**1. Introduction**

Ducted-fan aerial vehicles (DFAVs) have attracted much more interest of the academic and industrial communities worldwide due to their compact layout and high-security applica‐

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tions inseveraltasks suchas surveillance,data acquisition, andevenphysicalinteractionwithin unstructured environments. Among the different possible configurations, the vertical takeoff and landing (VTOL) ducted-fan aircraft is well-suited for a variety of missions in environ‐ ments cluttered with obstacles or close to infrastructures and humans. This fact is motivated mainlybytheducted-fanconfigurationinwhichthepropellerisprotectedbyanannularfuselage. Inaddition,aprominentadvantageofducted-fansystemisbetteroverallefficiencyatlowspeeds [1]. In this respect, also inspired by the previous works considering test-fly methods in control [2, 3], the aircraft considered here is a tandem ducted-fan vehicle configuration characterized by a very simple mechanical structure, composed only of two tandem contra-rotating propel‐ lers inside the vehicle's fuselage, a number of control vanes which deviate the propeller's air flow in order to obtain full controllability of the attitude dynamics (see also [1, 4]) and a set of auxiliary "direct force control" with small lateral electric ducted fans (EDFs).

Drawing inspiration from the potential of the well-designed VTOL aircraft, the focus of this chapter is on the systematic modeling, flight control development, and implementation methods of the aerial vehicle named BIT-TDF at Beijing Institute of Technology (BIT). A number of contribution focuses on the problems of feedback control design for such a class of systems. In [5], a dynamic inversion controller is proposed to govern the nonlinear dynamics of a ducted-fan aircraft. In [6], a framework of nonlinear robust control based on a path following strategy is applied to a ducted-fan miniature UAV . A structured two-loop feedback controller combined with a static anti-windup compensator is proposed in [3] for a ducted-fan aircraft. However, few research laboratories are carrying out advanced theoretical and experimental works on the system; among others, to mention a few, the European Community under the 7th Framework Programme through collaborative projects AIRobots [7], the HoverEye project of autonomous ducted-fan aircraft [8], and the Martin Jetpack [9].

To actually show the potentials in a real application scenario, the overall system design of a fully functional UAV has to be validated experimentally using a real setup, especially the proposed control techniques. The object is to consider a robust flight control design for our small-scaled ducted-fan aircraft. The proposed methods have been tested either in simulation, experimental, or both frameworks where the implementation has been carried out using the ducted-fan UAV known as BIT-TDF.

Throughout the overall development of the UAV, deriving a high-fidelity nonlinear model has been a challenging issue due to their inherent instability and large amount of high-complexity aerodynamic parameters. After the hardware construction of the ducted-fan prototype, we first obtain a comprehensive flight dynamics model based on an elaborated approach which integrates first-principle and system identification techniques. The frequency-domain identi‐ fication tool CIFER [10], which was developed by army/NASA Rotorcraft Division and is one of today's standard tools for identifying models of different aircraft configurations, has been used here to capture the key dynamics. With the identified model in hand, we then carry out to design a flight control system with two-loop architecture, in which an inner loop is for stabilization and decoupling the UAV dynamics and an outer loop is for desired velocity tracking performance. Specifically, we have combined (1) *H*<sup>∞</sup> technique; (2) nonsmooth optimization algorithm; and (3) custom-defined gain-scheduling to design a nonlinear flight control law and successfully realized the automatic flight test.
