<div class="htmlview paragraph">Helicopter tail rotors are subject to many types of divergent vibration. A classification of these various types is presented, and a description given of those which have been actually observed. The means used by manufacturers to predict and avoid such vibrations include various calculations in the design stage and special test stands for development work. Substantiation tests accomplished on these stands enable flight tests to be undertaken with a tail rotor free from divergent vibrations.</div>