Investigation of Practical Flight Control Systems for Small Aircraft

Abstract

Personal air transportation utilizing small aircraft is a market that is expected to grow significantly in the near future. However, seventy times more accidents occur in this segment as compared with the commercial aviation sector. The majority of these accidents is related to handling and control problems. In commercial aviation, Fly-By-Wire (FBW) technology is used to prevent these types of accidents. Instead of downscaling advanced and high-cost FBW platforms, a low-cost solution should be considered for the general aviation market. In the European project “Small Aircraft Future Avionics Architecture”, a FBW platform is developed specifically for small aircraft. In this environment, Flight Control Law (FCL) designs are needed that have robustness against model uncertainties, sensor bias, sensor noise and time delays, while being fast and accurate enough to accommodate the relatively agile dynamics of a small aircraft. FCL designs that meet these requirements are called practical FCL designs in this thesis. Based on a dynamic model of a Diamond DA 42 and a description of the dynamic properties of the FBW platform, two different FCL designs are synthesized and analyzed in this thesis. The first design uses classical control theory and the second design uses a newly developed nonlinear design method, based on backstepping, singular perturbation theory and approximate dynamic inversion. This latter method, called Sensor-Based Backstepping (SBB), uses no dynamic model information and relies solely on measurements. Both FCL designs are compared on sensitivity to parametric uncertainty, sensor noise, disturbances, time delays, handling qualities, design effort, certifiably and the option to add flight envelope protection. In the scope of this thesis, SBB is selected as the preferred FCL design. This method produces good aircraft responses without knowing the exact dynamic behavior of the aircraft during FCL synthesis, as long as the system is minimum phase, controllable and sufficiently time-scale separated.