European Union targets for simultaneously cutting jet engine noise and carbon dioxide emissions can only be met by technology breakthroughs, say engine manufacturers.
In Budapest this month the final workshop of the EU's Environmentally Friendly Aero Engines (Vital) propulsion programme heard Rolls-Royce and Snecma claim success in achieving the Vital goals of cutting engine fan noise and clipping 7% off CO2 emissions.
But the Vital targets areonly an interim step towards a more ambitious objective, set by the EU's Advisory Council for Aeronautical Research in Europe (ACARE), of halving perceived aircraft noise and also halving CO2 emissions for engines in service in 2020. To reach the ACARE targets,a far greater noise reduction for the engines, along with a specific fuel consumption reduction of 20%, will be necessary. Achieving both is beyond today's technology.
The fan is key, asits spin rate relates to engine noise output and the air flow it delivers contributes to the turbofan's cycle pressure, enabling good fuel burn. Experience showsthat larger fans can rotate more slowly, reducing noise output while boosting air flow and improving cycle pressure and fuel burn. But a larger fan is heavier, requiring mass reductions.
The relevant noise from an aircraft or engine and its fan is the frequencies that human beings most notice. Its unit of measurement is the effective perceived noise (EPN) decibel (dB).
Vital's effective perceived noise level reduction goal for the fan is 6EPNdB. However, to achieve ACARE's halving of aircraft noise the overall engine reduction has to be 10EPNdB.
Because the fan is so much wider than the engine core, driven air will go around the powerplant as well as through. The difference between the two is described as the bypass ratio (BPR). While BPR increases have gone hand in hand with noise and fuel burn improvements, through Vital engine manufacturers now know that beyond a ratio of 14 noise improves, but fuel burn, and therefore CO2 output, does not. Thus, new technologies are needed to enable improvements in both noise and SFC.
Vital is being extended by 15 months to conclude in 2010, as its partners have so far completed just over half of the planned tests. The remainder will be carried out this year, with much of the delay explained away by its partners as "the usual research unknowns".
Early declarations of success are based on predicted weight and noise improvements expected to be confirmed by comparisons with the two ACARE baseline engines. Analysed in the context of their operation on generic Airbus long- and short-range aircraft, the baseline engines are the R-R Trent 700, for long-haul flights, and CFM International's CFM56.
Competing against these two engines under Vital were R-R's direct drive turbofan (DDTF) and Snecma's contra-rotating turbofan (CRTF) designs. Snecma completed the CRTF fan's first series of tests at the beginning of March and its baseline testing at the Moscow-based Central Institute of Aviation Motors does not start until June. Only then can it compare results and confirm reductions. While declaring 4EPNdB, the company predicts further tests later this year will validate a 6EPNdB reduction.
This monththe Moscow institute completed 16 weeks of testing of its CRTF with a 10-blade front rotor paired with a 14-blade second rotor. The next version will have nine and 10 blades, respectively. A standard CFM56 has a 22-blade fan.
R-R led Vital's fan subproject aiming for a fan module 30% lighter, 6EPNdB quieter and 2% more efficient. It declared a success with a DDTF fan, mechanically tested at full scale at its Derby, UK plant, that can run 15% slower. It uses a composite blade with a tip stagger angle increased for aerodynamic optimisation. A scalemodel will be tested from June in Germany to confirm noise levels.
Snecma research and technology deputy vice-president Isabelle Dubois says Snecma could make composite blades with the geometries that R-R achieved under subproject two and that it would use them for its Leap-X engine - indicating that while Vital has its eye on 2020 its partners are equally focused on the near term.