Flight control systems are obviously very important for vehicles, such as aircrafts, helicopters, satellites, launch vehicles, missiles, hypersonic vehicles, airships, etc. Flight control systems play a crucial role in the stability augmentation of such flight vehicles. In the past, because most flight vehicles were assumed to be a rigid body, the flight control systems were generally transformed into linear systems by linearization. Consequently, traditional control methods proposed for flight control systems were mainly concerned with PID or gain scheduling-based techniques.
With the higher requirements placed on aeronautical and space systems, in both transient and steady state phases, the required control performances expected from the flight control system has become more demanding. In this case, the PID-based algorithms are not satisfactory. In fact, with the expanded flight envelope of modern vehicles, the dynamics of such vehicles are inherently nonlinear. These nonlinearities include couplings, uncertainties, unmodeled dynamics, etc. The performance of the flight control systems to some extent is dominated by the aforementioned nonlinearities. If the nonlinearities are not very prominent, PID-based algorithms may be still sufficient. However, when the nonlinearities are prominent, for example, when flight vehicles perform a large aggressive maneuver, the nonlinearities will play a dominant role in the dynamics and, in turn, affects the performance of the flight control system. Under these circumstances, robust nonlinear control algorithms are necessary. The objective of this special issue is to bring attention to the latest advancements in flight control systems. With this goal in mind, we have invited several well-known researchers to present their recent research results in robust and nonlinear controls. The special issue consists of six papers that cover several different flight control systems.
Garcia and Keshmiri  consider the safe control of the Meridian unmanned aerial system with abnormal conditions. A novel adaptive nonlinear model predictive controller is proposed and conceptually proven to ensure safe control of the Meridian unmanned aerial system in off-nominal conditions. Controller performance is improved by updating the physics-based model by using real-time nonlinear estimation of aerodynamic forces. The authors also show that the proposed predictive controller coupled with real-time parameter identification, exhibits robust characteristics and successfully mitigates the impact of nonlinear and unsteady aerodynamics while preventing loss of control.
The guidance control problem for a small unmanned aerial vehicle (UAV) is proposed in Liu, McAree, and Chen  so that the UAV will perform path-following under wind disturbances. A disturbance observer-based control approach is adopted. The wind information is first estimated by a nonlinear disturbance observer; then, it is incorporated into the nominal path-following controller to formulate a composite controller that is able to compensate wind influences. An initial flight experiment is performed, and some promising results are obtained.
The paper of Xia, Lu, Zhu, and Fu  considers the attitude control of a quaternion missile model, which is nonlinear in aerodynamics with atmospheric moment uncertainties, inertia uncertainties, bounded disturbances, and actuator failures. By integrating the back-stepping technique and sliding mode method, a robust sliding mode control algorithm is proposed to guarantee the state variables of the closed loop system to converge to a small region of the reference state with the help of the adaptive law by estimating the total uncertainties.
In the paper of Zong, Wang, and Tao , the problem of tracking control for a longitudinal air-breathing hypersonic vehicle (AHV) model with flexible effects and intricate couplings is investigated. To overcome the analytical intractability, a control-oriented model is constructed for the purpose of feedback control design. The high-order dynamic sliding mode control is proposed to force the velocity and altitude of the flexible AHV to the desired reference commands in finite time. The control method does not require to know the upper bounds of uncertainties.
A finite-time fault tolerant attitude stabilization method is developed by Hu, Huo, and Xiao  for a rigid spacecraft whose redundant actuators are mounted to the reliability of the attitude control system. Also based on the sliding mode technique, an adaptive fault tolerant controller is derived with uncertain inertia parameters, actuator faults, and external disturbances explicitly addressed. The key contribution of the method lies in that the design of the fault tolerant control does not require a fault detection and isolation mechanism to detect, separate, and identify the actuator faults.
The same problem mentioned previously is also discussed in Shen, Jiang, and Cocquempot , where the fault-tolerant control problem for a class of uncertain nonlinear systems with actuator faults is considered. By using the fuzzy control technique and the backstepping approach, an observer-based fault-tolerant control scheme is developed. The advantage of the method is to remove the classical assumption that the time derivative of the output error should be known.
As can be seen from the aforementioned text, these papers all focus on the nonlinear control theory with application to flight control systems. They have provided solutions for improving the performances of flight vehicles. Although some new results are proposed in this special issue, they can not cover all the control problems for flight control systems, and more innovative control methods should be further studied. We hope that this collection of papers will bring more attention to flight control systems.