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Composite Catapult Arm Design: A Case Study of Composite Design, Manufacture, and Education



In modern times, catapults have been used extensively at all levels as an educational tool. Recently, many communities, universities, and organizations have begun to hold contests where participants are tasked to build a catapult to throw pumpkins at targets or for distance. In 2013, Team ETHOS (Experimental Torsion Hybrid Onager System), a non-profit group promoting STEM education, began design and manufacture of a carbon-fiber throwing arm for their catapult. This article describes the process Team ETHOS and partners went through to design and manufacture a new arm for their machine while using the journey as an educational tool for themselves and student team members. The team developed requirements and models to demonstrate the impact of various design strategies to create a reliable structural arm. A hybrid filament winding process was chosen as a manufacturing method for the new arm. This approach consisted of building a semi-structural mandrel suitable for filament winding and incorporating pultruded unidirectional materials into the filament wind process. A unique ply-drop technique was developed in the filament winding process to address the need for skin thickness reductions from the arm root to tip. Experimental measurements of mass, center of gravity, mass moment of inertia (MOI), and internal strains of the new arm were quite close to those specified in the design process. The as-manufactured arm displayed a mass 1.80% higher and a center of gravity 15mm away from specified in the design of the 4.6m arm. Most importantly, the MOI about the arm pivot was less than 0.5% higher than predicted. Field-measurements of strain compare favorably with models.

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