An Integrated Flexible Aircraft Model for Optimal Control Surface Scheduling of Manoeuvre Load Alleviation and Wing Shape Control Functions

Kurzfassung

In the field of structural loads analysis, the focus is on accurate modelling of the lift forces, as they are the main driver of the structural sizing loads. Hence, in many aeroelastic implementations, forces acting in the longitudinal direction of the airframe are neglected. However, for flight mechanical assessments and cruise performance, the forces in the direction of the flow are essential. This issue can be addressed by either using higher fidelity methods or by extension of the potential flow based methods to account for longitudinal forces such as the induced drag. A common practice is to pass the deformations of the structural model to the aerodynamic code and vice versa the pressure loading is passed to the structural code, which in turn is used to solve the flight dynamics equations of motion followed by a aircraft trimming procedure. This paper proposes an extension of the Vortex Lattice Method (VLM) implementation used in classical loads analysis. The resulting closed-form integral model allows for a fast execution without tedious iterations between the interfacing disciplinary codes. The present implementation of the VLM accounts for the inherently nonlinear behavior of the induced drag and the dependence on the on-flow direction, while preserving the Aerodynamic Influence Coefficients and boundary conditions in matrix form, compatible with classical formulations. The integral aircraft model is then used for wing shape control by optimizing the control surface scheduling of the trimmed, flexible aircraft to minimize the induced drag in off-design flight conditions. Another important objective is to minimize the wing bending moments during pull up manoeuvres to reduce the sizing loads and the resulting structural weight of the wing. These optimized control surfaces deflections constitute the allocation for the Manoeuvre Load Alleviation (MLA) control function. Furthermore, the inclusion of induced drag allows also for a better representation of the characteristics of the flight dynamics such as the phugoid motion, which in turn affects the design of the baseline flight controller. The optimizations of the control surface deflections regarding wing shape and manoeuvre load alleviation control will be exemplified with a model of the FLEXOP aircraft, a high aspect ratio subscale demonstrator equipped with multiple trailing edge control surfaces.