TABLE OF CONTENTS Because of the aerodynamic performance gains achievable, current helicopter rotor blades have evolved into relatively high-aspect-ratio structures. From a structural dynamics standpoint such a structure is a simplification in that rotor blades can then be treated as one dimensional, that is, their elastic characteristics can be specified as functions of only the radius. A further consequence of this one dimensionality is that we need define the elastic deflections of rotor blades only in terms of bending deformations transverse to the blade, torsion, and, in some special cases, axial elongation and cross-sectional warping.
In this chapter the basic concepts relating to blade vibration characteristics are developed. These concepts include theory for and practical schemes for obtaining natural frequencies and mode shapes in both in-plane and out-of-plane bending and in torsion for relatively simple blades, that is, blades that have no pitch, twist, or preconing. Such simplified blade configurations produce what we will refer to as uncoupled mode characteristics. The uncoupled modal responses are characterized as having, for any one mode, all of the motion in either the in-plane, out-of-plane, or torsional degrees of freedom. In reality, rotor blades do have all of these complicating characteristics (pitch, twist, and precone), and material is therefore presented explaining how the basic uncoupled characteristics for the simplified blade are modified or coupled by these additional considerations.
The principal characteristics of rotating beams that distinguish them from conventional beams are that they function in a tension field that...
