UH 60 Tail Rotor Spar

The UH-60 tail rotor spar, which is manufactured with Cytec Rigidite® 5225 Unitape Structural PrePreg (modified epoxy resin on structural carrier) has consistently posed manufacturing challenges. Weight control, fiberwash, and porosity have been the main contributors to the defect rate in production. The UH-60 spar (Flexbeam) is designed to perform all dynamic motions (blade pitch, flapping, lead/lag) required for flight control. Laminated graphite (approximately 90 plies) provides high ratio of bending to torsional stiffness. Match metal molding along with press cure provides thickness control. Curing is an integral part of all composite manufacturing. Proper cure temperature, pressure and time ensure the finished product consistently meets the performance requirements of its design. At Sikorsky, the cure process must be better understood so it can be controlled. This is essential to taking a proactive approach in optimizing the manufacturing process to accommodate material and other variations. Statistical techniques were employed to understand the cure cycle of the spar and the cause of variation. Correlation, regression (best subset, stepwise), and statistical t-tests were some of the methods used to analyze the data collected during the manufacturing of the spar. The objective of the analysis was to pro-actively predict finished spar weight based upon both supplier and Sikorsky Aircraft process parameters and look at mold sensitivity as it relates to B-weight (resin and fiber), resin content and resin residual. We also intended to determine factors related to resin ‘bleed-off’, spar weight, porosity, and fiberwash. The results of the information will be utilized to adjust the process and optimize the yield. From the analysis it was found: - • Significant “batch to batch” variation (Points A, B, & C across rolls) • Positive correlation between B-weight and percentage defect (fiberwash) rate • Molds with fewer bleed ports have lower yields when combined with heavier a batches • Correlation of outside temperature and inside humidity to the resin residual • Porosity concentrated primarily in transition areas (cross-section decrease in thickness) of spar In addition, regression equations were developed relating resin content and resin “bleed-off” to the spar weight along with gel temperature and the ply thickness to the resin “bleed-off’. Statistical analysis on the data indicates a ± .1 standard error. Recommendations to improve the cure cycle of the UH-60 tail rotor spar include: - • Self-monitoring (close loop system) cure cycle • Develop standards for the molds (bleed ports, side gap, and cavity dimension) • Select molds based upon batch and kit weights • Work with vendor to control material properties – weight/fiber/resin • Use individual kit weights to make decisions regarding molds and plies • Run DOE to determine optimal combinations of weights, molds, etc. • Avoid certain molds when average batch weight exceeds a510 B-Weight To complement the close loop system expressions are being developed using isothermal tests for correlating the rate of degree of cure and viscosity with the degree of cure.