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ECO-design and manufacturing of thermoplastic structural fairings for helicopters

Final Report Summary - ECO-FAIRS (ECO-design and manufacturing of thermoplastic structural fairings for helicopters)

Executive Summary:
The Eco-Fairs project aimed to design and manufacture three structural fairings for helicopters using thermoplastic composite materials and with reduced environmental impact:
- Upper panel Rear Fuselage demonstrator;
- Sponson Fairing Demonstrator;
- Radome Demonstrator.
The three demonstrators were designed and produced according the technical specifications with a TRL6 Technology Readiness Level.
A robust methodology to design and manufacture thermoplastic composite structural components for the aerospace sector was defined in the first WPs, and a number of guidelines were collected. Then the materials and process selection was carried out. The analysis of the thermoplastic composites for the aerospace sector led to the identification of CETEX® (provided by TENCATE), a carbon T300 3K 5HS textile with double sided PPS film. The last important issue that was analysed in WP1 was the manufacturing process selection. The main processes to produce thermoplastic composite components were analysed and described. Among these processes, the compression moulding technique was chosen for the demonstrator manufacturing, because this technique allows a robust control process with good performances and high volume rates. The main guidelines of the compression moulding process were found and described and robust numerical and experimental tools were developed to define optimised process parameters. The final result of WP1 was the definition of a robust methodology for the design of the manufacturing process of thermoplastic composites for the aeronautic sector.
The critical issues regarding the manufacturing of components with complex shape makes clear the importance of the joinings for thermoplastic composites. The definition of robust and high performing joining techniques for advanced thermoplastic composites is the key point for the overcoming of this limit. In the first part of WP2 the joining methodologies for thermoplastic composites were analysed. Thermoplastic welding techniques were studied, because of the possibilities to obtain high performances of the joints, and induction welding revealed to be the most well promising technique. This technique uses the eddy currents generated by an alternating electromagnetic field to heat the material and allows very high performances with a robust control of the process. A new induction welding machine for continuous welding of thermoplastic composites was developed. A big effort in terms of numerical and experimental activities was required for the definition of this machine. The quality of the work performed was confirmed by the high values of the mechanical properties of the manufactured joinings.
In WP3, WP4 and WP5 the three demonstrators of the project were designed and manufactured according the guidelines found in the first part of the project.
In the second part of the project the activities related with evaluation of demonstrators performances were carried out. At first the NDI inspections were carried out according to the NDI plan on the three demonstrators. Then the evaluation of the mechanical performances of the demonstrators was carried out according to the building block approach, starting from testing of material coupon up to the full scale tests on the demonstrators. After the evaluation of mechanical performances the eco-quotation of the demonstrators was carried out, thus demonstrating the lower environmental impact for the thermoplastic components in comparison with the thermoset ones.
Due to the importance of induction welding technique technique for the exploitation of thermoplastic composites for the aerospace sector, in the last part of the project the induction welding process was optimised also for other advanced thermoplastic composites of interest for the aerospace sector, and namely PEEK and PEI composites. The optimised process parameters were found and the experimental activities to find the design allowables for PEEK and PEI carbon composites were carried out.

