Method and apparatus for varying the critical speed of a shaft

A system is provided for changing the critical speed of a shaft that includes: a first shaft (26) supported for relative rotational movement with respect to a stationary structure, a forward bearing (34) disposed at the forward end of the shaft, an aft bearing (40) disposed at the aft end of the shaft, at least one active bearing (36) disposed between the forward bearing (34) and the aft bearing (40), and means for changing the support stiffness of the active bearing.