An Integrated Flexible Aircraft Model for Optimization of Lift Distributions

This paper summarizes an integrated modelling approach for controlled flexible aircraft, applicable for flight loads and flight dynamics analyses. It is particularly suited when numerous repeated design analyses throughout the different design phases of an aircraft development need to be considered. An underrepresented aspect so far is the utilization of active control systems as design parameter already in the early design stages and not merely as mitigation strategy for emerging design deficiencies. An active control system can be beneficial for the reduction of structural weight by the use of load alleviation functions, as well as for improvement of the cruise performance and therefore reducing the fuel consumption. This achieved by appropriately optimizing the lift distribution for different use cases. Regarding the lift distributions with respect to loads analysis: the wing structural weight is driven mainly by the loads it has to sustain. One of the most important load conditions is the 2.5 g vertical pull up manoeuvre. A Manoeuvre Load Alleviation function shifts the center of lift inboard by deflection of the available control surfaces and hence reduces the wing root bending moment, which is in turn the primary driver for structural weight. Another very important load condition to be considered is the Discrete Tuned Gust, which might also be a critical case sizing the wing. An active gust load alleviation (GLA) system needs to redistribute the lift and damp out the structural modes excited by the gust by actively actuating the available control surfaces without introducing undesirable excitation of structural modes by the system itself. In the case of cruise performance, the induced drag depends on the spanwise lift distribution. Due to flexiblity, the wing shape changes during the mission. Active lift adaption by control surface deflections can help to reduce the induced drag. The focus of this paper is on assessing the applicability of the proposed modelling approach for the described use cases.