Flight Mechanics and Performance of Direct Lift Control: Applying Control Allocation Methods to a Staggered Box-Wing Aircraft Configuration

The objective of the present dissertation is to show how redundant control surfaces can be exploited to shape an aircraft dynamic behavior and obtain desired flight mechanics performance. This is achieved by introducing novel approaches and methods for flight mechanics and control, mainly revolving around original implementations of traditional formulations of the Control Allocation (CA) problem. Control surfaces and, more in general, control effectors are defined as redundant if they are capable to independently control the same motion axis of the aircraft.

Redundant effectors can be linked together, and to the pilot input, in many ways according to different optimality criteria and/or performance objectives. In particular, the research presented in this dissertation focuses on the possibility to achieve Direct Lift Control (DLC). The latter is intended as the ability to use control effectors to alter the aircraft lift "without, or largely without, significant change in the aircraft incidence, and ideally is meant not to generate pitching moment."

The ability to do so is essentially dependent on the position of the Control Center of Pressure (CCoP), which is the center of pressure of aerodynamic forces solely due to control surface deflections. In case of a single control surface dedicated to DLC, the CCoP coincides with the control surface itself. In case of redundant control surfaces, their deflections can be coordinated to induce the position of the CCoP towards some preferred location, as allowed by the architecture of the aircraft and the available control effectiveness.

The first three chapters of the dissertation are dedicated to establishing the societal, scientific, and technical background underlying the subsequent research studies, including an overview of the CA problem for redundant control effectors. The following four chapters present, in this order: an evaluation of the mission performance of a staggered box-wing aircraft model designed for commercial transonic operations; a comparison of different CA methods on the design of an optimum control surface layout for a box-wing aircraft, with control surface both fore and aft the aircraft center of gravity; a trim problem formulation which employs forces and moments due to the aircraft control surfaces as decision variables, to maximize control authority, minimize aerodynamic drag or obtain a prescribed pitch angle; a CA-based formulation aimed at altering the characteristics of the transient response of an aircraft by exploiting the properties of the CCoP.

The conclusive chapter presents a comprehensive, top-level recap of the main aspects and topics covered within the dissertation. It reflects on the classic meaning of DLC, and what it means to achieve it with redundant control surfaces that are not expressly dedicated to it. With some considerations on the needs of aviation market, it speculates on the practical role of unconventional aircraft configurations in the near future. Lastly, it provides suggestions for improvements and future research studies.