Model Verification of Force Determination for Measuring Vibratory Loads

Force Determination is a multiple regression technique for determining vibratory forces and moments using response measurements and dynamic calibration data. The response measurements are distributed throughout the structure and the calibration data are the partial derivatives of the response to each external force or moment. The method was verified with laboratory tests to determine vibratory hub forces on a helicopter dynamic model. The hub calibration data were obtained from both direct hub shaking and single‐point shaking at the nose of the dynamic model. Vertical and lateral forces were simultaneously applied to the hub of the dynamic model with electromagnetic shakers. These force magnitudes and phases were then determined to within 5% and 4 deg average absolute error, respectively, using the Force Determination method. Gross weight and center‐of‐gravity variations indicated that additional fuselage calibrations, reflecting the loading and c.g. changes, may be required to accurately determine the vibratory forees. An application of Force Determination to an Army UH‐1H helicopter is discussed. Promising results indicate the potential of applying Force Determination to full‐scale aircraft.