Determination of nonlinear aerodynamic coefficients using the estimation-before-modeling method

Abstract

Aerodynamic coefficients of a Schweizer 2-32 sailplane have been estimated from flight test data for angles of attack (α) up to 30 deg and sideslip angles (β) to ±17 deg. The nonlinear aerodynamic model has been identified by applying the estimation-before-modeling (EBM) technique to flight data derived from 15 maneuvers including 7 stalls and “poststall gyrations.” An extended Kalman-Bucy filter and a modified Bryson-Frazier smoother were used to estimate the time histories of the forces and moments from a 14 element discrete measurement vector. The optimal estimates and the measured control variables were sorted into 50 “subspaces,” and the aerodynamic modeling was performed using a multiple regression scheme in each subspace. The linear and nonlinear portions of the coefficients could be observed from the modeling results, providing insight into the stall dynamic behavior of the sailplane. The accuracy of the aerodynamic coefficients was assessed by comparing measured time histories of the sailplane with a six-degree-of-freedom simulation model response. A novel aspect of the research is the use of a quaternion-based mathematical model of aircraft dynamics. A microprocessor-based data acquisition system was used onboard the sailplane. © American Institute of Aeronautics and Astronautics, Inc., 1988. All rights reserved.