The Aeroelastic Stability Improvements of Soft-Inplane Tiltrotors By Active and Passive Approaches

Open Access
Author:
Paik, Jinho
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 10, 2009
Committee Members:
  • Dr Farhan Gandhi, Dissertation Advisor
  • Farhan Gandhi, Committee Chair
  • Edward C Smith, Committee Member
  • Joseph Francis Horn, Committee Member
  • Christopher Rahn, Committee Member
Keywords:
  • design optimization
  • whirl flutter
  • soft-inplane rotor
  • tiltrotor aircraft
  • feedback control
Abstract:
Soft-inplane tiltrotors in cruise mode have exhibited unacceptably low subcritical damping in the wing vertical bending mode as well as reduced critical whirl-flutter speed. However, soft-inplane rotor system is highly advantageous over stiff-inplane rotor system in terms of inplane dynamic hub loads which results in weight/performance penalties. Therefore, ensuring adequate aeroelastic stability characteristics is a prerequisite for soft-inplane rotor system to be used in future advanced tiltrotors. This dissertation constitutes fundamental studies of soft-inplane tiltrotors and appropriate methods to alleviate whirl-flutter instability. This study consists of four major investigations. The first investigation includes validation efforts of present analytical model against the recently available data for the Bell generic semi-span model in airplane mode and the SASIP model in hover mode. The second investigation addresses the approaches which have been employed to establish a physical understanding of the very low sub-critical damping phenomenon, which is consistently exhibited by soft-inplane tiltrotor configurations. Through analyses and comparison studies mainly between the Bell generic soft- and stiff-inplane semi-span models, the physics behind this phenomenon is emphasized. In the third investigation, parametric studies and design optimization of the rotor/wing design variables are performed in order to passively improve the whirl stability boundaries. For the last investigation, the effectiveness of active control through wing-flaperon and swashplate control inputs is examined in terms of stability improvement of soft-inplane tiltrotors. Scheduled gain and constant gain controllers are first compared for each actuation scheme and then output feedback controllers based on easily measurable wing states are compared with full-state feedback controllers. The baseline soft-inplane configurations used in passive and active studies are the full-scale Boeing Model 222 and the Bell generic soft-inplane semi-span model, which employ hingeless and gimballed hub respectively. Passive/active concurrent optimization is also investigated by cosidering active control gains and passive design parameters simultaneously to achieve both stability improvement and reduced control requirement.