3D printed carbon-composite wind tunnel model

One of the more significant applications in the aerospace sector is the new wind tunnel model of the Leonardo tiltrotor AW609 manufactured by CRP Technology for Leonardo Helicopter Division (Leonardo HD, formerly known as Agusta Westland).

This project allowed CRP Technology to highlight the perfect union between advanced 3D printing technology (Selective Laser Sintering) and Windform high-performance composite materials. Thanks to these materials, it was possible to complete and test the model in the wind tunnel within a very short period of time, with excellent results and with high-performing mechanical and aerodynamic properties.

The project related to the manufacturing of some external parts (nose and cockpit, rear fuselage, nacelles, external fuel tanks, fairings) of the 1:8.5 scale wind tunnel model for the prototype of the new Leonardo HD tiltrotor AW609, made by Selective Laser Sintering technology and Windform XT 2.0 Carbon-composite material, both supplied by CRP Technology. This wind tunnel model was designed, manufactured and assembled under the supervision of Leonardo HD by Metaltech S.r.l.for a series of dedicated low speed wind tunnel tests.

The low-speed wind tunnel tests were intended to cover a standard range of flight attitudes to be performed at the Leonardo HD wind tunnel facility and at Politecnico of Milan for the high angles of the flight envelope. During the different test sessions, various external geometries were changed and checked in order to understand all the aerodynamic phenomena. The main external components that were redesigned and manufactured include: fuselage and nose components, fairings, nacelles and spinner shapes, empennage, wings and flaperons.

Leonardo HD needed to meet a very short timetable, but with the highest level of reliability and commonality in order to manufacture the external parts for the wind tunnel model.The research of materials with high mechanical and aerodynamic characteristics for these components usually would have involved a classical composite material. Instead, the choice was to design and manufacture an aluminium alloy internal main structure that can easily be implemented with new geometries for the future aircraft versions or improved solutions.

It is crucial for the applied loads to be sustainable and therefore these could not be underestimated. Indeed, the aerodynamic loads of the wind in the tunnel are very high. The most critical aspect of the project is therefore the resistance to the loads, but also the need to maintain good dimensional tolerances of such a large-dimensioned component under load. It is important that the components of the external fairings don’t deflect too much under load. In addition, even when there are no external loads, the product must have dimensional characteristics in respect of the supplied specifications. It is important to remember that the performance of these components affects the final performance of the entire project, especially because the external fairings have to transfer the aerodynamic loads generated by the fuselage to the internal frame.

To ensure the model had the capacity to withstand the loads expected during the various wind tunnel testing phases, stress and strain calculations were performed. Such structural strength assessments were executed for all the critical model components and for the assigned loading conditions.

The envelope of the expected model load conditions, obtained by scaling the full scale reference values, was fundamental to enabling the requested structural evaluation. It also had to ensure that a component’s final design capabilities could guarantee full compatibility with both wind tunnel constraints (e.g. supports) and equipment (e.g. internal/external balances). Model components’ materials and related stress limitations, stress concentrations, fatigue, were also discussed during the design phase.

The tunnel model manufacturing technique has changed over the years. Historically, such components would have been made by a classical composite material technology. The biggest restriction of this technology was the long manufacturing time.Leonardo HD’s first wind tunnel models were manufactured using wood and metallic components and then changed to a mixed solution of wood and composite fibre materials. Today, all models are manufactured using a CADCAM approach; an internal structural aluminium and steel frame is milled and assembled and all the external geometries are obtained through 3D printing techniques. Advanced 3D printing combined with the Windform XT 2.0 material was chosen by Leonardo HD, thanks to its short manufacturing time and high-performance features.

Process and result

The activity of CRP Technology was based from the beginning on the maximisation and achievement of the requested goals. The work started from a careful analysis of the dimensional designs received from Leonardo HD. Thanks to the wide-ranging experience of CRP Technology in this market, and its detailed knowledge of the materials and the technology, it was possible to assist Leonardo HD in the choice of the best composite materials.

Choosing Windform XT 2.0 composite material was an in-depth process, with all the goals required by Leonardo HD considered, such as the importance of a short realisation time, good mechanical performances and also good dimensional characteristics. Windform XT 2.0 is a ground-breaking carbon fibre reinforced composite 3D printing material known for its mechanical properties, suitable for many applications such as use in wind tunnels, because of its high heat deflection (HDT = 173.40 °C;test method= ISO 75-2 TYPE A), superior stiffness and first-rate detail reproduction. It replaces the previous formula of Windform XT in the Windform family of composite materials: Windform XT 2.0 features improvements in mechanical properties including +8% increase in tensile strength, +22% in tensile modulus, and a +46% increase in elongation at break.

The first issue concerned the dimensions of the prototype: since some components were dimensionally superior to the construction volume of the 3D printing machines, it was necessary to manufacture the single parts separately. The experience and knowledge of this process by CRP Technology’s staff have allowed the analysis, the study and the consequent creation of such a complex project without any delay or problem for the client.

From the beginning the work was focused on the design of the components, with a correct split of the parts, considering of course the working conditions and the stress that the components would have to sustain.

Author
Paul Fanning

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