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Rolls-Royce Starts Gearbox Load Tests For New Large Turbofans

 - June 16, 2017, 10:02 PM

Rolls-Royce reported in late May that it had started power runs on the gearbox for its next generation of large engines at its Power Gearbox (PGB) facility in Dahlewitz, Germany, marking what the company said is another significant step in the development of its new UltraFan engine. The gearbox allows for a more efficient high bypass-ratio engine. PGB testing of a unit not under load started on September 1, 2016, heralding the eventual end of Rolls-Royce's three-shaft approach towards a “two and a half shaft” geared turbofan approach similar to that pioneered by U.S. rival Pratt & Whitney.

The Derby, UK-based manufacturer is working in partnership with Liebherr-Aerospace to develop the PGB, through their Transmission Technologies joint venture (which is based in Friedrichshafen). R-R leads to design definition/integration and testing.

UltraFan will be available from 2025 and will offer a 25 percent improvement in fuel efficiency over the first generation of Rolls-Royce Trent engine, said the manufacturer, which added that power would be “scalable for widebody or narrowbody aircraft.” It will ultimately be able to transmit up to 100,000 hp.

The plan for UltraFan includes new engine core architecture, reduced weight through a CTi fan system, and from titanium fan blades and a composite casing, plus ceramic matrix composites that require less cooling.

Mike Whitehead, chief engineer for Rolls-Royce Civil Aerospace and head of the UltraFan technology program, said, “We are continuing to deliver our UltraFan program and this latest achievement marks another milestone. Having successfully started tests on the altitude rig last year, we are now starting power rig tests.”

Trent Future

During a visit to Dahlewitz on March 28, journalists were able to tour the PGB facility. The Dahlewitz site, near Berlin’s Schonefeld Airport, employs around 3,500 people and has delivered over 7,000 engines over the years–in recent times it was part of the BMW-Rolls Royce joint venture that developed the BR700 family that now powers many wide-cabin, long-range business aircraft (such as the Gulfstream G650).

Phil Curnock, Rolls-RoyceCivil Aerospace chief engineer strategy & future programs, said that the Advance program to develop architecture for UltraFan was launched in 2014, based on the Trent XWB, with Vision 5 ‘Advance’ and Vision 10 ‘UltraFan/Open Rotor’. He said as the fan gets bigger and the bypass ratio higher, “the fan wants to turn slower but the turbine faster–so we’ve put a gearbox in the loop.”

Curnock listed various other technologies that would feature in the new engines–such as a lean-burn combustor, “blisks as appropriate” in the compressors, possibly titanium aluminide in the turbine. The idea is to offer the technologies in a scaleable architecture that gives maximum flexibility, so that the right engine can be developed as soon as an application is identified–when an airframer requires an engine for a new aircraft it has defined and plans to launch.

The Advance engine will not have the PGB and is intended to provide an engine that introduces all these technologies and efficiencies at an earlier stage, if required, with the UltraFan (or Open Rotor should it come about) being the next stage and introducing the gearbox for the first time in a Rolls-Royce engine.

The 80cm-diameter PGB operates differently to the gearbox in Pratt & Whitney’s PurePower engines as it has a sun gear and five planet gears but the casing is stationery, meaning the through-ratio of rotational speeds is 4:1 if there sun and planets are the same size (whereas it is 3:1 if drive is through the outer gear). Curnock, who illustrated this with a scale moving model of the gearbox (and a new iPad app), said the fan diameter would move “beyond 115 inches now with UltraFan.”

He said an initial flight test campaign was finished for the CTi fan system, and more than 50 simulated bird strikes (gelatin sacks are used now instead of dead birds) had been carried out. “There will be more testing this year with the case and the blades together,” he added.

At the large engine/Trent end of the spectrum the program is known as Advance 3, whereas for the mid-size engines that will power business jets in the future it is the Advance 2 program.

The second step in the Advance 3 strategy, explained Curnock, is “to prove the changes in the core,” eventually using a test engine composed of a Trent XWB front and T1000 back.

The first test core had been built at Bristol in the UK as of late March and was about to be moved to Derby for the tests starting “mid-year”. Curnock said, “It’s not an optimized engine, we’re just putting the core in a representative environment.”

