A study of the influence of predeformations on the vibrations of blades

Abstract

The influence of flapwise predeformations on the natural frequencies and mode shapes of rotating and non-rotating blades is studied, by using a new method presented recently by the authors. This method combines a principal curvature transformation, a velocity component transformation and the generalized co-ordinates approach. In the present paper it is used to analyze the small free vibrations of a blade superimposed on predeformations of finite magnitude. The influence of such parameters as the magnitude and shape of the predeformation, the presence of a root torsional spring representing control system flexibility and changes in structural properties are addressed and discussed. It appears that the influence of predeformations on the free vibrations of blades may be more difficult to anticipate than appears at first glance. The influence is sensitive to various parameters and, therefore, use of detailed numerical calculations will often be required to predict these influences correctly. Blades with predeformations will generally have substantial coupling among out-of-plane and in-plane bending and torsion. It is shown that use of mode identification criteria, such as the strain energy criterion, can be misleading for rotating, predeformed blades.