One of the most challenging areas of research leaded by the Green Rotorcraft Consortium (GRC) within the Clean Sky Program was the development of Active Rotor Technologies as the Active Gurney Flap (AGF) systems, which enable a helicopter to operate with a reduced tip speed of its main rotor whilst preserving the current flight capabilities. The on-going validation of innovative AGF systems by the GRC required the manufacturing and testing in wind tunnel tests of small scale composite model blades before their implementation at full scale. The integration of the AGF systems into the model blades (figure 1) demanded a precise process and tooling design in order to allow an accurate assembly for an efficient performance.
Figure 1 Scheme of rotorcraft model blade with AFG system
Due to the small scale, the dimensional tolerances of the model rotor blade were very tight (less than +/- 0,1mm on the aerodynamic profile). This fact represented a great challenge for process and tooling design. Actually, to fulfil those tight tolerances, not only the mould had to be machined with very high precision means, but also the cavity design had to be designed with special consideration for minimizing any shape distortion induced by the process due to the different thermal and chemical shrinkage of the materials.
In close collaboration with the GRC consortium, CIDAUT contributed with its expertise in composite materials processing, and an innovative tooling design methodology. This methodology was based in process simulations capable of predicting distortions and solving process related issues from the early design stage. The methodology also avoided the need for expensive and long lasting trial and error procedures. Process simulation models developed by CIDAUT in the ACCUBLADE project included thermal and impregnation simulations for the analysis and optimization of critical process related issues, such as shape distortions caused by the different CTE of the materials, temperature gradients, or potential resin flow defects.
Laboratory characterisation tests were carried out with the selected materials in order to determine the required input data for the modelling of the most relevant causes of distortions when processing composite materials, including the warping and spring-in phenomena. Process simulation models were well correlated with experimental results obtained through the processing of flat and C-profile coupons with different layups and curing conditions (figure 2).
Figure 2 Simulation of process induced distortions: warping and spring-in phenomena
Based on the results of process simulations, the design of the suite of tools required for the processing of the model blades was optimized. This included the selection of the optimum tool materials, the definition of the proper alignment and clamping systems, and the integration of an efficient and homogenous heating and cooling system with in-mould sensors. Also, with the aim of evaluating potential improvements in the manufacturing process in terms of quality and costs, the mould was designed and manufactured allowing the evaluation of two alternative processes, both aimed at producing the same net model rotor blade product: compression moulding and SQRTM.
All tools were manufactured by CIDAUT using very fine milling means, and inspected to guarantee the fulfilment of the requirement specifications before putting them at the disposal of the GRC consortium for the processing of the model blades. The first three model blades produced with the tools (figure 3) were used for the validation of process and tooling designs, being subjected to destructive and non-destructive inspection tests including mechanical substantiation tests with fully satisfactory results.
Figure 3 Rotorcraft model blade produced for the integration of AGF systems
Since April 2013, CIDAUT has been coordinating REMART project, carrying out the technical tasks in collaboration with 3 partners, CIDETEC, ITRB and PBLH and under the direction of AGUSTA WESTLAND as Topic manager.
This project has been developed in the domain of the CLEAN SKY JTI – Green Rotorcraft which has the aim to improve the environmental impact of the components in aeronautic and air transport sectors. Along the project, the REMART consortium has been involved in the optimization of the use of existing recycling technologies, designing an efficient and environmental recycling protocol for each piece or set of pieces of the HTS (Helicopter Transmission System).
The first step in this work was the development of a comprehensive survey of market surrounding the recycling of HTS. In parallel with the market survey of recycling processes a study and analysis of all materials and coatings used in the manufacturing of HTS was developed.
Once all materials were identified, just like their main coatings and surface treatments, each piece and their most suitable recycling processes were combined.
The next stage of the project was the quantitative evaluation of the recycling processes. With all this information, the development of a tool for designing and developing recycling protocols (as the one of the next figure for chemical stripping) for each component of the HTS was possible.
Example of Recycling protocol for chemical stripping
Two different demonstrator components (one from AGUSTA and other from AIRBUS Helicopter) were tested during the project (tail gear box and intermediate gear box see below figures) to validate the recycling protocols developed.
