Parameter Estimation of Short-period Low Order Equivalent System for Fighter Aircraft Under Extreme Flight Conditions

This paper presents the estimation of Low Order Equivalent System (LOES) parameters for the longitudinal short-period mode of a fighter aircraft, using high-fidelity nonlinear simulation data of the F-16. The complex flight dynamics of such aircraft, effectively a High Order System (HOS) due to the presence of flight controls and component dynamics, require simplified equivalent systems for evaluating flying qualities against military standards. This study focuses on estimating LOES short period parameters using nonlinear flight simulation data under extreme flight conditions defined by high angles of attack up to 30 degrees. LOES parameters were identified through a hybrid approach combining time-domain regression and frequency-domain parameter estimation using a Maximum Likelihood Estimation (MLE) algorithm, resulting accurate identifications characterized by low standard deviations. Model validation confirms that LOES output closely matches HOS nonlinear data using inputs in the time domain, with a coefficient of determination R² metric above 77% and fit error below 0.2. Additionally, the LOES model effectively represents nonlinear system responses in the frequency domain, yielding relatively low MIL cost and maintaining mismatch errors within the Maximum Unnoticeable Added Dynamics (MUAD) boundary.