On 06/16/2007 09:12 PM, gh.robin wrote:
> I notice a some strange behaviours with the Instrument Altimeter
> The Instrument Altimeter is unable to indicate more than 61831 feet
>
> Here the snapshot of property:
> instrument/altimeter/indicated-altitude-ft ==> 61831
> position/altitude-ft ==> 75793
I agree this is strange and deplorable ... but it is /not/ a
problem with the altimeter instrument!
Let's look more closely at the properties: Let's fly up to 100,000
feet and see what happens:
/position/altitude-ft = 100000 # so far so good
/environment/pressure-inhg = 1.94 # trouble in the environment
/systems/static/pressure-inhg = 1.94 # static system dutifully follows the
environment
/instrumentation/altimeter/indicated-altitude-ft = 61800
# altimeter dutifully follows the
static pressure
FYI, under ISA conditions, according to my calculations:
61,800 feet of pressure_altitude ==> pressure is 1.94 inHg.
100,000 feet of pressure_altitude ==> pressure is 0.32 inHg;
If you want to know exactly why FGFS poops out at approximately
62,000 feet, look at line 88 of Environment/environment.cxx
You can contrast that with the ISA table that goes up to 278,000
feet as found e.g. at the top of
http://www.av8n.com/physics/altimetry.htm
More generally, the code that calculates the /environment/pressure-inhg
is a mess from end to end, to put it politely. For example:
*) In the front-end gui, the Weather Conditions popup allows the
user to specify, layer by layer, the altitude AND the temperature
AND the altimeter setting. Almost all ways of filling in the
gui form will be inconsistent with the basic laws of nature.
*) In the back-end c++ code, there is quite a bit of code that
has never been tested. Indeed some of it has never been executed
AFAICT.
*) The back-end c++ code performs calculations that bear no discernible
relationship to the equation of state of the atmosphere. This is
particularly noticeable under nonstandard-day conditions.
*) Don't get me started about the documentation or lack thereof.
The atmosphere-modeling features need to be properly thought through,
and then re-implemented from end to end.
For example, in my private copy of FGFS, in the Weather Conditions
popup, all the numbers in the rightmost column (altimeter settings)
have been replaced by a single number representing the QNH of the
air mass. Alas this is only 1% of what's involved if you want to
do the job right.
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