Bigger engines, bigger challenges. Clean Sky’s ASPIRE resolves them
For large passenger aircraft flying medium and long-haul routes, Ultra High Bypass Ratio (UHBR) Engines will power the majority of airliners over the next three decades. But at twice the size of today’s engines, their aerodynamic and aeroacoustic interaction with the rest of the aircraft presents many issues. Clean Sky’s ASPIRE project has brought the best of Europe’s research centres together to face those challenges.
Everyone’s talking about electric, hybrid and distributed powered aircraft of the future – and their evolution is certainly gaining momentum. But for medium and large scale airliners, the prevailing power paradigm from around 2025 onwards – and probably for the next couple of decades beyond that – will be the Ultra High Bypass Ratio Engine (UHBR). In such an engine, the ratio of the air bypassing the core compared to the amount of air passing through the engine core is much higher (between 12 to 20 times) than it is in the best of today’s engines (at 10 to 12 times), for in UHBR engines the bypassed air only needs to accelerate a bit in order to generate lots of thrust. New carbon-composite lightweight fan blades and reduction gearbox technologies are key enablers to achieve these ratios, and they are projected to improve fuel consumption by 15-17% compared with reference ’Entry Into Service’ 2000 technology. But to accomplish this requires building engines with substantially larger diameters. ”Compared to the current generation of aircraft, the size of UHBR engines increases which leads to stronger interactions between the rotating engine parts (fan blades) and the airframe. And that needs to be reliably evaluated” says Michaël Meheut, Research Scientist and Project Manager at the Aerodynamics, Aeroelasticity and Acoustics Department at ONERA, the French Aerospace Research Centre.
”The objective of ASPIRE has been to study the aerodynamics and acoustics efficiency of new propulsion systems, for instance Ultra High Bypass Ratio engines. And more precisely, the objective is to demonstrate the ability of numerical (Computational Fluid Dynamics and Computational Aero-Acoustics) and experimental methods (dedicated to wind tunnel and flight tests) to accurately assess the aerodynamic and acoustic performance of such configurations”.
The project kicked off at the beginning of 2016 and was concluded last September, and in addition to ONERA brought together the leading research centres of Europe, including DLR, NLR and Russia’s TsaGI (which contributed to this project through national funding from Russia, not the EU).
Why has it been necessary to bring such extensive and diverse research capability to the task? It’s down to the sheer complexity of the challenge and the multiple facets that need to be addressed, especially the integration of UHBR engines under the wing due to their colossal size and their interactions with the aircraft’s nacelles, pylons and wings. ASPIRE has demonstrated the ability of existing numerical and experimental methods to accurately assess the aerodynamic and acoustic performance of such configurations thanks to a reliable modelling of fan/airframe physical interactions. Some specific technical objectives included the design of generic fan/OGV (Outlet Guide Vane) combinations representative of a future UHBR engine; demonstration of the ability of different CFD codes to predict aerodynamic performance of aircraft equipped with UHBR engines; demonstration of the ability of aeroacoustic methods to predict aeroacoustic performance of aircraft equipped with UHBR engines; and identification and assessment of the experimental capabilities for the characterisation of UHBR installation noise sources.
”It’s anticipated that most of the future generation of aircraft, with at least 150 passengers, entering into service between 2025 and 2035 will be equipped with UHBR engines” says Meheut, ”therefore the capability to accurately assess the performance of such configurations is a key driver for the design of efficient concepts in the coming years”. In terms of results, Clean Sky’s ASPIRE has produced a reference configuration (airframe with fan blades) with several derivative shapes designed by DLR and ONERA to support the numerical activities. Based on these configurations, ONERA, DLR and NLR compared the accuracy of their numerical methods used by European industry to assess the aerodynamic and acoustic performance of such complex configurations.
”The different partners compared their approaches and test benches used to measure the near field or far-field noise produced by this new engine generation. This work is particularly relevant in order to understand the ability and capability of each method for future engine and aircraft programmes” says Meheut.
Additionally, jet noise wind tunnel tests were performed at TsAGI to assess different experimental methods dedicated to jet noise measurement, with numerical and experimental methods developed and applied in the frame of ASPIRE on a full scale UHBR design.
”The progress made in ASPIRE regarding numerical and experimental methods will help the European industry to design more efficient engine and aircraft configurations in the near future, producing less noise around the airport” says Meheut. Clean Sky’s ASPIRE Project Officer Antonello Marino emphasises that the project, beyond its technical accomplishments, has also been a superb case study in demonstrating the potential of collaboration between different European research entities: ”It’s very important to underline the fact that this project, to achieve its challenging objectives, has demonstrated a really exceptional level of cooperation, facilitated through Clean Sky, between all these different research centres working together in European aeronautics towards a common objective. They’ve been sharing the ground hardware and software, and have made a comparison of the results achieved with different tools by using the same benchmark – they performed the same simulations or experimental simulations on the same benchmark in order to share results for the first time, to understand the pros and cons of the different methodologies. Really, it’s an exceptional cooperation” says Marino. ”There’s also been a large amount of dissemination activity – they’ve published more than 25 papers at international conferences and in journals and books, so in terms of impact for the European Community the ASPIRE project is very productive”.
As for mobility and environmental considerations in Europe, the findings of ASPIRE will contribute to the performance of current aircraft and those we’ll see in the near future. ”It will contribute to noise reduction, to the improvement of engine efficiency and reduced fuel burn efficiency compared to the best of today’s technology” says Marino.