TiltRotor Aeroelastic Stability Testbed (TRAST) Rotor Hub Analysis and Testing Results

Author(s):

Benjamin Cobb and Andrew R. Kreshock

Abstract:

The TiltRotor Aeroelastic Stability Testbed (TRAST) is a semi-span aeroelastic tiltrotor testbed, developed by the U.S. Army and NASA, used to investigate whirl-flutter stability of tiltrotor aircraft. Understanding the modal behavior of the rotor is an important step in successfully modeling the testbed as a whole. The TRAST model has two primary hub configurations: gimballed and hingeless. Ground vibration testing was conducted to provide baseline frequencies and trends for the four primary rotor modes (symmetric and asymmetric flap and lag) and to compare them with analytical data from RCAS, NASTRAN, and CAMRAD II (hingeless hub only) models to validate the models and demonstrate their efficacy in predicting the vibratory modes of the hub. Both configurations of the hub were experimentally excited using an impact hammer with blade pitch angles ranging from –5° to 65°. The frequency responses were measured using a series of accelerometers, allowing the frequency of each mode to be determined. The results of the test demonstrate reasonable agreement between analytical and experimental results for both the hingeless and gimballed. Additionally, the hingeless hub’s frequency response is demonstrated to follow the same general trends as the gimballed hub throughout the collective range, albeit with the large increase in frequency one would expect from the stiffer configuration.