Flutter and Post-Flutter Constraints in Aircraft Design Optimization
E. Jonsson, C. Riso, C. A. Lupp, C. E. S. Cesnik, J. R. R. A. Martins, and B. I. Epureanu
Progress in Aerospace Sciences, 109100537, 2019
Flutter is a dynamic aeroelastic instability driven by the interaction of inertial, elastic, and aerodynamic forces. It is an undesirable phenomenon in aircraft because it causes divergent oscillations that may lead to structural damage or failure, performance and ride comfort degradation, or loss of control. If flutter is discovered at the aircraft certification stage, costly redesign is required. Performing flutter analysis early in the design process can mitigate this problem. Furthermore, including flutter analysis as a constraint in multidisciplinary design optimization reduces the risk of costly modifications late in the design cycle. We review the methods for flutter analysis in the context of aircraft design optimization. We also include methods for predicting post-flutter limit cycle oscillations due to the increasing impact of nonlinear effects on future aircraft. While there has been extensive work in flutter and post-flutter analyses, developing design optimization constraints associated with these analyses has additional requirements, such as acceptable computational cost, function smoothness, robustness, and derivative computation. We discuss these requirements and review efforts in the development, implementation, and application of flutter and post-flutter constraints in aircraft design optimization. We conclude the paper by summarizing the current state of this field and the main open problems. Flutter constraints have been included in structural optimizations, but optimizing both the structural sizing and the aerodynamic shape remains a challenge due to the need to recompute the aerodynamic properties at each design iteration. Additional difficulties arise in the presence of large structural deflections and transonic flow conditions due to the dependency of the flutter point on the equilibrium state and the high cost of nonlinear computations. Post-flutter constraints have rarely been included into design optimization, but they are crucial in the prevention of undesirable limit cycle oscillations. Implementing such constraints requires the development of more efficient and robust prediction methods that can handle realistic configurations. While this paper focuses on flutter and post-flutter constraints for aircraft design optimization applications, the considerations and challenges are broadly applicable to the optimization of engineering systems including stability and post-critical dynamic constraints.