Lightweight structural design of UAV wing through the use of coreless composite materials employing novel construction techniques.

Abstract

A new structural layout was designed for an existing UAV wing with the aims of lightening the wing by eliminating the use of cored composite construction and reducing the manufacturing time of the wing by making use of waterjet-cut internal frames while satisfying strength and stiffness requirements. Two layouts, a traditional metal wing layout and a tri-directional rib lattice layout, were selected for consideration based on the literature surveyed. In order to present a valid comparison with the previous wing design the same composite materials were used in the design of the new wing layout and material tests were performed according to ASTM testing standards to obtain the mechanical properties of these materials. Load cases for the wing in flight were calculated according to FAR-23 standards and the loads on the wing were found using XFLR5 vortex-lattice methods. An empirical, spreadsheet-based initial sizing tool was developed to obtain initial layups for an iterative FEA-based optimisation process that employed the SolidWorks Simulation Premium software package and made use of the Tsai-Wu composite material failure criterion and empirical buckling equations. The iterative optimisation resulted in the traditional metal wing layout being selected and predicted a weight saving of 14% over the original wing design. A full scale prototype wing was constructed in the CSIR UAS Laboratory using wet layup techniques and laser cut internal frames as it was found that the waterjet cutting of thin composite frames was not practical as a result of the high working pressure of the waterjet cutter. The prototype wing showed an actual weight saving of 14% but took considerably longer to manufacture due to the necessity of constructing specialised jigs to aid in the bonding and alignment of the internal frames. The prototype wing was tested using a custom set-up whiffle tree rig up to its maximum limit load of 4.9 g and showed an average of 4% error between measured and predicted deflections thereby validating the FEA models. It was concluded that a UAV wing can be significantly lightened through a coreless structural design, but at the expense of an increase in construction time. It is hoped that this study will contribute towards a changed design philosophy in an industry where cored construction is the norm. It is recommended that the methods developed during this project be applied to the rest of the aircraft components in order to obtain a lighter overall structure.