Flexible all-plastic aircraft models built by additive manufacturing for transonic wind tunnel tests

Abstract

Wind tunnel testing is considered as a reliable tool, especially for the high-order non-linear aerodynamic problems of large aircraft with high-aspect-ratio wings at transonic speeds. Thanks to its capacity to manufacture complex structures quickly, the introduction of the additive manufacturing (AM) technique into the design and fabrication of testing models can improve the testing performance significantly. However, these AM-built models so far are limited to low-speed testing due to the low strength and modulus of non-metal materials, epoxy resins mostly, used in popular AM processes for aircraft models. The easy-deformation properties are usually considered as the major weakness and many methods are adopted to strengthen the plastic models for high speed tests. Taking advantage of the properties, however, this paper proposes a plastic flexible testing model with a specific pre-deformation that can be deformed into the desired state during wind tunnel tests. To obtain the pre-deformation quantitatively, an optimization formulation was developed based on the coupling of computational fluid dynamics (CFD) and computational structural dynamics (CSD). As a case study, testing models with the DLR (German Aerospace Center) F4 configuration were designed and fabricated by stereolithography (SL), a popular AM process. After the strength calibration, the plastic models were tested in a transonic wind tunnel. All the models performed normally in the harsh condition when the Mach number reached 0.85, and the resulting lift coefficients (CL) obtained by the plastic models showed good consistence with their metallic counterparts. This indicates that the plastic models of large aircraft made by SL could be used in wind tunnel tests at transonic speeds. However, the all plastic models can only be used in a single combination of testing condition. Further studies should be conduct to extend the scope of application of the models. In conclusion, due to AM's capacity to manufacture complex structures with low-modulus materials, flexible models could be designed and built quickly in an economic way. The method could be used in the conceptual design for configuration screening of high-aspect-ratio aircraft, and the paper would provide a new test scheme that is fast and reliable for aircraft design.