High-Fidelity Finite Element Modeling of Rotorcraft Shafting System for Critical Speed Prediction

An accurate critical speed prediction of shafting systems in rotorcraft is required during preliminary design, so that dynamic loads do not lead to premature component failure within the full operational speed range, i.e. from stationary to maximum operational speed. If all modes are above the operational speed range, the shafting systems is considered subcritical and when design constraints do not allow for this type of design and so one or more critical speeds must be passed through before the shaft reaches its maximum speed the shafting system is considered supercritical and deflection limiters are required. Analytical simulations of the critical speeds of a shafting system can not only assist designers in making decisions in the earlier phase of design, but also helps mitigate risk during the qualification testing, ground run and flight test of the aircraft. Moreover, there are three factors important for an accurate critical speed prediction, i.e. gyroscopic effect, centrifugal force (CF) stiffening effect, or stress stiffening effect, and boundary conditions. The current paper focuses on the first two factors, especially the stress stiffening effect, but the discussion of the boundary conditions will be in a follow up paper. Ultimately, with the advancements of finite element analysis software, and the parallel processing provided by High Performance Computing, high fidelity finite element modeling of rotorcraft shafting system to assess critical speed is possible and practical.