THE INFLUENCE OF MODE SHAPE ON FLUTTER STABILITY OF A SCALED COMPOSITE UHBR FAN

In literature, two common parameters to indicate flutter sensitivity are the reduced frequency and the amount of twist present in the combined first flap mode. A critical parameter is the plunge-to-twist incidence ratio, which is a function of both frequency and twist-to-plunge ratio in the mode shape. However, the limit of this stability criterion for real fan blades is not fully understood yet. This paper investigates the influence of plunge-to-pitch incidence ratio on a 3D transonic test case, the scaled composite UHBR fan of the CA3ViAR project, with the aim to understand the validity of the plunge-to-twist incidence ratio as a flutter parameter for transonic and three-dimensional fan blades. This fan was designed with the unconventional goal to achieve stable operation at the design point but flutter at throttled off-design conditions. The design was done by varying the CFRP ply material and orientation towards specific reduced frequencies and twist-to-plunge ratios to drive the aerodynamic stability towards the desired behaviour. Here, the aerodynamic damping of a set of 3D mode-shapes based on variations in the composite ply-layup are simulated using TRACE Harmonic Balance for different reduced frequencies. Flutter stability for different reduced frequencies and twist-to-plunge ratio is mapped for several operating points. The results show that the stability boundary only follows lines of constant plunge-to-pitch incidence ratio in some regions of the map but deviates in others. The key deviations are then explained by analysing the aeroacoustic conditions. The study demonstrates that the validity of the plunge-to-twist incidence ratio parameter depends on the flutter mechanism. It is not valid inside the region where the acoustic wave is cut-on upstream but cut-off downstream. To the authors' knowledge, this is the first time that limits of this stability criterion for fan flutter were explicitly shown and explained.