8
edits
Avionics and instruments: Difference between revisions
Jump to navigation
Jump to search
→Compass: Elaborated on magnetic variation and added an example magvar shell script
Hellosimon (talk | contribs) (reinstate kap140 page) |
(→Compass: Elaborated on magnetic variation and added an example magvar shell script) |
||
| Line 3: | Line 3: | ||
The compass aligns itself along the magnetic field that surrounds the earth. | The compass aligns itself along the magnetic field that surrounds the earth. | ||
Note that The magnetic north pole and the geographic north pole are not located at the same spot. | Note that The magnetic north pole and the geographic north pole are not located at the same spot. The difference between the magnetic field and true north is called "magnetic variation" or "magnetic declination". The magnetic variation is expressed as an angle east or west of true north. Not only does the magnetic variation vary with position on the planet, it also slowly varies over time since the magnetic poles are not stationary. | ||
When planning a flight it is necessary to know the magnetic variation along the flight path. Air navigation is generally done using magnetic bearings, and the VOR beacons also use that. Runway numbering is based on magnetic bearings. Even GPS's designed for air navigation uses magnetic bearings. Almost everything in aeronautical navigation is based on magnetic bearings. Wind directions from METAR however, reports winds with ''true'' direction. And that's why you need to know the magnetic variation: for calculating the wind triangle; how much you must hold up against the wind (your heading) when flying to the destination (your course). The Kelpie flightplanner for FlightGear also uses true bearings. | |||
Magnetic variations are expressed in "degrees east" or "degrees west". An ''easterly magnetic variation'' means that the magnetic north points a bit to the ''east'' of the true north. In FlightGear, easterly is a positive number, and westerly is negative. The magnetic variation at FlightGears default airport, KSFO, is about 15° east. (This is why the instrument settings of the directinal gyro in the menu is "-15" initially, when starting at KSFO. It shows ''magnetic'' heading.) | |||
The magnetic variation for a specific spot can be retreieved from the area charts or from other maps. | |||
Good area charts are not always easy to get (for free). So here is a quick and dirty shell script for retrieving the magnetic variation at a specific coordinate (at sea level and date of invocation) by shamelessly abusing the '''testmagvar''' utility from the SimGear source code: | |||
#!/bin/bash | |||
if [ $# -ne 2 ]; then | |||
echo "Usage: $(basename $0) lat long"; | |||
echo "East and north are positive."; | |||
else | |||
testmagvar $1 $2 0 $(date +"%m %d %g")|tail -1|tr -s [:space:] \\t|cut -f6; | |||
fi | |||
You ''can'' cheat and get the magnetic variation from the current coordinate when flying in FlightGear, from the property tree (/environment/magnetic-variation-deg). But being a good student you of course want to know the magnetic variation ''before'' rolling out from the hangar, already during the flight planning. You do plan your flights, right? | |||
=== Directional Gyro === | === Directional Gyro === | ||