Autopilot

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The autopilot setting dialog of FlightGear.

An autopilot (AP) is a mechanical, electrical, or hydraulic system used to guide a vehicle without assistance from a human being. Most people understand an autopilot to refer specifically to aircraft, but self-steering gear for ships, boats, space craft and missiles is often also called by this term.

The autopilot of an aircraft is sometimes referred to as "George".

Autopilot in FlightGear

We have a generic one and custom ones written for a certain aircraft, which are supposed to simulate particular autopilot types/models for a specific aircraft.

The generic one can be used for aircraft which don't have a custom autopilot implementation, and seems to work reasonably well on most aircraft using JSBSim. You can enable it with the "Autopilot Settings" which you can open using the F11-key or in the menu bar using the "Autopilot" item.

In real life, aircraft have specifically-programmed/customized autopilot system to cater for individual aircraft characteristics. To a certain extent, Flightgear has the possibility to simulate this as well.

So it is possible to write a custom autopilot.xml to override the generic autopilot implementation. This custom autopilot configuration file can be driven by either the standard autopilot dialog, custom cockpit panel hotspots or a combination of both. Also, it is possible to provide enhanced or completely new implementations of the standard autopilot dialog to cater for autopilot/aircraft-specific features (for examples, see the b1900d or Citation Bravo in Git/HEAD).

A custom Navomatic 400AP
A custom autopilot dialog

Some aircraft in FlightGear however only provide one means of interacting with the autopilot (i.e. autopilot dialog or panel hotspots). For example, the Seneca II and the Piper Comanche autopilots can be only used from the virtual cockpit using custom panel hotspots. This is in fact increasingly the case, as new emulations of autopilot systems are implemented using the built-in scripting language Nasal which provides for much more flexible system design and functionality, compared to the relatively static way of describing autopilot systems and their relevant and modes using only the XML-configurable autopilot system. In fact, to properly emulate more complex autoflight systems that provide support for more abstract flight modes, using a combination of XML-configurable PID controllers and Nasal is likely to be the most promising approach.

In general, it can be considered to be more complex to provide proper cockpit panel implementations than wiring up the autopilot properties to the standard autopilot GUI dialog, so if an aircraft's autopilot doesn't seem to work properly using cockpit panel hotspots, you may want to try using the standard GUI dialog instead.

Also, please note that not all aircraft/autopilot combinations provide full support for all features offered by the standard autopilot dialog. This may be due to an aircraft's completion status, but also due to technical limitations in certain aircraft/autopilot, so that only certain modes are provided. This may also apply to aircraft panels, that are seemingly offering functionality that may not yet be implemented.

In addition, JSBSim also features support for a standalone autopilot implementation, that works without any dependency to FlightGear/Nasal.

For further information about programing autopilots in FlightGear look at Autopilot Tuning Resources and http://www.flightgear.org/Docs/XMLAutopilot/.

Autopilot Settings

The AP Settings dialog can be found at Autopilot > Autopilot Settings.

Heading control

  • Wings Level: enable this to keep your plane horizontal- usually used at Go-Arounds.
  • Heading Bug: located on the compass rose of your instrument panel is a movable heading bug, the purple triangles that are pointing at your runways heading (283 for KSFO default). This bug is moved around the rose by setting the Heading Bug. The heading bug can be used several ways. When hand flying the aircraft turn the bug to your desired heading. This way you will have a constant visual reminder. If Air Traffic Control gives you a new heading move the heading bug to the new heading and you have your visual reminder.
  • True Heading: your true heading as shown on the compass (of the HUD) and MP Map.
  • NAV1 CDI Course: used for VOR-by VOR flying like in old days and used with ILS.

Velocity control

  • Speed with Throttle: speed will be regulated to the selected speed by controlling throttle.
  • Speed with Pitch: your plane will be pushed down or up to reach the selected speed. This can not be used during takeoff, landing or low altitude flights. Doing so could cause a crash.

Pitch/Altitude control

  • Vertical Speed: set the speed of vertical climb. Usually used as "feet per minute"
  • Pitch Hold: pitch degrees of your plane. Use a negative number to lower your nose.
  • AoA Hold: The Ange of Attack describes the angle of the wings compared to the direction of the circumfluent air. The lift of the wing depends on the AoA. A too high AoA will cause the aircraft to stall.
  • Altitude Hold: the desired altitude of your plane in feet (FL1=100 ft, FL25=2500 ft etc.).
  • AGL Hold: The Altitude about Ground Level. Usually the altitude is given about sea level.
  • NAV1 Glideslope: The vertical slope which led the aircraft from the interception point down to Runway level-used only with ILS.

FlightDirector Mode

The flight director computes and displays the proper pitch and bank angles required in order for the aircraft to follow a selected path. A simple example: the aircraft is in level flight on a heading of 045 degree and at an altitude of 15000 feet maintaining a speed of 260 kts, the FD bars are thus centered. Then the flight director is set to a new heading of 090 degrees and a new altitude of 20000 feet. The aircraft must thus turn to the right and climb. This is done by rolling to the right and pulling up. The roll bar will deflect to the right and the pitch bar will deflect upwards. The pilot will then pull back on the control column while rolling the aircraft to the right. Once he reaches the proper pitch and bank angle the FD bars will again center and remain centered until it is time to roll back to wings level (when the heading starts to approach 090). When the aircraft approaches 20000 feet the pitch bar will deflect downwards thus commanding the pilot to reduce pitch in order to level off at the new altitude.

The FD is generally used in direct connection with the Autopilot. Where the FD commands the AP to put the aircraft in the attitude necessary to follow a trajectory. The FD/AP combination is typically used in autopilot coupled low instrument approaches, (below 200 feet agl) or CAT II and CAT III ILS instrument approaches.

The exact form of the flight director's display varies with the instrument type either crosshair or command bars.

Route manager

1rightarrow.png See Route Manager for the main article about this subject.

The Route Manager in FlightGear is something like a very simple FMC- Flight Management Control/Computer. In FGFS you can input a list of waypoints like NAVAIDS, Fixes and Airports, which the aircraft flies along. You can also add the altitude which the aircraft should have at a certain waypoint. So it is possible to let the aircraft fly along a certain route.

At the moment, it is not possible to add velocities or other constraints.

See Also

Advanced Autopilot Tuning Tips

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