Conceptual Comparative Study of the Flutter Vibration Amplitude of Unstable Aeronautical Fans and Low-Pressure Turbines

Abstract Flutter design criteria in aeronautical fans and cantilever low-pressure turbines (LPTs) differ significantly. Modern LPT blades often exhibit aeroelastic instability across multiple nodal diameters but are designed to endure this instability by limiting the vibration amplitude through dry friction in the fir-tree attachment. In contrast, aeronautical fans are designed to be flutter-free, avoiding operational instability. This divergence is noteworthy given the typical minimum critical damping ratio (ξ) of LPTs is about ξ∼−1% whereas in aeronautical fans is ξ∼−0.1%, an order of magnitude lower. This paper addresses the understanding and rationale of such disparity in fan and LPT flutter design criteria and strategies. The vibration amplitude of unstable friction-saturated aeronautical fans is predicted using a previously established, calibrated, and simplified model for fluttering LPTs. The model is informed with the aerodynamic and structural blade characteristics, size, and operating conditions of the components. Despite their minimum critical damping ratio differing by a factor of about ten, the combined effects of changes in natural vibration frequency, blade size, material properties, and operation point are shown to be more significant. This study qualitatively compares the vibration amplitudes and alternate stresses of fan and LPT blades saturated by friction and their scaling with the engine size. Although aeronautical fans' critical damping ratio is much smaller than that of LPT rotor blades, fans cannot endure the resulting vibration levels, whereas high aspect ratio cantilever LPT blade can. This difference is due to key factors such as the larger size, distinct geometry, lighter materials, different mechanical design, and lower natural frequency of fan blades compared to LPT blades.