From: John Denker <[EMAIL PROTECTED]> > > I'm under the impresssion that the >
> property called "environment/pressure-inhg[0]" has a lag computed into
> > the final value. Is that correct or is that the "instantaneous"
value or > > pressure level the aircraft is at and what I'm looking for?
> Close.
> The trick is that altimeters and suchlike are not (and should not be)
> connected directly to environment/pressure. They are connected to
> /systems/static/pressure-inhg ... and *that* has an unrealistically
> huge lag built into it. I would argue that there should be "some"
> lag, just not nearly so much.
[...]
> Decreasing the static-system lag from 1 (the previous compiled-in
> value) to .1 or even .2 makes things look instantaneous or nearly
> so.
There are three types of altimeter in common use:
(1) Air data computers, which go to a lot of trouble to report the
instantaneous value. Aircraft with such instruments should use the
"environment" value,
(2) Basic barometric altimeters, which should have a long lag (second or so, in
my experience). Small aircraft manufactured before the 1970s tend to have
these, I believe, and this behavior is modelled by the "systems" value,
(3) Corrected barometric altimeters, which have a relatively short lag that is
just about observable by eye (therefore presumably around a tenth of a second)
when operated in an IFR-like way. Small aircraft of recent manufacture tend to
have these, unless the aircraft is aerobatic. As far as I know, we don't have a
model for this in FlightGear.
Correcting altimeters contain a small accelerometer that detects the vertical
component and, using a small bellows, injects a high pass filtered contribution
into the static pressure value. The idea is that an increase in vertical
acceleration (i.e. more than gravity) implies the aircraft is going up and so
the bellows temporarily sucks some air out of the static pressure and causes
the instrument to indicate a climb sooner than it would otherwise.
Equivalently for descent. This works great for operations in IFR, so normal
category light aircraft with an IFR panel tend to have these installed (unless
it is an old aircraft with original equipment). To the pilot in IMC, the
improvement is comparable to flying with a gyro compass instead of with a wet
compass.
Of course, the bellows trick of (3) goes wrong for (a) increased G maneuvering
and (b) unusual attitudes:
(a) If I make a 60 degree banked turn, so the aircraft pulls 2 gravities, the
bellows will (for a second or so) cause the altimeter to indicate a higher
value than is actually correct. However, providing I roll smoothly into the
turn, so the acceleration force picks up gradually, the altimeter information
continues to be useful and is not particularly odd.
(b) If I'm upside down, changes in apparent G force should have the reverse
effect on the altimeter. Some instruments limit out for zero or negative G, so
the correction is disabled in unusual attitudes. I suppose it's feasible that
there are some which use the absolute value of the apparent G force so they
work relatively well when upside down ... but I haven't come across that.
Hope that helps ...
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