Pictures of the demonstrators of the project
The validation of the recycling protocols was performed using quantitative parameters from the experimental tests, as for instance, time of each process, surface quality after stripping (see next figure) and the risks for the environment.
Microscope Surface quality after different stripping process
Finally, the last stage of the project consisted on the cost-effectiveness analysis of the recycling cycles for each material and coating (see example for zinc plated component in below figure).
As one of the conclusions of the project, the stripping stages during the recycling process are a good procedure to get the initial characteristics of metal base. Nevertheless, re-using components will not be a common solution for the aerospace industry due to the high cost of the stripping process. This limitation can always be overcame with stricter environmental policies or by breakthrough from others recycling processes.
Funded by the European Commission: FP7 – JTI – CS – Joint Technology Initiatives – Clean Sky
One of the aims of Clean Sky is to develop new technologies for green manufacturing through Integrated Technology Demonstrators (ITDs) within the frame of both Green Regional Aircraft (GRA) and Eco-Design (ED) platforms, with the objective to realize low weight/eco-friendly aircraft components featuring competitive manufacturing costs. To achieve this purpose ALENIA AERMACCHI, (Leader of the GRA ITD and Member of the ED ITD) has conducted several studies and launched initiatives aiming to develop, optimize and industrialize Liquid Resin Infusion (LRI) processes. Executed out of autoclave (without pressure), the required solution shall reduce weight, related environmental impact and reduce life cycle costs, for the one shot manufacturing of wing box stiffened panels in composite material.
Under these initiatives, CIDAUT has led the research project named “Panel Liquid Infusion Technology” (PLIT), (Topic Manager SICAMB), which was launched within the GRA “Low Weight Configuration” domain, and was set up to provide a scientific approach to the physics of the LRI process by the development of a “process simulation numerical model”, to study resin flow during the impregnation stage.
The PLIT project consortium was led by CIDAUT Foundation, in charge of development and validation of simulation models and the test bench. ITRB was involved in the tools detailed design and PBLH International Consulting was in charge of dissemination activities.
The LRI simulation model set up by CIDAUT allows identifying potential causes for non uniform distributions of resin flow that may cause injection process faults like dry spots, poor saturation of the pre-form, partially filled composite parts and other defects. An outcome of the model is shown in figure 1. Filling time was used to correlate experimental and simulation results.
Figure 1 Correlation between simulation and laboratory tests to determine permeabilities
Two main technical objectives were addressed by CIDAUT in the PLIT project:
- Development of an optimized LRI process simulation methodology especially suited for analyzing large parts and stiffened wing skin panels.
- Research to gain in-depth understanding of the flow phenomena based on experimental data and try-out.
The simulation model development required full understanding of the most significant phenomena in flow processing and development of dedicated methodologies in laboratory to characterize key material parameters of carbon reinforcements, epoxy resins and distribution media affecting resin flow in LRI processes (an example is shown in figure 2).
Figure 2 Correlation between simulation and laboratory tests to determine permeabilities
Compared to a full 3D resin flow computation, commonly used in infusion processes analysis, the optimized numerical model developed by CIDAUT leads to significant reduction in computation time, while accurately predicting resin flow from the distribution media through the laminate thickness. The model is parametric and user friendly. Case studies can be parametrically defined depending on the resin viscosity parameters, carbon fiber permeabilities, infusion process parameters (resin pot and oven temperatures) and impregnation strategy (i.e. number, location, diameter and length of the inlet and outlet pipes, location of distribution media, sequential fillings, etc).
The simulation model was numerically verified and experimentally validated against experimental LRI tests, carried out in large stiffened wing panel demonstrators manufacturing. For that purpose, a complete test bench was manufactured and delivered to the topic manager premises in Italy, where infusion tests were conducted and filling times were accurately measured at critical locations along panels. Experimental characterization of permeability and viscosity parameters were key factors for achieving a good correlation between experimental and simulation filling times.
