As the previous chapters have stressed, the ability to analyze rotorcraft for vibration requires a good knowledge of the dynamic characteristics of the fuselage. These characteristics generally include, as a minimum, the natural frequencies of the airframe in the general frequency range of the blade passage frequency. Of increasing importance are the mode shapes, the structural damping descriptors, and the various mobility matrices [i.e., frequency-response functions (FRFs)] in the frequency range of the fundamental blade passage frequency. Until such time as the dynamicist has the tools (and the skill for using them) for accurately calculating these dynamic characteristics, some form of vibration testing will remain as part of the development of new rotorcraft configurations. An adjunct to the direct vibration testing of the airframe and airframe components is the testing for component fatigue life. Just as in the case of natural frequency and mode prediction, the prediction of component fatigue life is an inexact science that requires some form of experimental verification. These two related types of vibration testing are similar to the extent that they deal with structures operating in an environment that is almost wholly in the frequency domain; therefore, some of the techniques overlap.