A MODAL-BASED PROCEDURE FOR EFFICIENTLY PREDICTING LOW VIBRATION ROTOR DESIGNS

Both analytical and experimental research has been performed to develop an analysis which can be used to predict helicopter blade structural properties leading to low vibratory response. The work consisted of four parts: (1) develop modal-based, low-vibration design criteria and incorporate it into a design optimization analysis; (2) evaluate alternative optimization algorithms and problem formulations to determine procedures for performing vibration-oriented design optimization studies; (3) perform a wind tunnel test to verify that the resulting optimized designs yield low vibratory response; and (4) conduct a detailed study, comparing predicted and measured dynamic characteristics of baseline and optimized rotor designs, to understand changes in vibration characteristics caused by blade structural design variations. Previously published results provide a general description of the vibration criteria and cover the work under the first three items. This paper more completely documents the vibration criteria selected and details the results of part four of the program. In this work, measured flatwise, edgewise and torsional moments along the blade span; hub vertical shear, yawing moment and overturning moment; pitch link force; and hinge flapping and lagging angles were used with calculated modal characteristics to yield generalized coordinates and the vibratory contributions for the pertinent modes. From the correlation effort, it was determined that trends in the vibration criteria were reflected by the measured data. This approach led to lower vibration levels being achieved by configurations with optimized designs, as a result of reduced contributions from each of the major modal contributors.