SkyDemon Forums

Great Circles

http://forums.skydemon.aero/Topic19428.aspx

By peter@luthaus.de - 7/27/2016 12:01:14 PM

Hello,
suggested by Tim I post this request in the forum for discussion. I noted (it was already mentioned in the forum) that SkyDemon uses generally rhumb lines to connect waypoints. I find this unsatisfactory and annoying. Industry standard and the wanted way is great circles. There are many reasons why great circles should be used (compared to none using rhumblines):
1) Great circle is the standard for GPS navigation, so built in certified GPS use it and Skydemon is not consistent with the track actually flown even producing off track warnings.
2) For planning the total distance is not so different for short legs, but the off course deviation can be quite large. E.g planning from EDLW to EPWR the rhumb line goes through EDDP Delta, the great circle passes it completely.
3) Most pilots use Lambert projection charts approximating the earth's convergency for planning. So the straight lines drawn on paper will be close to great circles and it is unexpected that skydemon shows a different route.
4) Radials are great circles
5) Airways are great circles
As I find the rhumb line really a nuisance and also other pilots I know do so I suggest changing to great circles. I hope this could be implemented soon.
Peter

By Cat - 5/29/2019 10:31:22 AM

No, Tim. A line on the paper chart is very definitely a great circle. If you measure a long east west track at the ends, you’ll see the tracks are not reciprocal, but differ by convergence.

That’s why we teach to measure the track at around mid point. To get an approximation of the rhumb line, for ease of flying. But the line remains a great circle.

Long haul flights would, typically, be given the initial true track and the mean magnetic track, but the FMS would happily direct the flight along the great circle.

Likewise, radio nav can only follow great circles (and the track should be measured at the VOR, for that reason, and for the local variation to which the VOR is offset. When tracking a long east west radial, the radial uses convergence and variation at the station and the aircraft will experience an apparent drift of convergence and the difference in variation at the start, gradually reducing as departure decreases.