Nonlinear Flight Regimes Research Articles

Nonlinear Flight Mode Identification by Applying Modified Ensemble Empirical Mode Decomposition and Hilbert–Huang Transformation

Identification of the “aircraft dynamic modes” is one of the researchers’ favorite subjects in the flight mechanics field to study the behavior of airplane in maneuvers and nonlinear regime. Technically, by measuring the flight test parameters, the flight dynamic modes could be estimated. Although several approaches calculate precisely the linear flight dynamic modes, there is not a known standard procedure to calculate the nonlinear flight dynamic modes. In this paper, identification of the flight dynamic modes in highly nonlinear maneuver conditions and particularly in the coupled dynamic spin maneuvers is investigated. A new approach based on Hilbert–Huang Transform (HTT) for identification of the flight dynamic modes is presented in this paper. To overcome the mode mixing problem during decomposition which leads to imprecise results in the estimation of the flight dynamic modes, a Modified Ensemble Empirical Mode Decomposition (MEEMD) algorithm for processing of the complex signals that originate from FDR is presented in this research. This new algorithm is able to make a precise reconstruction of the original signal and it performs the task of signal decomposition with fewer iterations and so with less complexity order rather than the competitor approaches. An innovative algorithm for calculating flight dynamic modes natural frequencies and the damping ratios which uses the IMFs produced by MEEMD is presented. The obtained approach was applied to flight data, which belong to spin maneuvers. The results show the correct performance of the presented technique in the nonlinear flight regime.

Adaptive Control of Advanced Fighter Aircraft in Nonlinear Flight Regimes

When advanced fighter aircraft fly at high angles of attack, unsteady aerodynamic effects, wing rock, and saturation of aerodynamic effectors can lead to difficulty in control and maneuverability. A novel adaptive output feedback control design based on dynamic inversion is investigated for aircraft which are operated in highly nonlinear flight regimes, where uncertainties in the form of both unmodeled parameter variations and unmodeled dynamics are common. The stability of the design is analyzed and validated with simulations using a modified NASA F-15 simulation. The simulation includes thrust vectoring and a differential stabiliator to provide increased control authority at high angles of attack and relaxed static stability to increase pitch maneuverability. The control designs include the use of pseudocontrol hedging techniques to exclude adaptation to control saturation.

Simulation of a Controlled Airfoil with Jets

Numerical simulations of a two-dimensional airfoil, controlled by an applied moment in pitch and an airfoil controlled by jets, were investigated. These simulations couple the Reynolds-averaged Navier-Stokes equations and Euler''s equations of rigid body motion, with an active control system. Controllers for both systems were designed to track altitude commands and were evaluated by simulating a closed-loop altitude step response using the coupled system. The airfoil controlled by a pitching moment used an optimal state feedback controller. A closed-loop simulation, of the airfoil with an applied moment, showed that the trajectories compared very well with quasi-steady aerodynamic theory, providing a measure of validation. The airfoil with jets used a controller designed by robust control methods. A linear plant model for this system was identified using open-loop data generated by the nonlinear coupled system. A closed-loop simulation of the airfoil with jets, showed good tracking of an altitude command. This simulation also showed oscillations in the control input as a result of dynamics not accounted for in the control design. This research work demonstrates how computational fluid dynamics, coupled with rigid body dynamics, and a control law can be used to prototype control systems in problematic nonlinear flight regimes.

Nonlinear adaptive control for high angle of attack flight

This paper presents a nonlinear methodology for the control of a high angle of attack aircraft, in particular, a modified F-18 aircraft. As a modern combat aircraft demands better maneuverability and performance over domains which include high angles of attack, research in high angle of attack is presently at an advanced stage. An adaptive controller is developed to maneuver an aircraft at a high angle of attack even if the aircraft is required to fly over a highly nonlinear flight regime. The adaptive, controller presented in this paper is based on nonlinear prediction models, and can be constructed to minimize the given cost function or the difference of a described Lyapunov function with respect to the control input at each step. A controller uses system identification parameters to calculate a command signal so that the output of system follows the reference trajectory. The control is calculated to let system follow the reference trajectory under some constraints. This paper shows that nonlinear adaptive control can be utilized effectively to control high performance aircraft such as the F-18 aircraft for rapid maneuvers with large changes in angle of attack.

High-alpha application of variable-gain output feedback control

This paper describes a recently developed variable-gain, optimal, discrete, output feedback design approach that is applied in a nonlinear flight regime. The flight regime covers a wide angle-of-attack a range that includes the stall region. The paper contains brief descriptions of the variable-gain formulation, the discrete-control structure and flight equations used to apply the design approach, and the high-performance airplane model used in the application. Both linear and nonlinear analyses are shown for a longitudinal four-model integrated design case with a = 5, 15, 35, and 60 deg and two flight control modes. Nonlinear simulations show good time response for both a longitudinal pitch-up and pitch-down maneuver and a regulation within 5 deg from the nominal angle during a high-ex lateral maneuver.