In 2001, the Advisory Council for Aviation Research and Innovation in Europe (ACARE) published the much referenced ‘Vision for 2020’ [1] which set targets of 50% reductions in fuel-burn and perceived noise, and 80% in landing/take-off NOx emissions, relative to year-2000 aircraft, to promote a secure European global leadership in the aviation market while simultaneously responding to societal needs. ‘FlightPath 2050’ [2], further expanded these targets to 75% fuel reduction, 65% perceived noise and 90% landing/take-off NOx emissions by 2050. The massive challenges embodied in these documents are far from an isolated issue for the European aviation community – for instance, similar initiatives proposed by NASA for the ‘N+2’ (service-entry 2025) and ‘N+3’ (service-entry 2030–2035) generations of aircraft demonstrate the heightened global awareness of the issues, and have stimulated significant activity in wide-ranging future technologies from operation and airframe to advanced materials and control systems.
The FlightPath 2050 and ACARE 2020 aims can only be met if the operational and aircraft efficiency are improved along with airline fleet renewal. Current passenger aircraft designs (tube and wing with underslung turbofans) are coming to a peak of performance with further improvements becoming ever smaller and harder to realise. In the longer term, there is a requirement for breakthrough technologies to enable step changes in performance. In the nearer term, incremental development of existing aircraft configurations helps to improve aircraft performance, as witnessed by the Airbus Neo and Boeing Max aircraft derivatives, which achieve ≈ 15% fuel burn reductions, where the majority of the performance increases are coming from a new generation of geared turbofans.
| Target per passenger km, compared to Year 2000 | ACARE Vision 2020 | FlightPath 2050 |
| CO2 | -50% | -75% |
| NOx | -80% | -90% |
| Perceived noise | -50% | -65% |
| Emission-free taxiing | – | required |
| Recyclable air vehicles | – | required |
Current powerplant development is further contributing in part by a move to increased engine bypass ratio, facilitating increases in propulsive efficiency through operation at reduced specific thrust. This is a major factor in the design of the next generation Ultra-High Bypass Ratio (UHBR) engines including Advance and UltraFan®. The UltraFan® concept developed by Rolls-Royce promises a 25% reduction in both fuel burn and CO2 with significant reductions in both NOx and perceived noise. The UltraFan® concept has a proportionally larger fan for each thrust class to reduce the specific thrust. Conventional nacelle design rules, based on performance optimisation at a single design point, will maximise potential propulsive efficiency gains at the chosen operating condition. However, the increased fan diameter associated with UHBR configurations, and the corresponding increases in fan cowl size, weight and aerodynamic drag, may serve to diminish these gains. These efficiency reductions may be mitigated by the application of multipoint, multi-objective optimisation methods configured to minimise integrated mission drag as opposed to that associated with a single ‘cruise condition’ operating point. Specific flight regimes where off-design behaviour is critical to the mission include take-off, featuring high angles of attack and the potential for external cowl separations; windmill, where very little flow passes through the fan and so massively increases spillage; and idle descent, where there is both increased angle of attack and reduced mass capture ratio. Currently, the design sensitivities of novel compact UHBR nacelles at off-design conditions are not well understood, and current industry CFD predictive capabilities to interpret measured data results from the UltraFan® flying test-bed nacelle are not validated against representative geometry.
Nacelles have historically been designed in isolation. However, the larger fans and nacelles become harder to integrate under the wing of current civil transports, which leads to a closer coupling with increased interactions that PART B. I of the Partner(s) Application/Proposal for IA/RIA/CSA (Technical Section) – ODIN 5 need to be understood. This can lead to structural efficiency and weight saving opportunities. It also means that the wing, nacelle design and exhaust system can no longer be considered aerodynamically separate, as each will influence the others to a significant degree. The engine installation onto the wing will impact the nacelle aerodynamics both at cruise and a range of off-design conditions including windmilling. In addition, a critical phenomenon affecting the engine operating condition is ‘nozzle suppression’ due to installation and the associated aerodynamic interaction between the engine exhaust and the wing. This affects the main exhaust metrics such as velocity coefficient as well as the important bypass and core discharge coefficients. This is particularly important at high-lift conditions, due to the deployment of control surfaces.
One key ACARE goal, which currently limits the operation of aircraft and even the development of airports, is that of perceived noise, where a 65% reduction from the year 2000 levels is targeted by 2050. Understanding how novel compact UHBR nacelle designs and exhaust jets contribute to overall aircraft sound levels, particularly with high-lift systems deployed on take-off and landing, when jet-flap interaction noise will occur, is a critical outcome of the LPA flight test demonstrator (FTD), with measurement methodologies and predictive computational aero-acoustic codes needing to be validated against known data. The objectives of the ODIN project play an important role in facilitating the introduction of the fuel-saving, emission reducing, close-coupled novel compact UHBR engines described above, including assessment through various critical flight regimes, rather than focusing on cruise conditions alone:
Objectives of the ODIN project:
- Deliver validated aerodynamic design guidelines for UHBR novel nacelles which take into account off-design performance and requirements including external cowl separation at low-speed high-lift conditions
- Deliver detailed understanding of installed UHBR nacelle off-design performance to aid interpretation of the UltraFan® FTD results. Establish a highly instrumented, mid-TRL nacelle section rig to take innovative
measurements of the key flow physics of nacelles at off-design conditions. The rig will also facilitate future nacelle external cowl research - Quantify the influence of exhaust suppression under low-speed high-lift operation at low flows for the closecoupled UHBR nacelle using a highly-instrumented nozzle rig in a commercial transonic wind tunnel
- Evaluate design constraints imposed by noise levels at take-off, approach and cruise by combining the tools of computational aero-acoustics and experiments in a commercial transonic wind tunnel.
- Deliver guidance on sensor setup on a flying test bed (FTD) for the validation of acoustic numerical simulations