Martin Olveyra writes:
> If I make a round trip, the HUD shows correctly the latitude and longitude,
> and aparently the Heading is always ok.
The HUD shows your magnetic heading.
> When I take off, the HUD and compass shows the same heading, and I adjust
> the giroscope so it shows the same heading as them.
The whiskey compass shows your magnetic heading. In some parts the
the world, it will be close to or identical to the true heading, but
in many others, it will not, especially as you move into more northern
latitudes. In Ottawa, Canada (approx 75.5W 45.5N), there is a
difference of 14 degrees between true north and magnetic north; in
Canada's northernmost settlement, Alert (approx 62W 82.5N), there is a
difference of 73 degrees. If you're not noticing this, then you're
flying somewhere where the magnetic pole is pretty much lined up with
the true pole.
The gyro compass shows any heading you want.
It's probably a bad idea to set the gyro to the true heading (from the
HUD) rather than the magnetic heading (from the compass), since
runways, airways, approaches, etc. are usually published with magnetic
headings (except in the far north, where the deviation is too great).
> At the very first I can note a derivation between the giroscope in one
> side, and the HUD and compass in the other side. This difference makes
> greater as the flight time passes.
Gyro compasses respond to changes in heading fast and are not affected
by roll and pitch, but they tend to drift over time, and Alex Perry
has worked very hard to model that effect in FlightGear. As in a real
plane, you'll have to adjust the gyro compass every once in a while to
align it with the whiskey compass (make sure you're flying
more-or-less level when you do).
> So it becames imposible to navigate with the giroscope or even the
> compass.
Not at all. The whiskey compass doesn't drift, but it is affected by
turns, climbs, etc., so you need to be straight-and-level to get an
accurate reading. The gyro drifts, but it is not affected by turns,
climbs, etc., so you can use it while manoevering. As long as you
sync up the gyro compass with the whiskey compass from time to time,
you'll be find.
> Apparently the HUD is the only reliable navigation "instrument".
Yes, except that it gives the true reading, so it is inappropriate for
most published approaches, airways, etc.
> First I wondered whether this difference was caused to the difference
> between the magnetic and geographic poles (the simulator model does really
> contemplate this difference?) but the compass derives even when I'm quiet.
You were close -- just looking at the wrong compass.
All the best,
David
--
David Megginson
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