Handling Qualities Optimization in Aircraft Conceptual Design

The handling qualities of an aircraft have always been a crucial field of study, being chiefly concerned with the safety and comfort of flight. Historically, the design-by-discipline approach has been used, thus relegating stability and control considerations to later stages of the design process. In recent years a new view emerged, which advocates the benefits of including handling qualities at earlier stages, such as conceptual and preliminary. The objective of the research is to develop a handling qualities optimization module to be fitted in the Initiator, a conceptual design tool developed at TU Delft. The module shall handle both unaugmented designs (bare airframe), as well as augmented design, in which a suitable stability augmentation system (SAS) is included. To this end, a common stability paradigm, based on a modified Routh-Hurwitz criterion, has been implemented as a set of nonlinear constraints on the design space. In essence, the criterion has been transformed from a test to a design procedure, taking the form of a general polynomial-based regional pole placement method. The methodology is concerned with conventional aircraft configurations, specifically by sizing the horizontal tailplane and positioning of the wing. Themodule is focused on designs that possess optimal short period damping ratio and Control Anticipation Parameter (CAP), while the objective functions to be minimized are tailplane induced drag, zero-lift drag, and weight. Static stability and controllability are ensured in the relevant flight regimes using Torenbeek’s X-plots, implemented in the form of additional constraints on the design space. Lastly, the stability augmentation system consists of a pitch damper and an angle of attack feedback. Two different methods have been investigated for the computation of the feedback gains: the first one entails the inclusion of such gains in the design vector. The second one makes use of an optimal control technique: the Linear Quadratic Regulator (LQR), augmenting the design vector with three required weighting factors. Hence, the module accepts three user inputs which specify the methodology: unaugmented design, augmented with method 1 (gains in the design vector) and augmented with method 2 (LQR). The optimization has been run for the three different user-specified methods on an Airbus A320-200. The results show that the procedure is successful, increasing the CAP by 100% for the unaugmented and augmented (method 1) designs. By using the LQR, the CAP showed an increase in 150 %. Moreover the unaugmented design achieved the required short period damping of 0.55, or a 34% increase with respect to the baseline value, while the augmented designs were capable of achieving a damping ratio of 0.76, which corresponds to an increase of 85 %. A convergence study has been done within the Initiator, to assess the performance of the optimized configurations concerning aerodynamic efficiency (L/Dmax) and maximum takeoff mass (MTOM). The investigation led to the definition of breakeven points, which are used to indicate the values of short period damping ratio for which, at a given CAP, the optimized designs have the same performance as the baseline aircraft. Two general design guidelines have been extrapolated: for unaugmented designs, the breakeven points move closer to the baseline values as the CAP increases. Furthermore, at the baseline short period damping ratio and CAP, the optimized configuration is more efficient. For augmented designs, it was revealed that the performance benefits achievable are substantially higher and independent of the required handling qualities. Hence, no breakeven points can be defined. Optimal values of L/Dmax andMTOMare achieved right from the baseline values of damping and CAP, while the gains are progressively increased to cope with the handling qualities demands. Due to the qualitative nature of these conclusions, it is deemed possible to extend them to other conventional configurations, thus providing general design guidelines.