title: Rotorcraft flight control for improved handling, loads reduction and envelope protection
Creators: name: Voskuijl, Mark
Publisher: name: University of Liverpool Repository
Year: 2023

description: The advent of fly-by-wire control systems has made it possible to design elaborate automatic flight control systems that can provide a whole range of functions from basic stabilisation to advanced envelope protection systems and autopilots. The research presented in this thesis is focused on the design of novel control systems for rotorcraft with the aim to improve handling' qualities, provide structural load alleviation and to provide envelope protection. The application of the control theory used for the design is considered to be novel. Methods used include nonlinear control and H:o control. Seven control laws are designed for three different rotorcraft types. They are designed first of all for the Bell 412 Advanced Systems Research Aircraft, a fly-by-wire research helicopter of the National Research Council in Canada. This aircraft is equipped with a full-authority simplex control architecture with single nonredundant set of fly-by-wire actuators, sensors and flight control computer and software. Inherent to the design philosophy of the Advanced Systems Research Aircraft's simplex architecture is a reliance on automated safety monitoring systems and a safety pilot to guard against system failure or operational flight envelope exceedance. Novel control laws can actually be flight tested on this aircraft. A highfidelity nonlinear simulation model of this aircraft was developed in the project for control law design. The other two aircraft for which control laws are designed are the XV-I5 tilt-rotor and the FLIGHTLAB Generic Rotorcraft, a nonlinear simulation model representative of the UH-60 Black Hawk. Nonlinear simulation models of the latter two were available at the start of the project. One of the key outcomes presented in this thesis is the design and flight test of a nonlinear control law for the Bell 412. The attitude quickness could be specified within a nonlinear element. High bandwidths were achieved in flight, which correlated very closely to the simulationbased predictions. Also, describing function analysis was successfully applied to analyse the stability of the control law. Other important results are the presentation of a new design method for structural load alleviation control laws, in combination with a newly proposed structural load severity scale that can be used for the analysis of these types of control laws. Flight envelope protection was investigated as well. A heave axis control law providing mast torque envelope protection was designed for the Bell 412. The design method used was the Boo loop shaping procedure in combination with a nonlinear outer loop. Stability problems with the inner loop were encountered during the first flight test with this control law. The control law was therefore automatically disengaged by the safety systems of the Advanced Systems. Research Aircraft. Piloted simulation suggested that the structure of the control law is useful, provided the inner loop tracks torque. Finally, a complete flight control system was built with additional outer loops in order to take on a challenging problem: the automatic landing of a helicopter on a ship using ''t-theory'. The variable 't is commonly used in optical flow theory ofvisual perception. It is essentially the time to contact given a certain speed and distance. Theory suggests that pilots use this variable for manual flight control. The height rate reference signal generated by the automatic landing system was based on the variable 't during the final landing phase (from hover above the deck until touch down). A successful automatic .landing was simulated in zero wind conditions. The height control in the final phase of the automatic landing seemed natural, as if a human pilot performed it. The 't-based height control loop is essentially a proportional controller with a time dependent proportional gain. This application of't-theory in control is considered to be novel. Keywords: Flight control, Flight test, Hoo control, Nonlinear control, Rotorcraft Modelling and Simulation, Envelope protection, Structural Load Alleviation, Handling Qualities, Shipboard landing, Bell 412 ASRA, FGR, XV-15.