Longitudinal System Identification for a Small Flying-Wing UAS

This paper focuses on identification of the bare-airframe, longitudinal dynamics of a small, flying-wing UAS, including both the short-period and phugoid modes. Servo dynamics from commanded to real control surface deflection angles are first identified, which is important for UAS equipped with low-bandwidth servos. A flight test procedure is developed for longitudinal system identification that includes the collection of trim data, elevator and throttle frequency sweep data, doublet data, and phugoid oscillation data. Throttle and elevator frequency sweep data is used to generate longitudinal frequency responses. Trim data is used to compute the longitudinal speed stability derivatives. Then, a multi-input, multi-output state space model is identified by holding the longitudinal speed stability derivatives constant and fitting the remaining parameters to the measured frequency responses. The identified model is shown to accurately represent the UAS response to elevator deflection and electric motor rotational speed. The identified phugoid mode dynamics show excellent agreement with results obtained from the time-history phugoid oscillation data. Finally, nondimensional stability and control derivatives and their confidence intervals are computed from the identified models.