## The autopilot simulation models

The systems APILOT1, APILOT2, and APILOT3 contain the simulation models of the 'Beaver' autopilot, coupled to the non-linear model of the 'Beaver' itself. The three versions of the autopilot simulation systems differ in their complexity and computation speed. Other closed-loop models are the systems PAH, RAH, and PAHRAH, which contain implementations of the Pitch and/or Roll Attitude Hold modes only (neglecting the influence of signal limiters).

For more information, see the references below and the help-texts for the subsystems Actuator & cable dynamics, Sensors / Add initial inputs, Symmetrical autopilot modes, Asymmetrical autopilot modes, and Mode Controller. Details about the autopilot initialization can be found in the help-texts for the routines APINIT and APMODE. See the descriptions of the blocks ILS and VOR for more information about the radio-navigation blocks.

See also the color definitions for the FDC toolbox.

### APILOT1

This system contains the basic autopilot modes:

- Pitch Attitude Hold
- Altitude Hold
- Altitude Select
- Roll Attitude Hold
- Heading Hold / Heading select

For reasons of computing speed, it does not contain:

- approach and navigation modes
- mode-switching loops for approach and navigation modes
- ILS and VOR blocks
- any noisy disturbances (sensors, atmospheric turbulence)
- a Moving Average filter for the Altitude Hold mode

APILOT1 can be used for fast evaluation of the basic autopilot results, using the ODE15s integrator, which is quite efficient for systems with relatively smooth signals (i.e. few discontinuities, or none at all). In that case, the simulations will be very fast, and the number of timesteps will be limited.

### APILOT2

This system contains all control laws of the 'Beaver' autopilot:

- Pitch Attitude Hold
- Altitude Hold
- Altitude Select
- Roll Attitude Hold
- Heading Hold / Heading Select
- longitudinal and lateral approach modes
- VOR-navigation mode

Due to the inclusion of the approach and navigation modes, it also contains:

- mode-switching loops for approach and NAV (call M-files with mode-switching criteria, which slows down simulations)
- ILS and VOR blocks

However, for reasons of computing speed, APILOT2 does not contain:

- any noisy disturbances (sensors, ILS noise, turbulence)
- a Moving Average filter for the Altitude Hold mode

APILOT2 simulations are slower than those of APILOT1 due to the additional blocks. ODE15s is still a suitable solver for this system, because it doesn't contain any noisy disturbances or sharp discontinuities.

### APILOT3

This system is equal to APILOT2, but it also includes:

- ILS noise in the block ILS
- a Moving Average filter for the Altitude Hold mode
- atmospheric turbulence

Due to the noisy signals, the time steps are all equal to their minimum value, which means very slow simulations (unless you have a very fast PC or workstation). ODE45 is a suitable solver for this system.

### PAH, RAH, and PAHRAH

These systems contain implementations of the Pitch Attitude Hold mode (excluding the turn-compensation loops and neglecting any signal limiters), the Roll Attitude Hold mode (including the turn-coordination loops, but neglecting the signal limiters), and a combination of both, respectively.

Compared to the APILOT systems, the appearance of these PAH/RAH models is more similar to the representation of typical control systems in textbooks and the figures from the FDC manual. The main advantage of this appearance is that these models are a little bit easier to understand; the main disadvantage is that it is much harder to enhance them with outer-loop controllers. The APILOT systems are more similar to the control law representation in the Flight Control Computer of the aircraft.

In general, simulations of these PAH/RAH models will be quite fast, as these models do not include any noisy disturbances or sharp discontinuities, and they do not contain any outer-loop blocks. ODE15s is a suitable solver for these systems.

### References

For a detailed description of the 'Beaver' autopilot, refer to the FDC user-manual. More information can be found in the following references:

1. M.O. Rauw: A Simulink environment for Flight Dynamics and Control analysis - Application to the DHC-2 'Beaver' (PART II). MSc-thesis, Delft University of Technology, Faculty of Aerospace Engineering, Delft, The Netherlands, 1993.
2. P.N.H. Wever: Ontwerp en implementatie van de regelwetten van de De Havilland DHC-2 'Beaver' (in Dutch). MSc-thesis, Delft University of Technology, Faculty of Aerospace Engineering, Delft, The Netherlands, 1993.