CRP Technology collaborated with Leonardo Helicopter Division on the construction of a wind tunnel model using 3D-printing technology.
This project allowed CRP Technology to highlight the union between advanced 3D-printing technology (Selective Laser Sintering) and Windform high-performance composite materials. Thanks to the Windform materials, it was possible to complete and test the model in the wind tunnel within a very short time, with great results and with high-performing mechanical and aerodynamic properties. The project related to the manufacturing of certain external parts (nose and cockpit, rear fuselage, nacelles, external fuel tanks, fairings) of the wind tunnel model (1:8.5 scale) for the prototype of the new Leonardo HD tiltrotor AW609, made with 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 Helicopter Division by Metaltech S.r.l. for a series of dedicated low-speed wind tunnel tests.These tests were intended to cover a standard range of flight altitudes to be performed at the Leonardo HD wind tunnel facility and at Politecnico of Milan for the high angles of flight envelope.
During the different test sessions, various external geometries were changed and checked in order to understand all the aerodynamic phenomena. Leonardo Helicopter Division’s main goals, and therefore the reasons why they have been referring to CRP Technology, have essentially been the following two aspects:
- The necessity of 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 component, which usually would have been made using a traditional composite material to design and manufacture an aluminium alloy internal main structure suitable for easy implementation with new geometries for the future aircraft versions or improved solutions.
This detail is crucial for the applied loads to be sustainable, and therefore they cannot be underestimated. In fact, the aerodynamic loads in the tunnel are very high. The most critical aspect of the project is therefore the resistance to the loads, but also the necessity to maintain good dimensional tolerances of such a large-dimensioned component under load. It is important that the components of the external fairings do not deflect too much under load. In addition, even when there are no external loads, the product must have dimensional characteristics with respect to the supplied specifications.
To assure the model can withstand the loads expected during the various wind tunnel testing phases, stress and strain calculation have been performed. Such structural strength assessments have been 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, is fundamental to enable the requested structural evaluation and allow for the model components’ final design, which must be able to guarantee the model’s full compatibility with both wind tunnel constraints (e.g., supports) and equipment (e.g., internal/external balances).
Model component materials and related stress limitations, stress concentrations, fatigue etc. were discussed during the design phase.
Process and result
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.
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. Identifying the parts to split was an operation undertaken with the CAD, evaluating the functional measures of the working volume but also the possibility to optimise such volume and minimise production time and costs. The CAD cut was made using a special technique in order to maximise the contact surface in the place where the structural adhesive would be applied, thus having, also for very big parts but with a relatively thin thickness, a great resistance to any kind of stress.
The final step was the complete model surface finishing, directly mounted on the rig assembly in order to optimise the small imperfections that could have come from the union of the single components. In this case, too, CRP’s know-how, which has to remain confidential, made it possible to execute this step in a very short time. It was therefore enough to flatten the surface of the whole model in an efficient way and treat it with a special liquid that has the double function to make it waterproof and prepare the surface to be painted without any problem. All the model parts were then mounted and adapted to the main model structure thanks to a dedicated rig and assembled by Metaltech S.r.l.
The final result, in line with the timetable and characteristics of the part set at the beginning of the entire endeavour, has been finally tested in the Leonardo HD wind tunnel facility at Bresso (Mi). As part of a thorough review of the aircraft behaviour, Leonardo Helicopter Division has performed a high-speed wind tunnel test campaign at NASA Ames Unitary Plan 11 by the 11-foot transonic wind tunnel, encompassing speeds between Mach 0.2 to Mach 0.6.