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Advanced Testbed for TILtrotor Aeroelastics

Periodic Reporting for period 1 - ATTILA (Advanced Testbed for TILtrotor Aeroelastics)

Reporting period: 2019-11-01 to 2021-04-30

The ATTILA project addresses the tiltrotor whirl flutter stability of the NGCTR through the development and testing of a scaled aeroelastic wind tunnel model. Whirl flutter stability is a complicated aeroelastic phenomenon that forms the limiting factor in the forward flight speed of tiltrotor aircraft. A thorough understanding of the whirl flutter dynamics is, therefore, a key element in the successful exploitation of the advances in sustainable mobility promised by the tiltrotor concept. The test activities will provide experimental verification of the NGCTR flutter stability boundary, as well as the high-fidelity scaled test data that is critical for the validation of the numerical tools used in the development of tiltrotor aircraft.

This project has received funding from the Clean Sky 2 Joint Undertaking (JU) under grant agreement No 831969. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union
The first critical step in the design of the ATTILA wind tunnel testbed has been the definition of the requirements specification. The specifications considered the full breadth of the design including but not limited to the requirements for the structural aeroelastic scaling, flutter excitation and online modal analysis, instrumentation and data acquisition, etc. The consortium partners contributed to the specification, leveraging their respective knowledge and expertise.

Following the requirements specification, the Consortium proceeded with the conceptual design phase, which consisted of design trade-off studies at system-level to identify the preferred design concepts, material selection and high-level system layout. Initial FEA models for structural dynamic analysis were set up to aid the concept design. In parallel, analytical models of the targeted ATTILA testbed were developed to determine the design loads and predicted model-scale whirl flutter stability characteristics in air and heavy-gas. The conceptual design phase was concluded by a Conceptual Design Review meeting with the TM.

The design is currently in the detailed design phase. The mechanical design for the cantilevered wing, rotor, nacelle and drive system is steadily progressing. The structural FEA model has been updated over multiple iterations to reflect the ATTILA testbed design status and used to define component-level requirements on stiffness and inertia. In support of the design process, material coupon testing was performed to verify/obtain the properties of the composite laminate and foam materials selected for the design of the wing, yoke and rotor blades. The instrumentation and data acquisition layout has been optimized for online modal analysis during the wind tunnel test. The related algorithms for mode identification, damping assessment and mode tracking are being verified using virtual flutter excitation test data generated by the analytical models.

In parallel to the design and analytical modelling activities, CFD grids have been generated for the rotor blade and wing airfoil sections to enable characterizing the aerodynamic properties at model scale in air and heavy-gas. Initial CFD calculations have been performed in full-scale conditions to verify the analysis by comparison to existing test data available at the TM.
ATTILA marks the first European research program in which a scaled tiltrotor model will be tested in Froude number scaled conditions with a potential towards Mach-scaled testing in heavy-gas, thereby providing the most realistic test environment short of full-scale flight testing. The knowledge and experience gained can be applied to upcoming novel aircraft development programs in Europe.

Contrary to past tiltrotor experimental research in Europe, the ATTILA testbed is intended, as far as practicable, to be designed to be modular and scalable in order to facilitate future configuration changes and alternative test objectives or facilities. In this manner, the ATTILA testbed can be exploited by the Consortium to support the European tiltrotor research needs of the future.

In addition to the established aeromechanics codes that are used in the design phase, ATTILA aims to further the development of the novel VAST code. VAST is a coupled system of models that are expressed as state-space models wherein the implicit system of coupling equations is automatically resolved. This approach allows generic methods for solving the system to be developed and makes it suitable for general multi-model simulations. The multi-model capability, through minimizing the overhead of maintaining multiple codes, offers the benefit of reducing overall development costs for the industry.

The ATTILA testbed will employ fibre optic sensors for strain measurements in the rotating frame. Fibre optic sensors are used increasingly often in industrial strain sensing applications, but have not found their way into widespread use for wind tunnel testing applications. They provide a variety of benefits over conventional strain gauges, including immunity to electro-magnetic interference, superior fatigue characteristics, smaller interrogator electronics, etc. Fiber optic sensors can also be used for direct deformation measurements.

The ATTILA rotor will be fitted with contactless rotating power and data transfer in place of a traditional mechanical slip ring. The data is transferred in digital format, which simplifies the use of third-party data acquisition systems, such as fibre optic strain sensing systems and transfer of data to the main test data acquisition system. The technology can also be exploited for future novel smart/active rotor system developments where robust rotating data and power transfer is essential for operational applications.

Whereas tiltrotor whirl flutter stability testing has typically involved a process of excitation and free-decay measurements, the ATTILA testbed will feature online modal analysis to enable continuous state estimation and damping assessment, potentially without the need for direct excitation. As such, the technology promises faster testing and reduced complexity of the test setup.

The advanced multi-disciplinary design, manufacture, testing and validation techniques developed in the ATTILA project to understand and design for the whirl flutter phenomena associated with high speed forward flight of tiltrotor aircraft are a necessary step in the development of the NGCTR and a key enabler for its successful introduction. Moreover, the experience gained and tools developed in ATTILA can be integrated in future novel aircraft developments and related test activities, improving innovation capacity in Europe.
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