Empirical Modal Analysis of a Stationary Tail-Rotor Blade of the Bell UH-1H Iroquois “Huey” Military Helicopter Augmented by Operational Structural Dynamics Modification

In the multi-degree of freedom portion of the undergraduate vibrations engineering course taught at the United States Military Academy and West Point, New York, in-structors ask the students (a) how we identify system parameters of effective mass and/or mass moment of inertia, stiffness, and damping as well as (b) whether our implicit as-sumption, that these properties were constant for systems in motion, was valid. Instructors then use concepts from several other engineering courses along with an introduction to experimental methods to answer these questions. One set of answers to these questions came from the analysis of the elastic vibration of a tail-rotor blade from a U.S. Army UH-1H “Huey” helicopter. This analysis required the integration of fields of vibrations engineering with dynamics and mechanics of materials. Use of modal analysis empirically determined three out-of-plane flapping modes as well a torsional mode of vibration for a nonrotating blade. For this modal analysis, a Brüel and Kjær 2034 signal analyzer was used to gather frequency response functions of the stationary rotor blade as excited by an impact hammer. The response to the excitation was measured with an accelerometer. The centrifugal-stiffening effect of the operational angular velocity of the rotor blade was simulated via structural dynamics modification. This problem presented many interesting challenges and provided an innovative means of coupling material from different undergraduate courses to solve a real-world engineering problem.