Demonstration of a Supercritical Composite Helicopter Power Transmission Shaft

An experimental program is underway to investigate the dynamic behavior of supercritical composite drive shafts for helicopter applications. Design optimization results have shown that the system of least weight is achieved by incorporating supercritical operation and advanced composite materials. Uncertainties in the ability to manufacture, balance and safely operate supercritical composite shafts motivates the experimental program to examine their dynamic performance. Results for experiments with an aluminum and an optimized graphite/epoxy shaft are presented. Both were successfully balanced through two critical speeds and it appears that they may be operated at higher speeds with the aid of sufficient external damping. Due to the relatively low bending stiffness of the optimized shaft, the critical speed behavior of the shaft is of great importance and is discussed in detail. This includes an assessment of the proximity of operating speeds to critical speeds, external damping requirements for the system, and the observed sensitivity of the shaft to unbalance. Areas of future research are identified. The benefits of this research we: (1) lighter weight drive systems due to a reduced number of support bearings and the implementation of composite materials; and (2) increased reliability and lower maintenance costs due . to fewer system components.