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Tomasz
07-31-2008, 06:18 PM
Hi!

After quite a long discussion with friend about how to fly direct courses we were left with a question how does the AP coupled to the GPS fly direct to courses?

Does it use straight line or does it calculate a track based on orthodrome (great circle)? It probably wouldn't make much sense on short flights, but on longer ones it may give you some savings.

It's probably more question about how GPS sends track to the AP but I couldn't figure out where to post this :-)

hevansrv7a
07-31-2008, 06:24 PM
It's hard to observe in small flights but if you ask it for 1,000 miles it becomes obvious.

paul330
08-01-2008, 12:59 AM
There is no such thing as a straight line on a globe!

Most map projections will give you a rhumb line if you draw a straight line between two points. This equates to a constant true track. Commercial autopilots fly a great circle between waypoints and I assume the experimental units will do the same . This is the shortest distance between 2 points. It will involve a changing true track. This is not normally noticeable but on long tracks or closer to the poles, it will become evident.

The easiest way to visualize it is flying from 0 to 180E at high latitudes. The rhumb line takes you round the line of latitude on a constant easterly heading. The great circle takes you over the pole with a north heading to start and a south heading after crossing the pole, and will be shorter.

Don't start arguing about it - it's true!!

Tomasz
08-01-2008, 05:24 AM
Don't start arguing about it - it's true!!
Nobody is arguing that. The question wasn't which way is better/shorer. The question was how current generation of avionics do it. Do they select constant heading from point A to point B and fly the AP this way or do they calculate great circle course and fly this one?

kentb
08-01-2008, 02:23 PM
Nobody is arguing that. The question wasn't which way is better/shorer. The question was how current generation of avionics do it. Do they select constant heading from point A to point B and fly the AP this way or do they calculate great circle course and fly this one?

My understanding is when the AP is couple to the GPS, it will be directed by the GPS to fly to the right or left of the current course. In this mode that AP doesn't know anything about where it is going. Most modern GPS units will be plotting a arc circle course between the way-points.

When the AP is not coupled to the GPS it depends what smarts the AP has. Most would fly a bearing and don't know if it will take to your destination.

Kent

breister
08-01-2008, 02:29 PM
That sounds correct.

For example with the TruTrack autopilot - if you are in the "course" mode it will follow true heading if a GPS signal is present and a magnetic heading if not. In the "track" mode it will attempt to follow the course line displayed on pretty much any Garmin unit and probably most others as well. About the only difference would be whether the GPS provides true GPS steering (anticipates intercept of the course; much smoother) or "homing" (tends to overshoot course, and zig-zag a bit keeping the aircraft on the course line).

Mileage may vary.