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| Meeting | ACGS Committee Meeting 125 - Virtual - November 2020 | | Agenda Location | 4 GENERAL COMMITTEE TECHNICAL SESSION
4.1 General Session
4.1.2 D.K. Schmidt & Associates | | Title | D.K. Schmidt & Associates | | Presenter | Dave Schmidt | | Available Downloads* | presentation | | *Downloads are available to members who are logged in and either Active or attended this meeting. | | Abstract | Can We Design More Controllable Composite-Aircraft Structures? - A Close Encounter With MDO
This presentation provides an overview of an exploratory NASA-funded investigation into whether a more controllable composite aircraft structure can be designed. The question was addressed by employing Multidisciplinary Optimization (MDO), a research topic in the structures community that started to gain popularity in the 1980s. The project relied heavily on collaborations with MDO researchers at Virginia Tech. MDO deals with the computationally-intensive numerical optimization of a structure’s geometry (e.g., wing planform), internal structure (e.g., rib and spar locations and dimensions), and composite-material properties (e.g., ply thicknesses and fiber orientations) to minimize an aircraft’s weight, drag, fuel burn, for example, subject to many constraints. The study’s MDO computational framework included structural and aerodynamic modeling software (e.g., NASTRAN), and new modules were incorporated for obtaining dynamic-aeroelastic models of the aircraft’s flight dynamics.
The control problem selected for the study was suppression of body-freedom flutter for an unmanned composite flying wing, and the control optimization was performed using impulse residues associated with the critical sensor-actuator pair to be used in feedback control. Residues represent useful metrics of modal controllability and observability, as well as other properties. Simple, fixed pure-gain SISO control laws were synthesized post MDO optimization, and closed-loop results were compared to those from a second non-control-optimized MDO design. Flutter and robustness analyses of the two controller designs revealed that the robust flutter speed of the control-optimized design was over 50% higher than that for the nominal non-optimized design. (The robust flutter speed was defined as the highest airspeed below which the controllers’ gain and phase margins remained greater than or equal to 6 dB and 45 degrees, respectively.) Inspection of the open-loop control-optimized structural design revealed that when compared to the nominal structural design, control optimization led to larger control surfaces, increased damping ratios of all aeroelastic modes, and increased frequency separation between the body-freedom flutter mode and the higher aeroelastic modes. These increases resulted from modifications in the structure’s geometry and mass and stiffness distributions, so as to optimally modify the free-vibration frequencies and mode shapes. Based on these results we conclude that yes, we can design more-controllable aircraft structures. |
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