Project Context and Objectives:
Composite materials have been utilized more and more in the last decades for aeronautic and aerospace applications. Starting from lightly loaded structures, the applications have been extended to secondary and primary structures, interested by higher critical requirements in terms of laod carrying capacity and structural reliability. Since the first application were delivered thermoset composites have been preferred over thermoplastic ones because different reasons, such as a more spread knowledge and the easiness of process, better performances at higher temperatures, presence of higher number of experimental data on thermoset materials, components and structures used in aerospace and aeronautic applications.
In the last time a big effort to promote the use of glass and carbon reinforced TPCs to substitute traditional thermoset ones in advanced aerospace structures have been pursued by many researcher and technologists, in order to develop proper methodologies for Ecolonomic design and manufacturing and to take advantage of their promising properties in terms of hot/wet mechanical properties, durability, short production cycle and joining efficiency, reduced tooling and production cost. Some examples of applications of TPC large structures have been developed in the last time, such as the welded wing fixed leading edge operating on A340 and A380, and new primary components are being developed, like the torsion box of the horizontal stabilizer by Fokker aerostructures which is going to get TRL6. Thanks to this big effort, new TPC have been developed using thermoplastic matrixes with higher mechanical and physical performances, both amorphous (like PEI and PES) and semi-crystalline (like PPS and PEEK),in the form of both textiles and unidirectional tapes. Moreover joining technologies have been investigated mainly based on welding processes and sometimes on mechanical fastening.
These examples demonstrate, with different TRLs, that TPC are competing with thermosetting ones, on the basis of their technological and physical advantages: higher toughness, easier recycling, weldability, reparability, infinite shelf life. Despite of these characteristics, thermosetting composites are still largely preferred, mainly due to a certain lack of knowledge and experience of design, manufacturing and validation of thermoplastic structures in aerospace and aeronautic fields, and by not yet fully established procedures for a proper design and exploitation of technological advantages in terms of short cycle and weldability.
All these aspects have lead the interest of the main industrial players of the aerospace and aeronautic sector towards the investigation of such new materials and processes for the development of components for greener aircrafts and rotorcrafts, thus going beyond the apparent difficulties in using TPC.
Following the considerations previously reported, the main objectives of the projects are:
- design of complex shapes (sponson fairing, radome) and functional structures (upper panel) with thermoplastic composites, developing and providing design guidelines and manufacturing procedures for the final demonstrators;
- manufacturing of TPC demonstrators by means of out-of-autoclave processes, for the production of cost-effective products;
- development of quality control procedures, by means of non destructive inspection able to detect any internal damage or defect due to manufacturing and assembly processes;
- select of advanced joining method for TPC parts (induction welding), in order to take profit of other technological properties of TPC and to overcome their bad adhesion ability with respect to thermosets;
- development of characterization procedures for the certification of TPC structures, and collection of new experimental data;
- assessment of the environmental impact according to ECO-quotation procedures.

Project Results:
The main results achieved at the end of the project are listed below:

