TABLE OF CONTENTS A distinctive characteristic of rotorcraft vis- -vis fixed-wing aircraft is the necessity to deliver rotational power to the rotors via a system of shafts and gear boxes over lengths approaching the length of the aircraft itself. This is especially true for tandem helicopters and tilt-wing and tilt-rotor VTOL aircraft. Even single rotor helicopters rarely escape the need to deliver power to the tail rotor via a shaft. Thus, relatively long shafting defines not only the requirement to analyze the system for torsional vibrations, but for lateral ( bending) vibrations as well. This chapter addresses both of these issues from the standpoint of analyzing vibratory responses. A subsequent chapter addresses the problem of torsional instabilities.
No matter how well a rotor shaft might be balanced, there will always remain some infinitesimal amount of mass imbalance (i.e., the mass center of the shaft system lies off the rotation axis) and/or initial curvature. If the shaft now rotates at a frequency equal to one of the bending natural frequencies of the shaft, the small imperfection in mass or curvature would be sufficient to excite the system in this mode. This condition commonly referred to as shaft critical speed operation is a form of resonance that can involve whirling motion in either the rotational or contrarotational directions. Furthermore, because of the presence of rotational elements having significant rotary inertia, gyroscopic effects can play a major role. The first order of business is to identify the characteristics...
