Robust Aircraft Longitudinal Control Using Model-Error Control Synthesis Badr N. Alsuwaidan * and John L. Crassidis † University at Buffalo, State University of New York, Amherst, NY, 14260-4400 In this paper a robust control for aircraft longitudinal motion is presented. The Model- Error Control Synthesis (MECS) is developed for this application, which consists of a nominal controller with a model-error predictive filter. The control input is updated di- rectly using the estimated model-error from a predictive filter to cancel the unmodeled dynamics or disturbance inputs. The predictive filter is used only to estimate the model error, whereas an extended Kalman filter is used to provide state estimates from noisy measurements. The nominal control is chosen to be an LQR/dynamic inversion outer/in- ner loop controller for the longitudinal direction. The control law robustness to unmodeled dynamics and external disturbances is verified when MECS is active and not active. From various simulation results, MECS is shown to have better performance characteristics over the stand-alone nominal control. Nomenclature J Performance index f Residual value vector x Variable value vector F T Thrust Force, N m Mass, kg V Airspeed, m/s q Angular rate, rad/s θ Pitch angel, rad α Angle of attack, rad θ Pitch angel, rad δ e Elevator deflection, deg/s Superscript ∧ Estimated value ∼ Measured value I. Introduction T he need for robust controllers has challenged researchers for different types of uncertainties and appli- cations. Each uncertainty requires different types of robustness to be provided by the controller that may not be suitable for other uncertainties. For instance, robust control laws such as H ∞ and µ-synthesis controllers are designed to be less sensitive to external disturbances. 1, 2 Meanwhile, adaptive control, neural network and fuzzy logic approaches are intellectual controllers. 3 Adaptive control allows for on-line parame- ter modification, which handles parameter variations. Fuzzy logic and neural network use learning adaptive rules. Model-error control synthesis (MECS) is another type of robust control. 4 It modifies the nominal control input rather than the model itself. MECS is designed to handle the uncertainty associated with * Graduate Student, Department of Mechanical & Aerospace Engineering, bna2@buffalo.edu, Student Member AIAA. † Associate Professor, Department of Mechanical & Aerospace Engineering, johnc@buffalo.edu, Associate Fellow AIAA. 1 of 12 American Institute of Aeronautics and Astronautics