Composite driveshaft prototype design and survivability testing

Rotorcraft drivelines typically utilize a multi-segmented metallic system to transmit power between the engine and tail rotor. The typical arrangement of metal driveshaft segments, hanger bearings, and flexible couplers contribute to a significant logistical footprint, maintenance downtime, and life-cycle costs. Thus, an innovative flexible matrix composite driveshaft design alternative is presented in this paper, intended to simultaneously reduce the number of couplers and bearings, as well as, provide high fatigue strain capacity. Through reduction in number of parts, the maintenance cost and time as well as weight of the system are reduced. Composite driveshafts, representing those used in utility helicopters, were designed using an optimization process that considers: (1) damping-induced self-heating, (2) whirling stability, (3) torsional buckling stability, and (4) lamina strength. The paper provides a ballistic comparison study between a baseline carbon/epoxy composite and flexible carbon/polyurethane composite driveshaft segments. One driveshaft of each material was torsionally loaded to failure without ballistic impact. Additionally, two driveshafts were impacted obliquely at zero torque with 7.62 and 12.7 mm armor piercing/incendiary (API) rounds. After impact, the driveshafts were loaded in torsion to failure. Residual torsional strengths were 17–21% and 13% of un-impacted strengths for the 7.62 mm and 12.7 mm rounds, respectively. For the small sample size, flexible driveshafts had a marginally higher residual strength compared to the carbon/epoxy counterpart. Residual torsional stiffness values were 83–86% and 52–59% for the 7.62 mm and 12.7 mm rounds, respectively.