Silent Wings has several autopilots that can be used in-flight while needing a break. They are also used for the auto-rudder function. The towpilot plane is flown completely by this autopilot system, and has to cope with whatever the pilot being towed does.
The autopilots are based on the PID-regulator. This is a very common regulator. There is basically 3 parameters for each regulator: Kp (Gain), Ti (Integrator time) and Td (Derivator time). Tuning a PID regulator is a science on it's own, so if you need to tune some of these try to proceed methodically without changing too much/many parameters at the same time.
Here are the entries from the autopilot.dat file from the Antares model, which contains the parameters that are needed by most models.
rudder_Kp = 3.0 rudder_Ti = 1.0 rudder_Td = 1.5
This is the auto-rudder autopilot that tries to keep the yaw-string centered.
rudder_heading_Kp = 0.1 rudder_heading_Ti = 5.0 rudder_heading_Td = 0.5
When flying very low, the auto-rudder tries to keep a straight heading instead of centering the yaw string. This is helpful when taking off in aerotow, and when landing.
auto_pitch_speed_Kp = 0.06 auto_pitch_speed_Ti = 3.0 auto_pitch_speed_Td = 0.8 auto_pitch_speed_maxpitch = 30.0 auto_pitch_speed_minpitch = -30.0
Auto_pitch_speed is the regulator keeping a constant glide speed, and it is enabled using ctrl-s. It is a controller that is controlling the nose pitch angle, not the elevator rudder. Notice the maxpitch and minpitch parameters: these are limits to the allowed pitch angles. Without these the autopilot might try to nose down 90 degrees or more to get up to speed quicker, which is not the best solution at all times.
apitch_Kp = 3.0 apitch_Ti = 3.0 apitch_Td = 0.05
The apitch regulator tries to keep a constant nose pitch angle using the elevator. When auto-speed is enabled, this regulator listens to the auto_pitch_speed regulator.
heading_Kp = 1.0 heading_Ti = 15.0 heading_Td = 10.0
This regulator is used to keep a constant heading.