Experiences In Nastran Airframe Vibration Prediction At Bell Helicopter Textron

Airframe vibration prediction is a complex task involving the analysis of the coupled rotor, pylon, and fuselage responding to distributed, time-variant dynamic and aerodynamic loads. At Bell, the NASTRAN analysis is used during the design process in order to achieve acceptable vibration levels with minimal design modifications during development flight test. The analysis involves tuning the pylon isolation system and fuselage structure to minimize vibrations and position the important, hard-to-move modes away from rotor excitation frequencies. NASTRAN is a valuable design tool for airframe vibration prediction and has been used successfully at Bell since its introduction in the early 1970's. Some case histories of the application of NASTRAN during the development of the AH-1T, the Model 214ST, the Bell ACAP, and the V-22 are discussed in this paper. In addition, under the NASA DAMVIBS research program, an understanding of higher frequency vibration correlation was achieved and modeling deficiencies identified to improve main rotor 4p vibration predictions.