Rolls-Royce may be a late starter when it comes to introducing composite fan blades into its large turbofans, but the UK manufacturer claims to have had good reason to stick with its hollow titanium blade designs until now.
"One of the questions we're often asked is, 'Aren't you coming to this a bit late compared with your competitors?'” says Alan Newby, R-R’s chief engineer for future programmes and technology.
“I guess one of things we've always been looking at during the development of this technology is how it trades against our existing fan blade system. So we clearly had a strong technology lead with our hollow, super plastically-formed and diffusion-bonded titanium fan blades, and it was really only recently that we've been able to see a carbon system begin to offer benefits over and above the titanium fan system.
“For example, we know the base ingredients for a carbon fan blade are lighter, but if you look at something like a Trent XWB [titanium] fan, that's about 40% hollow so it's not all metal. So we've got quite a hard target to shoot at."
Switching to a composite fan system (blades and case) requires a complex optimisation process that can affect the design of the airframe as well as the engine. In addition to fuel burn, environmental issues and maintenance support costs need to be taken into account. The business case for introducing expensive new materials into an aeroengine is impacted as the price of fuel rises or falls.
Also, as engine bypass ratios increase, the low pressure system represents a greater proportion of overall engine weight.
"Anything you can do to reduce the weight of the LP system is of paramount importance. For a large engine it can be anywhere between 500-1,000lb (250-450kg) you'd be shooting at to take out as a result of the lightweight fan system,” says Newby.
Lessons are also learned as the in-service Trent fleet accumulates ever-increasing numbers of flight hours and cycles, and the performance and reliability of new technologies can be accurately assessed.
R-R has progressed its composite fan development work to the point where it has completed tests on the first Trent 1000-based advanced lightweight low-pressure system (ALPS) demonstrator engine at its Derby, UK headquarters, and begun testing a second. These powerplants incorporate a carbon-titanium (CTi) fan system (titanium is used at the leading edges of the composite blades) as well as composite “rafts” that simplify the installation and maintenance of the engine’s pipes and cables.
The Trent 1000 uses a 112in-diameter fan with 20 blades, but this is reduced to 18 on the ALPS demonstrator.
"It's the next step in a major evolution of our technology for carbon-titanium fan blades,” says Newby.
R-R has been refining its CTi blade for about a decade to "get it down to a thinness that compares with what we can do with our best titanium blade”, he adds. “It's only now that the design and manufacturing technology has got to a level where we're really seeing a competitive blade, compared with what we can do with the titanium standard."
The new design combines the weight benefits of a carbon fan system with aerodynamic performance comparable to that achieved with a titanium blade. The thickness of a composite fan blade is largely driven by requirements related to bird strike, so development work has focused on 3D lay-up techniques to produce blades that are both thin but sufficiently strong.
The ALPS blade is not aerodynamically identical to Trent 1000's titanium blade due to the different mechnical properties of the materials.
"We rig-tested a version of this blade and we found that we can get as good as or slightly better than the Trent 1000 aerodynamic performance,” says Newby.
Manufacturing the blades and case involves an automatic tape lay process which lays down thin strips of carbon. "That allows us to very carefully tailor and control the shape of the aerofoil to get the right shapes we need, which are optimised from both a manufacturing and aerodynamic point of view,” says Newby.
Nearly 50 bird-strike tests on have been performed on individual blades, as well as fatigue and ballistic tests. "We've got quite a strong background of rig and subsystem and element testing under our belt before we put the blade anywhere near an engine,” says Newby.
Looking further into the future, the CTi blade technology could find its way onto an open rotor engine (see below).
"Fundamentally, the open rotor has very big propellers, so the requirements for lightweight technology are equally applicable,” says Newby. “I think a lot of the composites technology we're developing and manufacturing would be applicable to such a solution. The challenges and the design requirements are similar."
In addition to ALPS, R-R is running core demonstrators to investigate technologies to improve the thermal efficiency of the engine. These incorporate advanced materials such as ceramic matrix composites, and cooling schemes for turbine blades that require advanced manufacturing processes.
Three full-scale Trent 1000-based demonstrator programmes are underway, including ALPS which will undergo flight tests later this year using the company’s Boeing 747 testbed. A second demonstrator incorporating a low-NOx combustion system is due to be flight-tested next year.
The goal, says Newby, is to bring some or all of these new technologies into production engines by around the end of the current decade.
‘LOW-LEVEL’ WORK ON OPEN ROTORS
Although it is focusing the bulk of its research and development work on new technologies for the next generation of turbofan engines, Rolls-Royce continues to work on open rotor concepts albeit “at a relatively low level”, says the company’s chief engineer for future programmes and technology, Alan Newby:
"We've done quite a lot of work on the conceptual design of the open rotor and identified what the critical technologies are for that type of configuration.
“As a result of that we've completed a number of rig tests on the critical technologies that we would need to understand and that would actually be those required to make an open rotor concept successful. Noise and performance are the key ones.
“We've done work over the years on scale model testing of a contra-rotating propeller system – low-speed and high-speed testing – which essentially validates our performance and noise predictions. Those are the key risks, effectively, and we now understand those and I think we know how to mitigate those.
“The other one clearly is the gearbox system in there and we have run a test rig with some of our partners and that again allows us to understand that if you want a contra-rotating configuration we know what the gearbox would look like and we know what the design challenges would be.
“At the moment I think we understand the challenges and we've done quite a lot of mitigation work to understand what it would take to bring an open rotor engine to market, and we've done quite a lot or work maturing the critical technologies. At the moment we are working at relatively low level, trying to understand what the requirements are going forward."
R-R says it does not have firm plans to fly an open rotor demonstrator, but says it will be ready to do so when required by the manufacturers.