Paris Air Show

Paris 2011: Trent XWB newcomer is mature before its time

 - June 19, 2011, 5:00 AM

With positive early test results and an accelerating work schedule, Rolls-Royce is confident it can deliver the Trent XWB as a mature engine, ready for full production before the end of 2014. Related technology programs are said to be on track in terms of high temperature and thrust. The new engine is to power the Airbus A350 XWB, for which orders from nearly 40 customers number more than 570, having risen by over 60 in the past 12 months.

Later this year, the manufacturer expects to have optimized its final Trent XWB manufacturing methods, drawing on lessons learned from construction of eight development engines (see box). Rolls-Royce claims that its preparation for manufacturing is the “fastest ever” large-fan production ramp-up. “Proper production” will start toward the end of next year, with a maximum rate of almost one engine a day (315 a year) being achieved by late 2017 or early 2018.

The 118-inch-diameter fan is Rolls-Royce’s largest and provides a lower hub-to-tip ratio, enhanced by use of a narrower hub than that on the Trent 900, which powers the larger A380. The fan blades have an increased surface area –“as much as 50 percent” more–providing higher air flow and reduced drag-to-weight ratio, explained Trent XWB chief engineer Chris Young in a briefing ahead of the Paris Air Show.

The large fan is combined with a smaller core to produce a higher load differential between the various bearings in the three-shaft powerplant. The “optimized bearing-load management system” sees greater loads carried further forward, where larger bearing balls provide better mechanical loading, which in turn is better for fuel burn, said Young, who acknowledged that this “more capable” solution is also heavier.

The Trent XWB’s “rising line” compressor sees considerable innovation, with blades rotating at higher speed toward the rear of the intermediate-pressure stage. The long-term fuel burn will benefit from increased overall pressure ratios in the two-stage intermediate-pressure turbine (IPT).

The short low-pressure turbine (LPT) has been scaled from that in the Trent 900 and is nine inches shorter. The lighter LPT offsets the higher weight of the IPT. The rear fan case, made of composites material and supplied by new Trent partner ATK, has gone well, said Young. To ease portability of completed engines, Rolls-Royce has developed a transportation stand that enables a simpler separation of the fan case from the core before being are air-freighted to Airbus.

A major focus for Rolls-Royce has been its plan to deliver engine maturity during–or even before–production ramp-up. Trent XWB program director Chris Cholerton said the manufacturer has matured its design processes, looking at all disruptive events occurring throughout the Trent fleet and applying the knowledge of previous failure modes to establish reliability.

It has used failure-mode effects and critical analysis (FMECA) and the Pareto principle–the idea that, say, 20 percent of causal events generate perhaps 80 percent of consequent effects–on some 400 parts and has 1,200 mitigation plans in progress.

Huge investments have been made in computing capacity to support component and systems testing aimed at reproducing in-service operating conditions. Key technology contributions to the Trent XWB program have come from Rolls-Royce’s environment-friendly high-temperature and E3E two-shaft demonstrator engines.

There are four Trent XWBs undergoing tests, with a further four being built for completion by July. Testing momentum is increasing in anticipation of the Airbus A380 flying-testbed (FTB) first flight with a Trent XWB later this year. Cholerton claimed that initial testing has gone “very, very well.” The engine, which initially will provide 84,000 pounds thrust, has been operated at thrust levels of more than 100,000 pounds, said Young, with the first unit exceeding expectations. 

Rolls-Royce invested increased resources into the early stages of the program, including proof-of-concept work. The highly instrumented engines, from which more than 1,500 parameters–including overall gas flow–are measured, are validating all the design modules.

Young said the XWB has yielded the “best-ever Trent performance” results. Cholerton added that specific fuel consumption has been “excellent; better than expected.”

By late May, Rolls-Royce had completed 63 rig-test programs as it sought to ensure engine reliability, having begun investigation of mechanical behavior at an early stage and increased its diagnostic capability. Many elements of such testing have been pushed to “extreme limits,” according to Cholerton.

The company has completed a large proportion of emissions-control reliability testing. Inspections and product assessment tests covering humidity, thermal and vibration (such as that following blade loss) performance have been completed ahead of final humidity inspection and checks.

Functional operation is described as “excellent,” with “very good” surge margin demonstrated in the intermediate-pressure compressor on which one of the four rows of guide vanes are fixed. Young said final nozzle optimization, balancing specific fuel consumption with operability, is a key milestone in progress toward first flight of the Trent XWB.

Rolls-Royce claims that “real-time” digital X-ray equipment introduced for the Trent XWB program is the most powerful in western Europe, showing how all the engine parts are working compared with their intended operation. The system saves weeks of time compared with earlier film technology, according to the company.

Young said the digital X-ray results obtained so far will contribute to engine optimization and further improvements in performance. For example, in the first and second stages of the low-pressure turbine, the manufacturer has been able to observe movement of the blade tips relative to the blade seal; if tighter tip clearances can be obtained, then initial performance can be maintained further into the engine’s life.

The manufacturer said much has been learned during early maintainability validation tasks. All on-wing tooling has been validated ahead of the Trent XWB installation on the FTB, which must be approved by the European Aviation Safety Agency.

FTB operations mark a fourth “parallel stream” of testing, which began last year with rig work, followed by initial expansion of the performance envelope and the start of maturity testing. Function and performance testing now under way will be followed by certification-related activity.

The FTB will sport a brand-new pylon, which introduces a new mounting that hangs the powerplant from the fan and rear engine cases, rather than–as previously–“Airbus-style” from the core, which now carries no load. “We have felt for some time that this was the preferable mounting and have worked with Airbus from very early on” to develop it, said Young.