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GREen Turboprop Experimental Laminar Flow Wind Tunnel Testing

Periodic Reporting for period 3 - GRETEL (GREen Turboprop Experimental Laminar Flow Wind Tunnel Testing)

Reporting period: 2018-08-01 to 2019-07-31

Joint Technology Initiative (JTI) has devised a dedicated Innovative Aircraft Demonstrator Platform (IADP) in order to answer the societal needs and streamline the stakeholder's efforts. The Regional aircraft IADP (R-IADP) is offering a roadmap with concerted activities that will ensure technical excellence in the field and are expected to enhance the EU leadership in regional aircrafts, improve industrial competitiveness, create jobs and deliver an innovative, more efficient, greener and safer aircraft.
One of the major aspects of the R-IATD developments is advanced aerodynamics based on Natural Laminar Flow wing, turbulent skin-friction drag reduction techniques and load control to increase aerodynamic efficiency in cruise and off-design conditions (climb, descent). In this respect, wing morphing is considered a key enabler. The term Morphing typically refers to shape changing structures capable to adapt their shape according to the specific regime of the flight exhibiting optimal aerodynamic performance thus reducing engines gaseous/environmental pollutants emissions. Morphing structures allow a shape change without the generation of discontinuities, in other words without aerodynamic gaps. Past research in this area usually focused either on aerodynamic performance or system integration, with relatively little attention on the strict requirements imposed by the long lifetimes and extreme environmental conditions the structural materials are exposed to. Therefore, morphing structures have not yet made their way into serial production aircraft. The Clean Sky 2/ GRETEL project aims to change this.
The GRETEL project, as part of the R-IADP, will contribute to the objectives of increased fuel efficiency and noise reduction through the realisation of a large scale natural laminar flow, flexible wing model, with integrated innovative morphing active devices that will be verified and eventually tested in a large Wind Tunnel (WT). The proposed activities will mature the Technology Readiness Level (TRL) up to 6 and drastically de-risk the integration of the investigated solutions on future products, effectively resulting in reducing the direct operating costs for the airlines and minimizing the impact on the environment.
GRETEL aim is to address the topic JTI-CS2-2016-CFP03-REG-01-02 and to develop and demonstrate the technologies required to improve aerodynamic efficiency and environmental footprint of aircraft life cycle. In order to address the specific challenges, the project has the following technical development objectives:
• To design a large scale (1:3) Natural Laminar Flow (NLF) flexible wing model of a future regional aircraft that will feature innovative active devices for increased aerodynamic efficiency.
• To manufacture all components of the wing model and assemble them observing the provided specifications in dimensions and other tolerances. The wing model apart from the active devices (droop nose, morphing trailing edge, morphing winglet) will integrate the necessary sensors for the subsequent wind tunnel testing.
• To perform Ground Vibration Tests in order to validate the aero elastic predictions and ensure the safety of the model during the wind tunnel testing.
• To assist the wind tunnel testing by providing the experimental support definition, the test planning and data analysis and reporting.
The objective of the project is to develop a fail-safe scaled down flexible wing (scale 1:3) concept that allows an easy installation, transportation and maintenance while keeping an eye on the costs and the wing structural elasticity.
An iterative process for the wing’s stiffness calculation and mass distribution was also developed using optimization algorithms. A generic wing structure was developed and dimensioned using the aforementioned optimization tool. A scaled wing was proposed relative the flight profile and the scaling methodology in order to achieve the goal results for the final wing tunnel model. Following the scaling down procedure and the development of the mathematical model of the wing, GRETEL worked towards the preliminary technical design of the project as defined by the requirements, system design, and modeling. Design of scaled wing box was defined using the following inputs:(1) Loads assessment based on lift coefficients and velocities provided by Topic manager, (2) Dimensioning of wing box ensuring structural integrity taking into account holes and cut-outs. (3) Aeroelastic analysis (flutter-free check) to ensure safety during wind tunnel testing.
This work enabled informed decisions regarding to informed decisions with respect to a composite material wing box with very similar stiffness characteristics to the full scale one, ensuring it is flutter free for the velocities considered, allowing definition of mechanical interfaces with the wing tunnel test facility and morphing devices. Based on this dimensioning, a first iteration of a Manufacturing-Assembly-Integration plan has been completed and a proposal for wing test model instrumentation was completed. The GRETEL CFRP Wing structure is subjected to a “build-to-print”-approach. With respect to the individual design the components, they consist of: (1) a structural CFRP wing box consisting of an integrated upper wing structure and leading edge and trailing edge spars (monolithic structure) with included interfaces for wind tunnel attachment (root rib) and morphing elements (LE, TE). (2) the lower wing structure will be provided as a monolithic part to for integration after sensor integration (3) ribs inside the wing box structure and the interface for the morphing winglet (tip rib) will be also provided and integrated into the wing box assembly. Adequate quality assurance is one key aspect for the manufacturing of delicate composite hardware. The global QA for the manufacturing and assembly of the structure is also established. The project has implemented a system of work instructions describing the requirements, process procedures and parameters for all materials.
The main progress beyond the state of the art and associated impact are summarised below:

1. The state of the art automated wing scaling approach is enriched with a series of aerodynamic and structural parameters and a computational fluid dynamics/computational structural mechanics optimization is employed. This developmental effort addresses all elements: from the conceptual phase all the way through to WWT, with significant impact towards reducing development and certification costs.
2. The manufacturing of the composite tooling that employs tooling resource efficient OoA manufacturing processes. Such processes are not established in the series production of aerospace parts. Especially due to the very strict geometrical tolerances to the outer surface of wing structures, such innovative production processes need to minimize effects that cause surface waviness, spring-in and spring-out effects. One source causing such effects is the mismatch of the CTE between CFRP part and metal tooling. CFRP tooling with integrated heating will be minimize this effect and allow for an energy efficient manufacturing process due to the low thermal capacity of such CFRP tooling.