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Final Report Summary - IDOHAP (Innovative design of hubs and propellers)

The European aeronautical industry faces the challenge to counter the environmental impact due to the expected growth of air transport with low-fuel burn innovative products. Increasing the by-pass ratio of turbofan engines was a successful measure in the past decades. The ultimate in this sense is the counter rotating open rotor (CROR) which offers a potential of 20 % fuel burn reduction relative to the classical turbofan engine in service today. There is a need to obtain comprehensive knowledge on the aero-acoustic behaviour of CROR blades through experimental characterisation. Within the European community, CROR tests planned in the coming years will be conducted by means of CROR powered wind tunnels models. In particular, one-fifth- and one-sixth-scale rig testing of installed CROR systems will be performed. To support this programme, IDOHAP hub designs for a CROR rig have been delivered by the Aircraft Research Association (ARA) located in Bedford, United Kingdom (UK) to Airbus. To insure that rig interfaces were correctly maintained and controlled, the hub designs were researched and developed in close collaboration with the Office National d'Etudes et Techerches Aerospatiales (ONERA) and the Nationaal Lucht-en Ruimtevaarlaboratorium (NLR) who were responsible for the design of the CROR rig and propeller blades respectively.

Project context and objectives:

The original topic description for the IDOHAP project was for the detailed design of two CROR models in the form of hubs and propellers to be fitted to an existing rig with the task input to be the supply of all necessary rig interfaces, CROR geometries and a detailed model specification. At the formal initialisation of the project a significant change in the scope of the project was introduced. The main changes were the following:

(1) instead of the design of two sets of hubs and propeller blades, the detailed design of up to three hubs was required with the blade designs to be provided by NLR;
(2) the integration of the CROR hubs and propellers had to be with a new contra-rotating rig.

The formal model specification was issued by Airbus in January 2010 and subsequently subject to a number of revisions, culminating in a final document in January 2011. Further re-definition of the scope of the programme resulted in only a single CROR model being required, but the programme would be complemented with design support for an iterative process where the rig specification was no longer fixed at project start but interactively developed by all partners as part of the design process. One impact of these changes was to reduce the number of specified formal deliverables from 6 to 3. The project started in January 2010 for a final duration of 18 months.

The change in emphasis and scope of the project necessitated a radical change in the design process for the hubs in as much as design iterations were now required as the interface detail with:

(a) the CROR rig;
(b) the rotary shaft balance;
(c) the design of instrumentation packages;
(d) the propeller blade design;

had to be carefully and methodically checked and the hubs regularly modified as the detail design evolved in each of these keys areas. This process was required to ensure that perfect interface compatibility was maintained and kept up-to-date throughout the design phase. This process, whilst subject to the inevitable timing difficulties as work progressed at varying rates in a number of organisations across Europe, eventually proved to be very successful and culminated in a critical design review with all partners held at ONERA in August 2011 at which the hub designs were able to be finally frozen.

Project results:

ARA has successfully developed the designs for a pair of one-fifth scale hubs for the Airbus new Z49 test rig. Deliverables in the form of computer-aided design (CAD) models, manufacture drawings and a structural analysis report have been provided to Airbus and also uploaded to the Clean Sky participant portal. The front hub contains eleven propeller blades and rotates anticlockwise when looking forward and the rear hub nine propeller blades and rotates clockwise when looking forward. The material for the manufacture of the hubs has been selected as titanium Ti-6Al-4V to provide adequate strength and a significant weight reduction when compared with alloy steel. The hub designs include innovative solutions for the simple, accurate and repeatable setting of blade pitch angles and for the retention of the blades in the hub. Due to the critical nature of the blade retention concept in the overall design, this aspect of the design has been validated by practical trials using representative test pieces. More details including CAD pictures, diagrams and photographs are presented in the attached PDF.

Potential impact:

The IDOHAP project has provided one essential enabling design technology for the acquisition of a CROR model for wind tunnel testing. By the very nature of the overall project, the full impact of the programme will only be realised after the proposed CROR model has been manufactured, tested and experimental results analysed. The aerodynamic and acoustic scientific findings resulting from this analysis are expected to provide important baseline data to support the development of fuel efficient and thus more competitive passenger aircraft as well as to achieve a reduction in noise levels over current standards. The hub designs developed during the IDOHAP project have been fully disseminated to Airbus and to those Clean Sky partners involved in the development of the CROR rig (ONERA) and the propeller blade manufacture (NLR). The project has advanced ARA's experience and capability in the design and analysis of complex contra-rotating open rotor hub assemblies. The hub designs have incorporated a number of innovative design features, most notably the method of blade retention and blade pitch angle setting and ARA will be able to exploit this advanced design capability as opportunities arise in the future.