INVESTIGATION OF DIFFICULT COMPONENT EFFECTS ON FEM VIBRATION PREDICTION FOR THE AH-1G HELICOPTER

Under the NASA-sponsored DAMVIBS (Design Analysis Methods for Vibrations) program, a series of ground vibration tests and NASTRAN finite element model (FEM) correlations were conducted on the Bell AH-1G helicopter gunship to investigate the effects of difficult components on the vibration response of the airframe. Previous correlations of the AH-1G showed good agreement between NASTRAN and tests through 15 to 20 Hz, but poor agreement in the higher frequency range of 20 to 30 Hz. Thus, this effort emphasized the higher frequency airframe vibration response correlations and identified areas that need further R&T work. To conduct the investigations, selected difficult components (main rotor pylon, secondary structure, nonstructural doors/panels, landing gear, engine, fuel, etc.) were systematically removed to quantify their effects on overall vibratory response of the airframe. The entire effort was planned and documented, and the results reviewed by NASA and industry experts in order to ensure scientific control of the testing, analysis and correlation exercise. The approach used in this investigation proved to be successful in quantifying the effects of selected difficult components on the airframe vibration response. In particular, secondary structure and damping had significant effects on the frequency response of the airframe above 15 Hz. Also, the nonlinear effects of thrust stiffening and elastomeric mounts were significant on the low frequency pylon modes below main rotor 1p (5.4 Hz). This paper summarizes the results of the ground vibration testing and NASTRAN FEM correlations, including highlights of the findings and areas needing further work that were identified in the investigation.