Figure 3 Stiffened wing panel made by LRI (Courtesy of ALENIA AERMACCHIand SICAMB)
As one of the last activities carried out within the WASIS FP7 Project, Cidaut performed the vibro-acoustic characterisation of two components, firstly one test panel and secondly the largest fuselage section (1m diameter prototype). In both cases the study covered low and high frequency ranges. The aim of this activity was to validate FEM/BEM models for low frequency range and SEA models for high frequency.
The panel dimensions correspond to the real scale size of the aircraft fuselage. The idea was to learn about the panel behaviour before addressing the 1:2 scale aircraft fuselage. Two test methods were used to identify the behaviour at low frequencies: inertance tests and experimental modal analysis. For the high frequencies the Transmission Loss and Radiation Factor were obtained. Trough these parameters coupling loss factors associated with each phenomenon can be derived.
To characterize the barrel, two different tests have been designed aiming to reproduce the noise field and acoustic loads the fuselage section would be exposed to in real conditions. In these tests the transmitted energies between different parts of the specimen are measured. Besides, the Transmission Loss and radiation factor were obtained.
To complete this task, vibro-acoustic models of filament winding structures were developed. The results of these models have been correlated with the results of structure characterization. Once the validation of both models was finished, a new model of a full scale filament winding fuselage was carried out.
All these models have helped characterize the vibro acoustic performance of Wasis Composite Prototypes, enabling the project Consortium to assess not only the mechanical performance, but also other factors such as the transmission loss and radiation factor.
Stiffened panels are required in structures which can be obtained by different processes. They can be made by attaching stiffeners to a thin panel or by producing integrally stiffened panels. An innovating manufacturing process based on Liquid Resin Infusion (LRI) can be employed for obtaining integrally stiffened panels. It is based on moulding a dry NCF (Non Crimp Fabric) pre-form of Carbon fibre plies, which is bonded by a one-shot injection process to high stiffness, pre-cured pre-preg T-section stiffeners. This method presents benefits like lower costs in machining and fewer assembly operations.
The structural behaviour of integrally stiffened panels is normally better than those panels with attached stiffeners, but the difference is difficult to quantify by analysis, and is dependant on the manufacturing technology. Especially, the major interest is to clarify the structural behaviour of the panels, and more specifically their critical mode of failure.
The immediate solution could be to carry on comparative structural tests on different coupons moulded by different manufacturing methods, but it must be taken into account that habitually employed strain and stress measuring systems are limited to specific predefined points or have limited resolution. As the manufacturing process and materials are expensive, and last a long term, few coupons are available. Therefore, carefully combined measurement systems must be employed to obtain as much information as possible during the test, and also recurrent information is desirable to correlate results obtained by different sources.
As an answer to this scenario, the ACID project was launched to explore and analyze some of the previous factors, trying to study comparatively the mechanical properties and behaviours of different panels obtained by different manufacturing processes.
To achieve this goal, a testing matrix was accomplished, based on 3 LRI coupons. Two of them are panels with attached stiffeners and the other one is an integrally stiffened panel. It is expected that the results obtained in the tests help to clarify the panels’ behaviour and allow comparing the mechanical advantages versus economic benefits of the manufacturing processes.
The main objectives of the project were described as follows:
- Carry on large scale structural tests for obtaining ultimate properties and failure modes of components manufactured by different processes.
- Measure strain and stress information during the test in a recurrent manner to combine and correlate the obtained signals which define the structural behaviour of the panels throughout the test.
- Analyze the obtained results, establishing a comparison between the behaviours of panels with attached stiffeners and integrally stiffened panels.
- Analyze the obtained results, establishing a qualitative comparison between the mechanical advantages versus economic benefits of the manufacturing processes.
The main achievements of the project were the validation of the novel techniques for composite manufacturing due to the result obtained in the tests. The final mechanical response of the differently implemented panels shows great similarities in the main mechanical characteristics (failure load, stiffness, failure mode).
This validation serves as a starting point for further methodologies development and means a widening of the possible applications or fields of Composite materials.
At the same time, the cross comparison of the measurement devices is useful when deciding the most convenient measurement system for each project. The pros and cons are highlighted and an estimative error between systems is obtained.
The major environmental benefit is the validation of the novel cleaner manufacturing composite methodologies (less energy needed, less wastes, lower costs) against conventional procedures in representative playground.