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Meeting	ACGS Committee Meeting 126 - Virtual - March 2021
Agenda Location	5 SUBCOMMITTEE A – AERONAUTIC AND SURFACE VEHICLES 5.3 Designing a More Controllable Composite-Aircraft Structure For Active Flutter Suppression via MDO
Title	Designing a More Controllable Composite-Aircraft Structure For Active Flutter Suppression via MDO
Presenter	David K. Schmidt
Affiliation	DK Schmidt & Associates
Available Downloads*	presentation
	Downloads are available to members who are logged in and either Active* or attended this meeting.
Abstract	This presentation provides an overview of a 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. MDO deals with the computationally-intensive numerical optimization of the internal structure (e.g., rib and spar locations and dimensions, ply thicknesses and fiber orientations), and composite-material properties to minimize an aircraft’s weight, drag, or fuel burn, for example, subject to many design constraints (buckling, static margin, etc.). Virginia Tech’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, and are directly associated with a feedback system’s loop-transfer function. Hence, adjusting the residues provides a means to shape the loop transfer. Simple, fixed pure-gain SISO control laws were synthesized post MDO optimization, and closed-loop results were compared to those from a baseline 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 nonoptimized 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 baseline structural design, control optimization led to larger control surfaces, decreased optimal control gain, 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.