Control of Rotorcraft at Low Speed

Abstract

Nonlinearities, uncertainties, and disturbances challenge automatic control systems. For rotorcraft at low speed, nonlinearities can include reversals in control trim and effectiveness gradients with airspeed. These phenomena are uncertain as is the three-dimensional airflow field on which they depend. Furthermore, wind gusts and aircraft motion directly affect airflow. In this dissertation, a nonuniform rotor induced velocity model is adapted and applied, resulting in control gradient reversals and qualitative agreement with flight data. There has been little previous mention in the literature of cyclic effectiveness gradient reversal. Trim feedforward control and model reference adaptive control are developed and tested in simulation. Ultimately, incremental nonlinear dynamic inversion (INDI) control is applied in simulation and flight of a low disk loading tailsitter with a 2 m diameter rotor. The induced velocity model is used in simulation, providing consistency in controller behavior between simulation and flight. A significant result is that attitude control performance with the INDI controller is markedly better than with the existing controller. The INDI controller promises to make landing onto a small deck reliable in winds and gusts extending across the flight envelope, whereas the existing adaptive controller performs marginally in moderate but gusty wind.