Rotor speed control for a demonstrator helicopter engine

Since military helicopters of today must operate in hostile environments and carry out intricate maneuvers, it is required that their engine and rotor systems be under automatic control. During early concept testing of a demonstrator engine for such applications, a simple controller is needed to maintain a fixed power turbine speed and insure the general stability of the engine and rotor. A mathematical model for the helicopter plant is a necessary first step in the design of a controller suitable for demonstration testing. In this thesis a linearized model of the plant was developed by expanding the governing equations about a quiescent operating point. This linear model was then analyzed in both the frequency domain (Bode) and the time domain to determine the plant's characteristics. Special attention was focused on the two major destabilizing effects for an engine and rotor system, which occur at the rotor and power turbine natural frequency and upon autorotation. Controller design goals were selected to include a phase margin in excess of 45°, gain margin of 6 dB, and steady-state accuracy within 15%. The uncompensated plant was analyzed first and different types of simple compensation were selected to fulfill the design goals. A second order notch filter was added to the controller to satisfy the torsional stability requirements without compromising the rotor speed governor's transient response. Good system performance was achieved with a bandwidth, evaluated at the gain crossover frequency, of 3 rad/sec (0.48 Hz). The simple controller designed here provides a suitable governor for the demonstrator engine and forms a baseline for continuing studies involving sophisticated control techniques applied to more advanced helicopter systems.