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Full Fairing Rotor Head Aerodynamic Design Optimization

Periodic Reporting for period 3 - FURADO (Full Fairing Rotor Head Aerodynamic Design Optimization)

Reporting period: 2019-03-01 to 2020-02-29

Europe has defined very ambitious goals in the HORIZON 2020 work program: maintaining global leadership in technology development, meeting the societal and market needs for affordable, sustainable, and seamless connectivity, namely by providing cost effective, green, safe and secure air transport. Rotorcraft could play an important role in this context, because they can perform short or vertical takeoff and landing (STOL/VTOL). Hence, they are predestinated for seamless mobility since they do not require large infrastructure on the ground. However, rotorcraft are neither as safe nor do they achieve the cruising speeds of fixed wing aircraft. Therefore, expanding the flight envelope towards higher cruising speeds would be an important innovation in the rotorcraft domain. Within Clean Sky 2, the cruising speed limitation is addressed through the Fast Rotorcraft Innovative Aircraft Demonstrator Platforms (IADP). One of these platforms is represented by the Rapid And Cost Effective Rotorcraft (RACER), which incorporates a compound helicopter developed by Airbus Helicopters and European project partners. The RACER demonstrator combines the beneficial characteristics of fixed wing aircraft and rotorcraft. It enables fast and efficient forward flight and allows for vertical takeoff and landing. Due to the high cruising speed of the RACER compound helicopter, aerodynamic efficiency is one of the major challenges within the demonstrator development.

The strong demands on safe and efficient flight operations in context of society needs and environmental impacts enforce continuous improvement of the performance of aircraft. The RACER demonstration program includes key technologies for compound rotorcraft configurations aimed on the “development of future products fulfilling expectations in terms of door-to-door mobility, protection of the environment and citizens’ wellbeing better than conventional helicopters.” A key issue enhancing the rotorcraft efficiency in terms of reducing fuel consumption and emission is the improvement of the aerodynamic performance by passive or active means. Here, the focus is on the application and adaptation of CFD methods on enhancement and prediction accuracy with respect to rotor head aerodynamic performance.

The FURADO project is well aligned with the required innovation for high-speed rotorcraft. The project focusses on the aerodynamic design optimization of certain fairings for the rotor head of the compound rotorcraft RACER. Due to the high cruising speed, drag reduction is one of the major goals within the RACER development. The investigated fairings comprise the blade-sleeve fairings, the full-fairing beanie and the pylon fairing. In order to be able to develop these fairings, a sophisticated optimization tool chain has to be created allowing for fully automated aerodynamic shape development. Moreover, the wake flow generated by the developed fairings has to be assessed in order to detect any wake issues causing undesirable excitation of the rotorcraft structure. Finally, the best combination of fairings is selected and mechanically designed, which enables their integration on the RACER demonstrator platform.
At the beginning of the FURADO project, the method development for subsequent optimization tasks was at focus. Therefore, a sophisticated optimization tool chain was developed, which allows for fully automated aerodynamic shape development by means of CFD simulation. The software was integrated to the topic leader’s work flow and the personnel was trained. This delivery concluded WP2 and the derived optimization software paved the way for subsequent optimization tasks. Within WP3, the aerodynamic design optimization of the RACER rotor head fairings was at focus. At first, the shape development of the blade-sleeve fairings was conducted. For this purpose, a multi-objective genetic optimization algorithm was employed. Promising designs from the final population were thoroughly investigated. One design offering the best compromise between the applied objective functions was selected and examined on an isolated five-bladed rotor head with cyclic pitch movement. The newly developed FURADO fairing was compared to the current RACER blade-sleeve fairing and any benefits in terms of performance improvement were highlighted. Furthermore, different full-fairing beanie shapes were investigated. Within a next step, two rotor head fairing combinations were examined on the full RACER configuration. The aerodynamic loads acting on the rotor head fairings were determined. An aerothermal risk assessment was performed for the fully airtight RACER rotor head. The aerothermal flow simulations included the external flow about the helicopter and the internal flow through the upper deck. The pressure and temperature field inside the RACER upper deck were investigated. Based on the obtained simulation results, aerothermal risk mitigation measures were discussed together with the topic leader. Therefore, the flow simulation results of the FURADO project directly contribute to the aerothermal risk mitigation concerning the RACER demonstrator program when commencing ground and flight tests.
This project significantly enhances the skills in numerical simulation of rotor head aerodynamic problems. Especially the combination of complex external and internal flows, including thermal effects, is still subject to current research interest in the numerical field. Integrating such complex numerical investigations into the early development process of new products is not state-of-the-art. Especially, combining aerodynamic and thermodynamic numerical simulations at low TRL levels is not standard. Thus, the FURADO project will significantly contribute to acknowledge the virtue of including such complex scenarios into early design stages. Furthermore, the automated optimization scheme provides advances beyond the current state-of-the-art. FURADO contributes to the simulation of complex geometries in relative motion within an automated optimization scheme, which is not established as a standard tool in the process development yet. FURADO will contribute to a significant enhancement of the design process for new helicopter developments. In addition, the mechanical design effort will contribute to progress beyond state-of-the-art. However, the remaining issue to be solved is achieving a sufficient sealing level of the fairing whilst maintaining the practicability of the design. This has yet not been achieved and therefore no full fairing concept is flying on current production type machines. Thus, FURADO will contribute to progress the maturity of full fairing concepts beyond current state-of-the-art. This project will enhance the performance of low impact, fast and efficient helicopters due to a rotor head full fairing concept applying advanced design and CFD methods. The appropriate solution should result in less fuel consumption and noise reduction. A positive impact on vibrations may be found as well. The numerical approach will provide detailed insight into the complex flow structures associated with the rotor head flow expanding the existing knowledge and foster new solutions. The outcome will support the development of full fairing rotor head components and design guidelines for optimized aerodynamic shapes of maximum performance.
Optimization framework
Investigated RACER rotor head fairings
Visualization of the vortical flow field for the full RACER configuration in cruise flight
CAD model applied for the design optimization of the RACER blade-sleeve fairing
Flow field on the isolated rotor head with the optimized blade-sleeve fairings
Temperature difference to the outside air temperature (OAT)