Active Spanwise Lift Control: A Distributed Parameter Approach

Abstract

Structural load alleviation has been a very active research topic since the 1950s for many reasons. By mitigating the effect of gusts on the wing, the maximum loads can be effectively reduced. This capability would lead to substantial benefits, such as reduced structural weight, better fuel burn performance, and improved passenger ride comfort. Instead of controlling the structural response, however, it can be argued that the aerodynamic behavior of the wing should be primarily controlled. Since the gust loads are caused by disturbances in the lift distribution, it is possible to mitigate the gust loads by controlling the shape of the lift distribution profile along the span. In contrast to previous approaches, this research builds on concepts from Distributed Parameter Systems (DPS), which is indeed the case of aerodynamic surfaces. The unsteady aerodynamic behavior of the 3D flow around a wing is modeled using two approaches: the Unsteady Lifting-Line Theory (ULLT) and the Unsteady Vortex-Lattice Method (UVLM). Then, modal identification techniques are used to identify spanwise aerodynamic mode shapes in terms of local lift coefficient along the span. These shapes provide an optimal basis for model order reduction and also for spatial control. The lift distribution is decomposed as a linear superposition of these shapes, with each weighted by a shape coefficient. By controlling a set of shape coefficients, the overall lift profile can be effectively controlled. In this work, the shape control problem is addressed using a Linear Quadratic Tracker to dynamically follow any desired reference lift profile. The gust alleviation problem is investigated using a similar controller with a special observer, able to decouple the state estimation from the gust input.