TABLE OF CONTENTS In the previous chapter the basic tools are identified (if not as yet completely formulated) for analyzing the helicopter/rotorcraft for vibration. The basic vibration analysis problem boils down to knowing the following items with sufficient accuracy:
the aerodynamic environments of both the rotor and the fuselage;
the aerodynamic and aeroelastic response characteristics of the (rotating) rotor in the form of integrated hub loads;
the structural characteristics of the (nonrotating) fuselage, in the form of accurate frequency response functions ( fuselage mobilities); and
the rotor-fuselage interactions, in the form of rotor impedances.
Once each of these separate items can be predicted accurately, coupling tools exist for combining them to give reliable vibration predictions. Such predictive capability, if available sufficiently early in the design process, should lead to the quantum reduction in vibration (with minimum weight) that is so highly prized.
However, the required knowledge to achieve this state of affairs is not yet at hand, and aircraft design, fabrication, and certification must continue without the benefit of this foreknowledge. Thus, methods that have necessarily been devised for modifying the aircraft design "after the fact" will continue to be used to reduce vibration to some tolerable level. This chapter describes major methods devised and presently either used outright or seriously being considered for controlling vibration.
Generally, the structural modification methodology available to the designer is of three basic types: 1) detuning the structural responses through passive structural changes, 2) using (passive or active)...
