Rolls-Royce’s development of an open-rotor engine for the next generation of midsize airliners has taken a giant leap forward after wind-tunnel tests revealed its design would comfortably meet current Stage 4 noise regulations.
“We’ve moved from a physics problem to an engineering challenge,” said Robert Nuttal, vice president for strategic marketing at Rolls-Royce. “Acceptable noise was the first of the five major open-rotor issues we wanted to de-risk.” He told AIN that the other four–the gearbox, blades, pitch change mechanism and aircraft/engine integration, “can all be solved.”
Earlier in the year, a series of tests were completed on its “Rig 145” installed at the low-speed DNW windtunnel in the Netherlands using a one-sixth-scale electrically driven rotor to simulate the huge, 170-inch diameter contrarotating rotors that would power the new-generation aircraft. A number of rotor configurations with different blade numbers, lengths, rotational speeds and blade spacings were tried and an optimum configuration found.
“We have confirmed that our computer model works,” said Nuttal. The model is now undergoing high-speed tests at the Aircraft Research Association transonic windtunnel in the UK.
The success of the trials mean that Rolls-Royce, the only engine manufacturer besides General Electric/CFM International to be working on an open rotor, can move on to the next stage of development confident that there are no engineering “show-stoppers” to an open-rotor-powered replacement for aircraft in the 120- to 220-seat range. “We’re a lot more confident it can be done,” said Nuttal.
Rolls-Royce believes the airline community is beginning to show real interest in the environmental advantages of open rotors and has been asking for regular updates on the program. “We’re out there briefing them all the time,” he said. “We’re also listening very carefully to what they’re saying about the operational implications of this engine”.
Clean Sky Initiatives
Nuttal argued that an open-rotor-powered aircraft could save the same amount of carbon dioxide as planting 250,000 trees during its lifetime, or as much as that produced by a reasonable-sized country. “The prize is so big that open rotors can’t be ignored,” he insisted.
The development program now moves on to a go/no-go decision around 2013-14, when the results of the open-rotor component of the European Clean Sky Joint Technology Initiative will be known. “We need to get to what we call technology readiness level 6,” he said. “That’s when we can tick the box that says we have proved our open-rotor concept is suitable for operation in the real world and we can begin designing an engine for a specific airframe.”
The time scale for entry into service of the next generation 120- to 220-seater has been slipping and is now set for around the end of the next decade. Rolls-Royce is looking at both two- and three-shaft open rotors and two- or three-shaft turbofans, the latter carrying the generic program names RB282 and RB285, respectively.
Under its “Options 15/50” strategy, technologies are fed into the various programs as they become available, the Option 15 engine being at the level of the Trent XWB and Trent 1000, which are around 15 percent more fuel efficient than a mid-1990s technology engine such as the International Aero Engines V2500. Option 50 is at the other end of the scale, at “enterprise level,” with the potential for 50-percent fuel savings when airframe, air traffic management and airport improvements are taken into account.
Option 20 is an advanced three-shaft turbofan–the RB285X– with a 20-percent fuel-burn saving and 20dB lower noise level than Stage 4, for service entry in 2018. Option 30 is an open rotor with a 30-percent fuel saving and 10dB lower noise than Stage 4, also for potential service entry in 2018.
While the remaining open-rotor engineering issues are solvable, each presents its own challenges. The gearbox driving the contrarotating rotor incurs extra weight and complexity, but is seen as the best way to achieve the maximum propulsive efficiency from the big rotors. “Rolls-Royce has plenty of experience with high-power gearboxes,” said Nuttal.
The pitch change mechanism will be sophisticated, requiring “some very careful design,” while the open rotor blades, to be developed by a yet-to-be-revealed partner, will be considerably larger than those of today’s biggest turbofans, and much slower turning to avoid supersonic tip speeds. “We’re looking at heights, spacing and numbers,” said Nuttal. The configuration of the Netherlands test model is currently “about 10 blades in each row.”
Unlike turbofans, propellers cannot be contained and so are designed with an “infinite” lifetime of one failure per 100 million takeoffs. The lifetime of the open-rotor blades will be an order of magnitude higher, said Nuttal. “We’re beginning to discuss certification issues with EASA and the FAA–this is going to be a tough certification ride and we need to start evolving a set of rules.”
Integration of the open rotor to the future aircraft is a “major issue,” said Nuttal. Airbus and Boeing are both working hard on designs and in early May Rolls-Royce, Boeing, Ruag Aerospace and Deharde Maschinenbau kicked off a collaborative research program to test a model concept airframe in 2010 at Ruag’s low-speed windtunnel in Emmen, Switzerland.
While Boeing stresses it has made no decisions as to the type of propulsion system for its future medium-size airliners, it says the trials “will help us to better understand the interaction of open-fan propulsion with a candidate airframe concept, and how much fuel savings might be possible.”
Rolls-Royce aims to have a flightworthy open-rotor demonstrator ready in four years, using an existing core engine to power the contrarotating propulsion. “That would allow us to put another six years into development of an engine that could enter service in 2018-20,” said Nuttal. “The way forward has been cleared by the Rig 145 noise results. The next four years will tell us whether we can be confident enough to propose this engine for commercial service.”