This class is the autopilot base model, which the autopilot models designed for different aircraft types extend. This autopilot model translates the commands that are used to define a reference flight trajectory, namely altitude, total velocity and track angle, into control actuator commands for an aircraft with conventional wing configuration, which includes elevator, ailerons and rudder deflection angles, as well as a command for throttle position. The autopilot architecture is shown in Figure 1 with its six PID controllers and surrounding models for actuator and aircraft dynamics.

Figure 1: Autopilot architecture. [1]

The altitude is controlled by the elevator deflection (δ_e) with two LimPID controllers in series, of which the first outputs a reference pitch angle (θ_ref) based on the error between the measured and reference altitudes, and the second outputs the δ_e based on the error between the measured and reference pitch angles. Similarly, the track angle is controlled by the ailerons deflection (δ_a)  with two LimPID controllers in series, of which the first outputs a reference roll angle (φ_ref) for the second, which outputs the δ_a. The reference pitch and roll angles can be constrained by the maxTheta and maxPhi parameters, respectively.

The total velocity is controlled by the throttle position (δ_T) with one LimPID controller. The rudder deflection (δ_r) is used to control the sideslip angle (β) from a constant zero signal as the reference sideslip angle, i.e. the rudder is only used to minimize the sideslip angle in order to reduce drag and lateral accelerations.

This autopilot model can also be used to partially control only the selected actuators by disabling control of the other actuators by the elvControl, throtControl, ailControl and rdrControl parameters in the Control Settings tab. All control gains of the LimPID controllers are gathered to the Control Gains parameter tab.

The measured altitude, total velocity and track, pitch, roll and sideslip angles are inputs to this model through the FlightDataIn connector.

References

[1]  Erä-Esko, N. (2022). "Development and Use of System Modeler 6DOF Flight Mechanics Model in Aircraft Conceptual Design."
      Available at: modelica://Aircraft/Resources/Documents/EraeEskoThesis.pdf.