TABLE OF CONTENTS In the preceding chapters the emphasis is placed on the diverse methodologies for obtaining the dynamic responses of rotorcraft structures, with particular attention being paid to the rotating substructures (rotors and shaft systems). In this chapter we address perhaps the most important and directly discernible consequence of these responses: fuselage vibrations. To approach this problem, we must proceed from the concept of dynamic responses of substructures to that of the dynamic loads impacting on them, with emphasis on the influence of the rotor loads on the fuselage. Furthermore, because the rotor and the fuselage are both dynamic structural subsystems, they, in effect, talk to each other, and the concept of rotor-fuselage coupling must therefore be addressed. Finally, it must be stressed that the general problem of designing rotorcraft with fuselage vibrations that are not only acceptable but "jet smooth" is still unsolved.
This state of irresolution is caused by two principal difficulties:
Precise knowledge of the loads (in particular, those of aerodynamic origin) acting directly on the rotor and acting indirectly on the fuselage is far from complete.
The ability to calculate the structural dynamic characteristics of the airframe in terms of natural frequencies, mode shapes, and mobilities does not yet exist with sufficient accuracy. Consequently, at present heavy reliance must be made on the use of various vibration alleviation devices to bring any given rotorcraft airframe design to a point of at least having a moderately acceptable vibration level.
