Flight-Test Determination of Longitudinal Stability Using System Identification

Abstract

In the present work, system identification methods were applied to the flight data from dynamic maneuvers in order to obtain an aerodynamic model of the longitudinal dynamics of a propeller-driven aircraft, including a model for the elevator hinge moment. As a result, the model allowed numerical computation of elevator deflections and stick forces required to trim the aircraft at different flight conditions, including variations of the center-of-gravity location. From these results, the stick-fixed and stick-free neutral points were computed. When compared to a conventional flight-test technique, the new method provided similar mean values for the neutral point location and reduced the associated uncertainty by about 50%. The data collected for system identification required only a fraction of the flight time used in conventional techniques. Moreover, the aerodynamic modeling revealed nonstandard dependencies, such as the linear influence of the airspeed and thrust coefficients on the aerodynamic coefficients. Therefore, propeller slipstream effects were automatically embedded into the neutral point results obtained.