power to the Trent 1000 with at least one major difference – power for the aircraft systems will be provided by bleed from the IP compressor, which has been requested by the aircraft manufacturer.

The Trent 1000 design may be considered a step towards the More Electric Engine (MEE). For this engine the advantages include a more balanced power taken from the IP and FIP systems in certain flight regimes and a smaller core. For starting, the FIP system is connected to the IP system with the starter operating both shafts. Of the 16 engines required for Trent 1000 testing, seven of these will be used on the ground with the majority undergoing multiple rebuilds. One will be installed in the Rolls-Royce flying test bed with certification just ahead of flight testing on the first of four Boeing 787s.

Trent variants are also making valuable contributions to the company's research efforts, particularly the Trent 500 which is being used for two rebuilds embodying aspects of the company's R&D Vision 5,10 and 20. The first which was completed in 2005 participated in ANTLE (Advanced Near Term Low Emission) for some years and made important contributions to meeting the environmental goals. The second rebuild includes many contributions from companies involved in the European More Electric Engine (MEE) project known as Power Optimised Aircraft (POA) and testing is hoped to be started later this year.

Terry Ford