Multidisciplinary design optimization of flight control system parameters in consideration of aeroelasticity
In the process chain of aircraft development, a detailed model of the flight control system (FCS) is considered only at a late phase. Many parameters as the number and dimensions of the control surfaces or their effectiveness are fixed or at least limited within certain bounds at that point. Thus, only a small design space is available for the FCS development. Structural mechanics, aerodynamics, load determination and flight control system are disciplines, which interact with each other and must therefore work closely together. Depending on the structural stiffness, it might be necessary to take aeroelastic coupling effects between aerodynamics and structural mechanics into account as well. In this regard, developing a FCS is a multidisciplinary task. The FCS parameters, or flight control system degrees of freedom (FCS-DOFs), must properly be determined from the design space limited by the various disciplines, to meet the demanded control power. The former can be ratios of control surface deflections for a respective flight envelope, the latter is given by the control surface configuration and may be quantified in terms of control moments or its built up rates. Particular attention must be paid to the allowable ranges of the FCS- DOFs in order not to violate FCS constraints given by stability or robustness. In a complex system as a FCS, minor changes to one parameter might have major changes on others. Therefore, for its development, usually a complex, iterative parameter design process must be carried out. The techniques of multidisciplinary design optimization (MDO) were developed to solve problems, where multiple disciplines are involved, various interactions amongst them occur and solutions must be found, which fit ranges of constraints. Handling the interactions usually results in a solution of best compromise between the involved disciplines. Thus MDO is a suitable approach to solve the formerly described problem of determining the parameters of FCS-DOFs. For a complex FCS, applying methods from MDO leads faster to a FCS design, that fulfills the constraints given by the FCS design engineer and other disciplines. Thus, MDO may enhance and improve the process chain of developing a FCS. The first basic step is the analysis of the engineering problem. Determining parameters of the FCS follows objectives and constraints. Hinge moments, occurring due to control surface deflections, commanded by the FCS lead to mechanical stresses in the structure, which must not exceed certain allowable values. Depending on the structural stiffness, aeroelastic effects might affect the mechanical problem as well. Minimizing the stresses can be formulated mathematically as a multidisciplinary optimization problem. There are general thoughts to be kept in mind, when applying MDO to the design of a FCS. When changes to the aerodynamics or the aircraft structure occur, the FCS design must be adapted as well. Therefore it must be designed flexible, i.e. the parameters of the FCS-DOFs must not be determined as fixed values but rather as ranges. As a result it is crucial to determine the sensitivity of changes in the FCS-DOFs to the behaviour of the FCS. Multiple simulations of the modelled FCS must therefore be carried out and analysed. Numerically, the solution of an optimization problem may depend on the applied optimization algorithm and its parameters. Thus, a sensible selection of the optimizer means better solutions of the FCS optimization problem. The FCS designed with MDO can then be compared with a FCS designed in a more manual way. Performance and development time are key parameters for this comparison. The FCS of a generic MALE UAV is modeled in SCADE Suite, a model-based development environment for embedded software. This model aircraft demonstrates the theoretical idea of designing a FCS using MDO for an example roll manoeuvre. The DOFs to be optimized are the spoiler and the aileron deflection, commanded by the FCS. It must be cleared how best to deflect the spoiler and/or the aileron for the respective roll manoeuvre. An appropriate optimization solver helps to determine a best suitable ratio between the spoiler and aileron deflections of the generic MALE UAV, with respect to objectives as e.g. minimizing structural stresses. Results of the optimization runs will be presented and discussed. Using optimization algorithms in order to determine parameters of degrees of freedom of the FCS can then be compared with the conventional procedure. Not only numerical values of the design, reached with the respective design methodology, but also the time of development need to be compared therefore.