Adaptive gust alleviation for a tilt-rotor UAV operating in turbulent airwakes

Vertical Takeoff and Landing (VTOL) UAVs are particularly susceptible to aerodynamic disturbances when operating in environments with unsteady turbulent airwakes. The vehicle's autonomous flight control system must compensate for the disturbances due to the unsteady flowfield. This paper investigates an adaptive gust alleviation control law designed to improve command tracking and disturbance rejection when operating in an airwake. The system is tested using an advanced simulation environment for a ship-based tilt-rotor UAV. The study builds off previous research on airwake compensation control laws designed for full-scale aircraft operating in a shipboard environment. The unsteady component of the ship airwake is treated as a stochastic process similar to the von Karman model of atmospheric turbulence. A compensator is tuned to reject these disturbances using H₂/H_∞ control synthesis and is incorporated into a model following control system. An adaptive control system is developed in which the spectral properties of the airwake are identified and the compensator is synthesized on-line as the vehicle operates in the airwake. Two different identification methods are evaluated: an autoregressive model approach and an on-line FFT algorithm with least squares power spectrum fitting. The controller was embedded into a fully autonomous trajectory control system, and the adaptive airwake compensator was found to greatly improve trajectory tracking when operating within the airwake while not significantly increasing control activity. While the adaptive airwake compensator is designed to alleviate the unsteady disturbances due to the airwake, an online learning algorithm is also being developed that identifies the more deterministic time-averaged characteristics of the flow field. Initial feasibility studies of this developing algorithm have already been conducted in simulation.