This paper examines the potential of graphite composite materials in airframe structures by developing data on the weight of various structural elements fabricated in graphite composites, and then applying this data to a study of the weight of the X-22A V/STOL aircraft. It is shown that over 30% of the weight of the current aluminum structure could be saved, resulting in a 100% increase in payload. Improvements of this magnitude may be sufficient to make the V/STOL aircraft a feasible and economical vehicle.
The structural element studies include such elements as tension members, stiffened and unstiffened compression panels, shear panels, sandwich panels, columns, beams, torque tubes, etc., and in each case the weight is presented as a fraction of the weight of the corresponding aluminum structural element, for a range of values of loading. As appropriate combined loads and multiple loadings, in various ratios, are considered and each point on each curve includes an optimization of the construction with respect to both geometry and fiber layup and orientation. Various potential failure modes such as buckling, fiber tensile strength, resin shear strength, fiber to resin bond strength, and resin strain limitations are all considered, as well as practical fabrication limitations such as the number of layers, layer thickness, etc. The curves presented thus provide a substantial amount of data for other studies of graphite composite applications.
The weight of joints, connections, and fittings has not been considered in the present study, but is is evident that the large potential gains from composite materials could be seriously reduced if appropriate joining methods are not developed. Recommendations to this effect are, therefore, made in the paper.