Because of their higher aerodynamic efficiency and lighter structural weight, thin composite wings with large aspect ratios are increasingly being used for civil transport aircraft wing design. However, the increased flexibility can induce several performance issues, such as high deflections, reduced control authority, coupled flight dynamics, and, possibly, a reduction in the flutter speed. Therefore, including control surface and control law design at the conceptual design stage is becoming increasingly important. This paper presents a novel approach for integrating a control objective into an aircraft design optimization at the conceptual design stage by proposing the use of the eigenvalues of the controllability Gramian to enhance the controllability of an aircraft. The individual eigenvalues of the controllability Gramian quantify the energy required to move along the corresponding eigenvector, which is observed to be dominated by one state-space variable. A higher eigenvalue implies lower energy requirements for controlling that eigenvector, thus the corresponding state-space variable. This approach does not rely on a control law design but on modifying the open-loop system such that it is easier to control. The approach is applied to conduct multidisciplinary design analysis and optimization (MDAO) of a composite flying-wing test aircraft. A previously developed MDAO framework is used to design the internal structural layout, using curvilinear spars and ribs (SpaRibs) and straight spars and ribs. It is found that the optimal control designs have up to 8% higher flutter speed with a 3.7% weight penalty. Moreover, the frequencies of the vibration mode shapes are higher for the optimal control designs. The active flutter suppression controllers are designed after the MDAO process, and it is found that the optimal control designs have significantly higher gain and phase margins. As a result, the optimal control designs yield higher robust closed-loop flutter speed as compared with the weight-minimized design.
Controllability Gramian as Control Design Objective in Aircraft Structural Design Optimization
AIAA Journal ; 58 , 7 ; 3199-3220
2020-06-02
22 pages
Article (Journal)
Electronic Resource
English