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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - GRETEL (GREen Turboprop Experimental Laminar Flow Wind Tunnel Testing)

Teaser

Summary

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:
â€¢ 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.
â€¢ 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.
â€¢ 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.
â€¢ To assist the wind tunnel testing by providing the experimental support definition, the test planning and data analysis and reporting.

Work performed

The work carried out so far involved 1) Three trade studies and three wing structural concepts and interfaces of the WT scaled down wing 2) CAD and FEM model requirements as a means of quality assurance planning 3) Automated methodology for scaling down developed, beta tested on a generic wing structure 4) 1:4 scaled test tool (master moulds) for the process design created.

Three different fail-safe solutions for the front and rear spars, internal ribs and tension bars were designed in CATIA. The objective of this study is to develop a fail-safe scaled down flexible outer 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.
All design features were examined in order to describe and demonstrate the feasibility of the concept, from mechanical, stress, manufacturing and integration point of view. Detailed design features were not considered in this stage.
A fully parametric mathematical algorithm for the wing scaling has been developed. The algorithm calculates the basic parameter values such as (wing mass, eigenfrequencies etc) regarding the full-scale data (dimensions, mass etc) and the physical properties of the air at cruise level and during take-off. The algorithm uses similarity laws and was developed in MATLAB. An iterative process for the wingâ€™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 will be proposed relative the flight profile and the scaling methodology in order to achieve the goal results for the final wing tunnel model.
Regarding tooling, a full geometry has been chosen to allow a better overview of potentially problematic areas of the wing shape. In order to allow a risk minimized manufacturing process for the full size wind tunnel model while minimizing efforts for this additional action. The model has been sized to a 1:4 scale. All manufacturing trials will be made with scaled ply books and foam core thicknesses. The composite tools will not be scaled with regards to the ply book in order to allow a good assessability of the resin flow which is essential for the reproducibility of small details.

Final results

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.