The planned core changes include taking the HP compressor up from six to ten stages, while the IP gets shorter–which is the “more compact architecture we will need on the UltraFan,” said Curnock. “So Advance 3 is being readied for test in Derby… the UltraFan program is on track and we’ll see the concept freeze gate for the demonstrator later this year.”

Whitehead said that developing the future engines was about “protecting our market share, which is currently over 50 percent [for large engines]. We want to stay competitive,” but he added it was also due to environmental targets, such as ACARE 2050. He also played down the risk saying the principle was not new, as epicyclic gearboxes are used “in household drills.” However, what is new is the scale, loads and temperatures.

Nobody has ever built a gearbox of this size and power density before–so early on we decided we would need our own dedicated test facility,” said Whitehead. The test facility for the PGB at Dahlewitz cost around €84 million ($94 million) such is the scale of the requirement, with the second test PGB having a transmission capability of around 150,000 hp (100 MW dynamic torque) though testing would go up to 100,000 hp. “That’s the equivalent of 100 Formula One cars,” said Whitehead.

The main test rig sits on top of a 2,400 metric ton concrete block on springs and dampers, to isolate the rig. Whitehead said that a small engine gearbox test facility had been commissioned in Indianapolis.

Asked about the oil system in the light of cooling issues Pratt & Whitney reportedly experienced with its Geared Turbofan engine, he noted that the oil system was still in development; “We’ve not decided if we’ll use the engine oil system yet. We prefer not to do a different one but we can try out different types of oil.” He also said simulating the range of oil temperatures that would be seen in service was “a challenge” given that aircraft “need to be able to operate on a hot summer’s day in the Middle East.”

Advanced Concepts

Looking further ahead, Curnock told reporters that under its “‘Vision 20 and beyond” program, Rolls-Royce is looking at “How we get where we need to be for 2050.” He said that although “there is more we can do with gas turbines, we are increasingly looking at the aircraft and engine together…distributed engines, hybrid, embedded.” He added “electrification is a key trend.”

Rolls-Royce opened a new center at Cranfield University in the UK in Febuary this year. A joint initiative with Airbus, the Aerospace Integration Centre will look at a “more collaborative approach” between airframers and engine manufactures, as it becomes clear future aircraft could be far more integrated than today’s pod-on-the-wing approach.

One possible configuration could be one gas turbine driving a generator that drives electric motors distributed around the airframe. A stepping stone to this could be an aircraft with a gas turbine and “remote electric propulsors augmenting the main engine.” But he said, “If you look at electric you need to look at the total carbon footprint–how is the electricity produced?” He also noted “batteries stay the same weight throughout the flight” and don’t burn off like fuel, and the cabling and is also heavy–but upsides include having motor/generators that can charge the battery on descent, for example. Yet another issue will be if such aircraft (such as the Airbus/Rolls-Royce E-Thrust concept) would fly slower, which could create ATC difficulties as airliners at present tend to cruise at around the same speed (around Mach 0.8).

Curnock said that Rolls-Royce Bristol took a Rolls-Royce Adour engine and put a starter/generator in it, as a phase I demonstrator. “We are moving forward our electrical capabilities,” he added.

Supersonic Bizjets

Rolls-Royce (Chalet 93) is also looking into supersonic concepts and is closely following programs that are developing supersonic business aircraft, so as not to miss out on what could be a lucrative market. Dr. Dean Roberts, market analysis executive for business aviation, said, “We’re unique in that we have supersonic civil aircraft experience, and can draw on our fighter experience as well.” He noted that until regulatory hurdles could be overcome to allow supersonic flight over land, the “halfway house’ would be a hybrid SSBJ.

The company has done detailed analysis, Robert said, and which showed the hybrid approach is “quite an attractive proposition–looking at the routes you could fly there are very clear benefits.”

On the economic side Rolls-Royce’s analysis suggests, “the further you can go [in terms of range] the more people will pay for speed. What we think is that if you radically increase speed, you will get an exponential not a straight line relationship so you can substantially increase the price”–which would be needed to make such programs viable. Thus “it will not destroy the subsonic world” as the number of units would be small–but this ties in with the increasing number of billionaires.

That U.S. company Aerion selected GE Aviation last month as its engine supplier for the AS2 supersonic business jet could be seen as a blow for the UK company, but Aerion admitted technology transfer issues (U.S. ITAR) were one concern it had.