Airframe vibration and damping characteristics of flight and laboratory test structures

Vibration damping levels over a wide frequency band for complex built-up structures are extremely difficult to predict. In this work, rotorcraft airframe structures were targeted for vibration based Structural Health Monitoring (SHM) technology development and implementation. Damping is a critical parameter for understanding the vibration characteristics of a structure and realistic damping levels must be established for the application of relevant vibration based damage detection simulations and laboratory testbed based assessments to be made. The goal of this investigation was to quantify the vibration and damping characteristics of an actual flight system structure, compare and contrast the results to laboratory development test structures, and identify any important trends and discrepancies. Experimental modal analysis and decay techniques were applied to a UH-60 Blackhawk airframe in a "flight" configuration. Structural mobility, resonance frequencies, modal damping, and high frequency one third octave band decay based damping levels were measured and quantified. Similar measurements were also made on laboratory test structures used for technology development and verification testing. The measured (flight configuration) airframe damping loss factors varied from approximately 0.01 to 0.10 over the frequency range 2 to 10,000 Hz. The flight vehicle damping characteristics were compared and contrasted with laboratory test structures, quantifying the damping effects due to built-up structure complexity. Methods for enhancing the laboratory test structures were also investigated showing positive results.