Definition of a robust methodology for the design, material selection and manufacturing of thermoplastic composite components for helicopters.
In the first part of the project the fundamental issues to define a robust methodology to design and manufacture thermoplastic composite structural components for the aerospace sector were found and represented in explicit form, and a number of guidelines for the designer were collected in the first deliverables.
The advantages and disadvantages of thermoplastic composites in comparison with thermoset ones, regarding their physical, mechanical, processing, testing characteristics, were found and outlined: starting from this point the differences in design, processing and testing of these two different classes of materials were made clear. Consequently, the methodology currently used to design, manufacture and test the thermoset composites for aeronautic field was modified and integrated for thermoplastic composite materials. This new methodology is widely described in the first deliverable of the project, and was used with good results in the further parts of the project.
The objective of the second task of WP1 was the materials and process selection. At first the thermoplastic composite materials potentially usable for the aerospace sector were identified and analysed, and their datasheets were collected. On the basis of the materials properties, CETEX® PPS was selected to be used for the demonstrators manufacturing. CETEX® PPS is a thermoplastic composite provided by TENCATE, having carbon fibres T300 3K 5HS textile with double sided PPS film. This material was chosen for its high mechanical performances and for its relative easiness of process. Moreover thermoplastic composites provided by TENCATE are the only ones qualified for aeronautic sector. At last, there are some applications of the use of CETEX® PPS for the manufacturing of primary and secondary structures for aeronautic sector.
In the third task of WP1 the manufacturing process selection was carefully and deeply analysed. At first the main processes used to manufacture continuous-fibre reinforced thermoplastic component for aerospace sector were described. Among these processes, the compression moulding technique was chosen for the demonstrator manufacturing, because this technique allows a robust control process with good performances and high volume rates.
The compression moulding process was described in detail and the main guidelines to define optimised process parameters for compression moulding were outlined. Special care was ensured to the study of the main deformation mechanisms to be considered for the thermoforming of different component shapes. In fact the main limit of compression moulding process for advanced thermoplastic composites consists in the reduced geometry complexity of the components that can be manufactured without defects like wrinkles and thickness variations. For this reason some important numerical and experimental tools were described for the prediction of such defects, and among the several compression moulding techniques, the rubber forming technique was deeply analysed and described. In this technique one of the moulds used for the component forming is made with a rubber-like material, in order to ensure a suitable distribution of the required consolidation pressure, thus allowing an increase of the complexity of the shapes that can be manufactured with the compression moulding processes. For this reason it was used for the sponson fairing and radome manufacturing.
Definition of a robust and high performing joining technique for advanced thermoplastic composites for the aerospace sector.
In the WP2 the joining methodologies for thermoplastic composites were analysed, focusing on joining techniques for aerospace sector. Even if bonded, mechanical and mixed bonded-mechanical joinings were considered at first, a special attention was paid to thermoplastic welding. In fact these techniques ensure the best performances in terms of mechanical properties and easiness of process, exploiting the possibility for thermoplastic materials to be melted and welded. The main results of this task were the complete survey of the joining technologies potentially of interest for aerospace sector and the selection of the joining technology that was used for demonstrator manufacturing, that is induction welding.
The induction welding technique is the most well promising technique to join thermoplastic composites in aerospace sector. Since the main disadvantages of this technique were the lack of experimental data and machine providers, Cetma (together with the Italian company SINERGO specialised on the development of new induction welding machines) decided to develop a new induction welding machine for continuous welding of thermoplastic composites. With this machine it is possible to obtain the working parameters suitable for thermoplastic composites welding and to make full-scale joinings.
In this machine a robust control system was developed to ensure the wanted temperature distribution within the materials to be welded. The machine is equipped with a cooled cylinder, required to apply the consolidation pressure, and with an air cooling system useful to remove heat where required, for example in the edges in order to avoid the edge effect. The working parameters of the induction welding machine (such as working frequency and maximum power) were established through a number of tests carried out to fix the best compromise between frequency and power.
To optimize the above listed parameters for the demonstrator manufacturing (upper panel and sponson fairing), developing a robust methodology for the induction welded joining in aerospace sector, numerical analysis were carried out by means of Comsol Multiphysics Finite Element software with a multi-physics approach. The developed numerical model was verified through a huge number of experimental tests.
strength values very close to the maximum values reported in literature were found. These high values of maximum shear strength are much higher than the maximum values that can be obtained using structural adhesives. It is important to notice that in the last part of the project the induction welding parameters were investigated and optimized also for PEEK-carbon and PEI-carbon thermoplastic composites, in order to have a complete survey of the capabilities of induction welding technique for the thermoplastic composite materials of interest for the aerospace sector.
The most important result of this task was the definition of a solid methodology for the design of the induction welding process, that was used for the design and manufacturing of the joinings required for the demonstrators.
Manufacturing of the three demonstrators
In WP3 the thermoplastic upper panel was designed and produced. The prototype is a significant portion of the existing Upper Panel of a Agusta Westland helicopter tail, and in the prototype manufacturing it is possible to evaluate all the process critical aspects that can arise during the manufacturing of the real part.
The flat laminate of the Upper Panel was manufacture thorough isothermal compression moulding. Also the flat strips required for the L stringers were manufactured with the isothermal compression moulding process. Starting from these strips, the L stringers were manufactured through the not-isothermal compression moulding process.
Finally the four stringers required for stiffened panel manufacturing were bonded to the flat laminate by means of the induction welding. The main result of WP1 was the complete manufacturing of the first demonstrator of the project.
In the WP4 the sponson fairing was designed and produced. The sponson fairing demonstrator consists of a skin and two ribs. The thickness of the skin panel is 1.24 mm, while the thickness of the rib is 1.86 mm.
The flat laminates of the skin and of the ribs of the sponson fairing were manufacture thorough isothermal compression moulding. The fundamental stages of such process are the same used for the upper panel. Starting from these laminates, the skins and the ribs are manufactured through the not-isothermal compression moulding process. The rubber forming technique was used to shape the skin of the sponson fairing. In order to achieve the optimized definition of the manufacturing process for the sponson fairing the numerical tools developed in the first work packages were used.
For the manufacturing of this component different critical issues were studied and overcome:
Manufacturing of the skin of the sponson fairing  manufacturing of thermoplastic components with U-shape and high height/width ratio;
Manufacturing of the rib of the sponson fairing  manufacturing of thermoplastic components with complex shape;
Induction welding of the skin and the rib  induction welding of thermoplastic components with complex shape.
In WP5 the radome demonstrator and its process were designed. Also in this case the rubber compression moulding technique was used, and numerical tools were used to optimize its manufacturing process.
In WP6 the plan of experimental tests required to evaluate the performances of the three thermoplastic demonstrators according the building block approach was defined. At first the tests on material coupons were defined and described. Then the test on sub-components were described, and finally the full-scale tests on the demonstrators were completely defined. The main result of the first task of WP6 is the definition of the test matrices to evaluate the performances of the three thermoplastic demonstrators.
In the last part of the project the test matrix to evaluate the performances of induction welded joinings manufactured with PEEK-carbon and PEI-carbon thermoplastic composites was defined.
In the second part of the project the activities related with evaluation of demonstrators performances were carried out. At first the NDI inspections were carried out according to the NDI plan on the three demonstrators.
The main results of NDI inspections are listed below:
• Ultrasonic method used for thermoset composites could be applied on the thermoplastic components;
• The three demonstrators satisfied the requirements of AW specification commonly used for thermoset composites.
In the WP7 the experimental tests required to investigate the material properties were carried out. At first the tests on material coupons were carried out: in this case the specimens were obtained from flat laminates with all the laminas oriented in the 0° direction, and the tests were carried out along warp and weft directions, according to DOT/FAA/AR-00/. Then the tests on sub-components were carried out according to the building block approach.
In the last part of the project the tests to evaluate the mechanical performances of the induction welded joinings manufactured with PEEK-carbon and PEI-carbon composites were carried out.
In the third task of WP7 the full-scale experimental tests to evaluate the mechanical performances of the three demonstrators were carried out. Both the stiffened panel and the sponson fairing were tested to evaluate their mechanical performances, such as buckling load and post-buckling behaviour.
After the evaluation of mechanical performances the eco-quotation of the demonstrators was carried out. Life Cycle Assessment was conducted by applying the LCA methodology according to the guidelines given in the ISO 14040-43 series, with the SimaPro 7 software from PRé Consultants (Netherlands).
The results of the eco-quotation activities demonstrated that the thermoplastic demonstrators are characterized by a lower environmental impact in comparison with thermoset ones.

