Multi-objective optimal design of composite rotorcraft driveshaft including strain rate and temperature effects

Single-piece composite driveshafts offer the potential to reduce weight, parts inventory, and maintenance costs in rotorcraft, compared to multi-segmented drivelines with flexible connectors. This innovative application of composites is enabled by the high cyclic strain capacity of composites, which allows operation under misaligned conditions. Trade space visualization was used to demonstrate how material, ambient temperature, bending strain, and torque/speed combination (for constant power) affect the design space. Design variables included stacking sequence and number of layers and hanger bearings. Failure criteria included overheating, whirl instability, torsional buckling, and material failure. The design goals minimized mass and maximized the lowest factor of safety by adaptively generating solutions to the multi-objective problem. Rate- and temperature-dependent viscoelastic properties and temperature-dependent strengths of five composites were experimentally characterized and used as model inputs. Design space inspection highlighted shaft whirling as a common limiting factor in addition to fiber direction compressive strength for flexible composites. Mass reductions of 15.15 and 13.65 kg were realized for rigid and flexible composite driveshafts respectively (original mass of 31.3 kg) in a case study, primarily due to the elimination of mid-span flexible couplers and one hanger bearing. Larger mass reductions were demonstrated by trading operating speed and torque.