Flutter Analysis of a Flexible UHBR Fan at Different Flight Conditions

Abstract

Aero engines operate at a wide range of atmospheric conditions. In particular, changes of altitude and flight speed induce changes in total pressure and total temperature. During the design process and to enable the comparison of different operating conditions, a so-called "reduced" operating point can be specified. From an aerodynamic point of view, this reduced operating point typically represents the inlet conditions for rig testing. In theory, when adjusting the rotational speed and mass flow, the overall engine characteristics, the pressure coefficient and Mach number distribution remain identical. In reality, not all similarity parameters are unchanged: e.g. Reynolds number effects are not taken into account. Reducing the flight conditions to one unique ground test environment is sufficient to investigate and compare the performance and efficiency of an aero engine with very stiff blading.

A flexible UHBR fan is investigated at different flight conditions with a focus on static deflections and aeroelastic stability. Operating points at varying inlet conditions, which are comparable according to the Mach similarity principle, are investigated. However, not all the aerodynamic characteristics remain identical and aerodynamic damping of mode shape vibrations is changed.

When steady deformations of the fan blades are taken into account, the deviation between different inlet conditions increases further. This is mainly due to torsional deflections, changing the effective angle of attack and causing a general shift of the compressor map.

Even though the subsequent changes in flutter predictions are not severe for most parts of the compressor map, the behavior at the boundaries is sensitive to the real flight condition.

As shown, the Mach similarity principle is not suitable for investigating aeroelastic stability throughout the whole flight envelope, especially when the static blade deformation is not neglectable. The reason for this can be found in the complex interaction between dimension-less numbers (Mach, Reynolds), sized values (pressure difference or aerodynamic loading, natural frequency) and their dependency on each other.