On 10/28/2009 05:53 PM, Ron Jensen wrote: > On Wed, 2009-10-28 at 09:56 -0600, dave perry wrote: >> Tim Moore wrote: >>> On 10/27/2009 01:49 PM, dave perry wrote: >>> >>>> Tim Moore wrote: >>>> >>>>> On 10/17/2009 07:42 PM, Ron Jensen wrote: >>>>> >>>>> >>>>>> On Sat, 2009-10-17 at 11:24 -0600, dave perry wrote: >>>>>> >>>>>> >>>>>>> With current cvs update on my notebook, I am not getting the correct >>>>>>> field elevation when I set the altimeter to match the real weather >>>>>>> altimeter setting. It is off by 160 feet today at KLMO (field >>>>>>> elevation >>>>>>> 5052). With my desktop which was last updated from cvs about 10/3, it >>>>>>> is very nearly correct even with radically high or low pressures. >>>>>>> >>>>>>> >>>>>> I've seen this problem going back into the middle of September: >>>>>> >>>>>> http://www.mail-archive.com/flightgear-devel@lists.sourceforge.net/msg23719.html >>>>>> >>>>>> It would be nice if someone figured out how to fix this... >>>>>> >>>>>> >>>>> With Ron's help I've committed a fix for this. Please check it out and >>>>> report any >>>>> further problems. >>>>> >>>>> Thanks, >>>>> Tim >>>>> >>>>> >>>>> >>>>> >>>> Hi Tim, >>>> >>>> With the altimeter set to the ALT (in hg) in the weather conditions >>>> table from the Environment gui, the field elevations I checked were >>>> correct. But the number in this table no longer matches the real >>>> weather scenario metar number. I believe Torsten made significant >>>> changes to the real weather code. >>>> >>> I just did a test with real-weather-fetch at KSLC. When I set the altimeter >>> using the QNH >>> in the metar -- 29.76 -- the altitude is right on, as far as I can see. >>> It's true that >>> this pressure doesn't match the pressure shown in the Environment >>> Conditions gui, 29.814, >>> but that is to be expected; that pressure is now the true pressure at sea >>> level, not the >>> altimeter pressure. >>> >>> Does this jibe with what you're seeing? >>> >>> Tim >>> >>> >> Hi Tim, >> >> Yes. After an update of fgfs and simgear this morning, I started at >> KEGE (Eagle County RGNL with a field elevation and TDZE both equal to >> 6540 ft using real weather. The QNH was 29.37 from metar. When I set >> the altimeter to this, it indicated 6528. Since I was at the threshold, >> not the TZE, that is close enough and much closer that with the >> altimeter set to the ALT (in hg) from the gui weather conditions. >> >> ALT (in hg) from the gui weather conditions used to be the QNH and one >> could reset it in the gui while flying so that the pressure altitude and >> the indicated altitude with the altimeter set to the QNH were not the >> same. In that case, one could do an approach and the minimums were >> realistic. It seems that we can no longer do this when using the manual >> input as the metar source. Why does the sim pilot need to know the >> sealevel pressure? In the above manual scenario, the sim pilot does >> need to know the QNH. >> >> