The introduction of aircraft with advanced counter-rotating open rotor (CROR) powerplants will improve specific fuel consumption by 10-15% relative to equivalent turbofans.
In the 1980's, prototypes of the first generation of open rotor engines were developed and tested. One of the findings was that the noise generated by these engines, even in the en-route flight phase, was significant, thus hazarding public acceptance. Since then significant effort was dedicated to improve their aeroacoustic design and the new generation of CROR engines currently envisaged are much quieter than their predecessors.
The NINHA project will assess whether noise issues away from airports (i.e. during high altitude operations) will hazard the introduction of this new generation of powerplants designed to improve fuel burn and reduce CO2 emissions beyond the ACARE 2020 objectives.
The DREAM programme is addressing the noise of CRORs around airports. NINHA addresses en-route noise. Existing long-range noise prediction models and CROR noise source prediction models will be developed in NINHA for their application to the prediction of en-route noise levels, and validated with A400M flight data and CROR noise data obtained in projects like DREAM. Since aircraft might be audible en-route in areas with very low background noise levels, a review will be performed in NINHA of issues such as noise metrics, annoyance and perception. EASA have agreed that their recent study on en-route noise levels of current aircraft can serve as a reference for the NINHA project, allowing a comparison with the predicted en-route noise of advanced CRORs. Based on the findings in NINHA, recommendations will be given to ICAO/CAEP on possible future en-route noise evaluation processes.
Open rotors: Removing barriers to entry
Aircraft with advanced counter-rotating open rotors (CRORs) offer the potential for significant reductions in fuel burn and carbon dioxide emissions relative to turbofan engines of equivalent thrust. An EU-funded project provided valuable data regarding their en route noise.
The EU has devoted much effort to reducing aircraft noise and emissions. The bulk of the work has been focused on turbofan engines commonly used in modern airliners and noise near airports during take-off and landing. The EU-funded 'Noise impact of aircraft with novel engine configurations in mid- to high altitude operations' (NINHA) project was initiated to extend these studies. Scientists sought to scientifically evaluate en route noise issues (during mid- to high-altitude operation) associated with novel CROR systems.
One major challenge for researchers was to validate the long-range noise propagation models at cruising altitudes. To this end, they performed near- and far-field measurements of noise attenuation between a propeller aircraft with a conventional engine and the ground. These measurements matched those of the prediction models that also involved locally varying atmospheric conditions.
Another major challenge was to extrapolate near-field CROR noise, measured in wind tunnels, to the far-field. Researchers applied two computational aero-acoustics methods to produce a comprehensive database for the far-field. The source noise and propagation models were implemented in the SOPRANO aircraft noise prediction software suite to predict en route noise levels as perceived on the ground.
Researchers also developed an en route noise impact model. Noise data for turbofans, turboprops and CROR aircraft were obtained from the EASA BANOERAC study, NINHA dedicated measurements and SOPRANO predictions, respectively. Combined with en route air traffic data generated in NINHA, the en route noise impact was established for various aircraft and engine configurations (CROR share). Based on the impact calculation results, researchers made a first assessment regarding the challenges the new-generation CROR aircraft may face.
NINHA established that at cruising altitudes the CROR noise with current technology is expected to be significantly reduced compared to that of the unducted fan of the 1980s. The maximum cruise noise level should be equivalent to that of today's turboprops. Further reductions in open rotor noise are expected before they enter service in the late 2020s.