I want to ask a basic question,I want to know why the system’s natural frequency vary with the rotor speed.Can you tell the reason resulting in the change of frequency.As before I always think the natural frequency is fixed。
There are a couple reasons. First, the stiffness of a blade is effectively increased as a result of centrifugal loads i.e. centrifugal stiffening causes the blade frequencies to increase with increasing rotor speed. Second, the natural frequencies of the blades are influenced by the frame of reference–the frequencies one would see in the rotating frame of the rotor are different than what one would see in the fixed frame of reference. The multi-blade-coordinate (MBC) transformation is useful for transforming all modes into the same (fixed) frame.
I hope that helps.