Noise Tests Keep Promise Of Open-Rotor Engines Alive

Research seeks answers to lingering questions about viability of open-rotor engines

Printed headline: Key Developments

The spike in oil prices of 2008 may have rekindled interest in open-rotor engines, but it will be several years before the renewed research effort determines whether they are viable candidates to power the next generation of single-aisle airliner. Rolls-Royce is studying a geared open-rotor engine, the RB2011, with a baseline pusher layoutCredit: ROLLS-ROYCE Fuel prices have subsided, and airline attention has been distracted by other economic woes, but the pressure to reduce and eventually reverse the growth in aircraft emissions has ensured that open-rotor studies continue. Research is focusing on the key issue of noise from the exposed blades, and whether there can be an acceptable trade between marginal acoustic improvements and substantial fuel-consumption and emissions reductions. In the U.S., initial results from a scaled General Electric open-rotor test rig show noise reductions from redesigned blades. Rolls-Royce has also completed initial scaled open-rotor acoustic tests. The issue is whether an open rotor that comes in under today’s Stage 4 noise limits will be acceptable later this decade when competing advanced and geared turbofans will offer far greater reductions in noise from existing levels. In Europe, two open-rotor demonstrators are planned under the Clean Sky initiative’s Sustainable and Green Engines (SAGE) project. Led by Rolls, SAGE 1 is a geared open rotor while SAGE 2, led by Safran, is a direct-drive engine. SAGE 1 has a transmission system between the free power turbine and contrarotating propellers. The baseline configuration has the open rotor at the rear. SAGE 2 has a contrarotating turbine directly linked to the propellers, a layout similar to GE’s Unducted Fan. Clean Sky is hedging its bets with SAGE 4 led by MTU, which is a geared turbofan with an advanced fan drive system developed by Avio. Both companies are partners in Pratt & Whitney’s PW1000G geared turbofan program.