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Open rotor set to become viable

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Novel aeroengine technology promises to be quieter and more efficient

Silent flight: The new open rotor could be quieter than the A320 family

Contra-rotating open rotor engines that are quieter than today’s turbofan engines could be developed in the near future, one of the top engineers at Rolls-Royce has predicted.

 

The aerospace giant has been working on the development of the novel engine technology as part of the pan-European Clean Sky initiative. And Ric Parker, director of research and technology at Rolls-Royce, said the progress that had been made had convinced him that contra-rotating open rotor engines offered real potential.

Speaking at a conference in London on disruptive aerospace technologies, organised by the IMechE, Parker said: “We believe we can make an open rotor significantly quieter than today’s aircraft and significantly more efficient. We are talking about an open rotor that could be 13dB quieter than today’s A320, which I think should be quiet enough for most people.”

Contra-rotating open rotor engines, which feature uncased blades positioned on two stages that rotate in opposite directions, are of interest to the aerospace sector as they promise increased propulsive efficiency relative to the turbofan. But attempts to develop the design have previously been hampered by noise levels, as there is no fan case to muffle the sound of the blades.

Clean Sky believes that by changing the shape and configuration of the rotor blades, noise could be brought down to levels that comply with regulations. Engineers have spent several years cutting noise by looking at the optimum distance between the two contra-rotating stages, the number of blades on each stage and the profile of the blades themselves. Shifting the position of the engines from the wing to the rear of the fuselage also cuts noise.

Ground tests of a contra-rotating open rotor engine developed by Clean Sky partners, which include Rolls-Royce and Airbus, will begin in early 2016, with flight tests to follow in 2019.

Parker used his speech at the disruptive aerospace conference to discuss the best options for open rotor engine/airframe configurations. “The open rotor formation comes in two forms: one is a pusher sitting at the back of the aircraft, and this is the style we have been working on most recently with Airbus. There are some advantages with it sitting at the back, certainly in terms of cabin noise, but there are disadvantages around the overall installation and getting the centre of gravity of the aircraft in the right place.”

An alternative option would be to have an open rotor mounted in a more conventional position under the wing. “But we are talking about rotors that are probably 4m in diameter, so you are either going to have an incredibly long and heavy undercarriage, or a high-wing variant of the aircraft. You can get some added benefit if you get some disruption to the flow over the wing. But you also get some enhanced lift if you mount the rotors a bit closer to the wing, so that they accelerate the flow over the top of the wing as well.”

Looking further into the future, Parker said that open rotors might one day be installed on a blended wing bodied design. In such a configuration, he said, open rotors could be mounted above the wing with the advantage of deflecting noise away from the ground. And on boxtail designs, an engine could be fitted with cowls around the rotors for noise suppression.

He said that the technology obstacles were unlikely to be the biggest hindrance to the future development of open rotors, citing political issues as the biggest single restricting factor.

“Noise is a very emotive issue. If you ask the general public to rank noise, nitrogen oxides and carbon dioxide in order of importance, they will mostly put noise at the top of the list, which in terms of short-term inconvenience is probably where it should be. But in terms of the long-term good of the planet, then it should be in reverse order,” he added.

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