Potential Impact:
Potential impact and dissemination activities
A short and a medium-long-term impact is expected from the ECO-Fairs project. In fact, at the end of the project the following important results were achieved:
• Development of a methodology for the Design of complex shapes and functional structures with thermoplastic composites: the guidelines collected in the first WPs will be the basis for the design of new thermoplastic structures in the aerospace;
• Manufacturing of TPC demonstrators by means of out of autoclave processes: the three demonstrators were produced with cost effective and high volumes processes. For this reason a strong interest from the main companies in the aerospace sector was appreciated in the last part of the project, when the results of the project were discussed in scientific congresses and in technical meetings.
• Development of quality control procedures and development of characterization procedures for the certification of TPC structures: these important issues are fundamental for the exploitation of the thermoplastic composites in the aerospace sector. In fact the results of NDI inspections and of the mechanical tests were used to demonstrate the quality of the work performed. This approach was highly appreciated from the aerospace companies during the dissemination activities.
• Development of the new induction welding technique for thermoplastic composites: the development of a new induction welding technique for thermoplastic composites (PEE_carbon, PEI-carbon and PPS carbon composites), characterized by high performances and high automation level is one of the most important results of the project. The exploitation of this new technique could be soon extended at an European level. At the end of the project CETMA received five information requests from companies operating in the aerospace sector at an European level.
• Assessment of the environmental impact according to ECO-quotation procedures: the lower environmental impact of the thermoplastic demonstrators in comparison with the thermoset ones is an important added value, since this result matches with the aim of the GRC.
The objective of this task is to ensure that appropriate measures for absorption of results by the potential industrial end-users are put in place.
The actions taken for the dissemination of the results of the project are listed below:
• Submission to scientific congresses of papers discussing of the results of Eco-Fairs project;
• Participation to exhibitions;
• Participation to specifics meetings with important end-users in the aerospace sector;
• Publications in magazines focused on composites materials;
• Specific sections on CETMA website.

List of Websites:
www.cetma.it

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