Context and Challenges

The philosophy of the COBRA is to use the complementarity of EU-Russian expertise in “ducted architectures”; it is expected that this will provide advantages in dealing with the noise and installation challenges required to overcome the insufficient noise performance of the counter-rotating fan tested in the VITAL project. Higher bypass ratio designs (use of gearbox becomes mandatory) with much lower blade tip speeds and blade counts will be explored, whilst at least maintaining and maybe increasing the very good aerodynamic efficiency already achieved.

The goal of COBRA is to promote an effective cooperation between EU and Russian scientists. The project is structured to benefit from the existing skills of EU and Russia in the design and the multi-disciplinary optimisation of UHBR CRTF architecture to assess at the whole engine level the capabilities of the UHBR CRTF in terms of performance and acoustic levels. Comparisons will be made with the VITAL CRTF results and with the strategy for the CROR. These activities will include extensive CFD-CAA and structural/mechanical computations and experimental studies.

 

An environmental challenge

In the near future, aviation will have to tackle various challenges imposed by a continuously increasing global mobility demand; from private as well as business travel. Fuel prices will further increase due to limited natural resources and emissions will have to be reduced to contribute to the global efforts against the climate change. Although air traffic accounts for only 2 percent of the global CO2 emissions, the aircraft industry has defined ambitious targets to reduce the CO2 emissions per passenger kilometre by 50% until 2020 and up to 75% until 2050 (both compared to the 2000). The Advisory Council for Aeronautical Research in Europe (ACARE) identified the new research needs for the aeronautics industry, as described in the new Strategic Research and Innovation Agenda (SRIA – Vol 1 ), published in September 2012. The targets for CO2 emissions as set originally by ACARE for 2020 were split into component areas: airframe, engine, ATM and operations. SRIA goals have been re-calibrated to reflect the achievements assessed by the AGAPE (ACARE GoAls Progress Evaluation) report and a new mediumterm goal for 2035 which is a significant point for aircraft fleet renewal has been defined.The perceived noise emissions of flying aircraft should be reduced in 2020 by 50%; and in 2050 by 65%. The SRIA roadmap adds a new medium-term noise target for 2035 taking into account the actual achievements.

 

COBRA, at the crossroads of European programmes

The engine sector has significantly contributed to the ACARE goals by achieving breakthroughs research and technology development programmes. Yet, there are still potential and opportunities for further significant improvements, especially regarding CO2 emissions and fuel burn as well as regarding the reduction of the perceived noise. These global objectives have promoted a number of European projects, collaborations between industrial entities, research centres and universities.

VITAL, and more specifically the WP2.4 of this programme, where the partners SNECMA – ONERA – DLR – COMOTI – CENAERO – UPMC - CIAM have studied the Contra-Rotating-Turbo-Fan concept. Along this project, the definition of the CRTF baseline design (BPR = 10) was achieved by SNECMA and tested at the C-3A test facility of CIAM which represents the first step of a whole aero-acoustic validation programme.

 

 

 

Contra-Rotating Open Rotors programmes (e.g. JTI Clean Sky –SAGE & SFWA and DREAM) showed that this concept can provide a good agreement with objectives in terms of performance and fuel consumption. Their major drawbacks come from the installation effects and acoustic level, which remains too high. However, these projects are useful for the validation of aerodynamic, acoustic and mechanical tools adapted to the counter-rotating configurations in terms of design, optimization and high-fidelity computations.

 

Many different projects also included R&D activities on geared turbo fan concept (VITAL for instance). This concept could strongly increase the bypass ratio. Indeed, using a higher bypass ratio, the fan size increases and fan pressure ratio decreases, and thus fan tip speed have to be controlled to reach the optimal regime. For this, the direct drive cannot comply with this constraint, giving way to the gear box strategy. This technology is applied on Pratt & Whitney PW1000G (using high bypass ratio up to 12). However, some technical experts think that the benefit of a geared architecture is questionable for by-pass ratio below 15.

 

Relevant Russian background
From the Russian side, the UHBR CRTF has already been studied and is developed since 1990 by Kuznetsov for use on small-medium range aircraft: Ilyushin Il-96 and Tupolev Tu- 204 (Tu-214) airliners and Tu-330 (Tu-204-330) freighter.The NK-93 gearbox turbofan engine has been designed with a bypass ratio of 16,5 (fan diameter is 2950 mm) developing 18 tons thrust at take-off .
Implementation of such an engine scheme was possible thanks to the unique skill of the KUZNETSOV in design and development of a highly loaded planetary gearbox for the NK-12 engine with 16 000 hp power, and also thanks to the scientific-technical background of the AEROSILA company in development of highly effective fans with blades made of composite materials.


The NK-93 engine fan has a casing, the fan is co-axial, two-row, with variable pitch counterrotating stages and reversed thrust. In the front row rotating in clock-wise direction, 8 blades are installed. In the rear row rotating in counter-clock-wise direction, there are 10 blades. Engine thrust reverse is provided by rotation of fan blades by angles ensuring the necessary value of reverse thrust. The mechanism of blade rotation is hydraulic.

The NK-93; seven-stage axial low-pressure compressor; eight-stage high-pressure compressor; annular combustor; single-stage high- and low-pressure turbines, three-stage propfan turbine and non variable jet nozzle. The engine has the electronic control system backed up by the hydraulic mechanic redundancy. Prior to flight tests, 8 full scale NK-93 engines were tested at KUZNETSOV test benches.