Whirl flutter instabilities have been a matter of concern for propeller-driven aircraft, aircraft having tilting proprotors, and helicopters ever since two Lockheed Electras crashed more than 40 years ago. A significant body of literature exists, which details the analyses used to predict whirl flutter and the experiments that have validated the analyses. The factors that contribute to whirl flutter are reviewed, and with use of simple analysis methods, their effect on stability are demonstrated. Whereas previous efforts in the literature have tended to concentrate exclusively on propellers, proprotors, or helicopter rotors, a unified treatment of all three configurations is presented.