Airbus is encouraged by preliminary data analysis of CFM’s RISE engine but the airframer’s propulsion engineering chief insists that the open-rotor technology has still to cement its case for selection against geared-fan alternatives.
“It’s a given ambition, but it’s not a given,” says Frank Haselbach, adding that it remains premature even to describe RISE as the “preferred solution” for Airbus’s future single-aisle programme.
“It could be,” he says. “You still have to look at the what the amount of commercial risk you’re taking, what’s the amount of industrial risk you’re taking to going down this route. So it’s a far bigger question than just a technical question. But technical is the entry ticket.”
Windtunnel testing of the engine model – with and without partial wing structure, studying performance, acoustics and interaction with flaps and slats – is wrapping up in France and the Netherlands after more than 500h, and full aircraft model tests will commence next year. In parallel with the windtunnel testing CFM is moving from the key component to subsystem work.
Haselbach says Airbus is “halfway” through the broad assessment programme to understand technological, integration, safety, certification and maintenance concerns associated with open rotor “whether it would be on the rear or on the wing”.
“We’ve seen some very good stuff coming out of it,” he says. “On community noise and cabin noise, we’re quite happy.”
Acoustic target zones were defined, he states, and preliminary data “makes us believe that we would actually hit these targets zones” – although it all requires validation.
The windtunnel testing has given Airbus “a good feel”, adds Haselbach. He hopes the further integration testing will “retire some of the residual risks” because the open rotor is “clearly the best value proposition”.
But the technology presents integration challenges. “It’s a small core, it’s a huge [low-pressure] system. It’s a gearbox in between. You have to mount it in the right way…we’ll have new sorts of loads on the aircraft, which we never have because we normally guide the air to the engine.”
Haselbach points out, however, that the computational fluid dynamic capabilities are “mind-blowingly different” compared with 20-30 years ago, and supercomputing power can analyse the rotors, stators and wing integration.
With the absence of nacelle containment for blade loss, fuselage shielding will be part of the design consideration. “There is preventive detection in there,” says Haselbach. “But you will have to have an element of fuselage protection.”
Passenger perception is a less-predictable issue, he says, but adds: “I think we’ll cross that bridge if we are at a point where we say this concept is sound enough to make an architectural decision towards it.”
Geared-fan alternatives to RISE, with increased bypass ratios of 14 or 15, remain contenders, Haselbach states. “They’re good as well. They’re not as good as the RISE.”
But he says the crucial architecture selection decision – still a couple of years away – will hinge on whether Airbus feels more reassured by mitigation of risk from open-rotor technology or the relative maturity of a more conventional design. He adds: “If you don’t prepare yourself to actually do that, at least mentally…you will never advance.”
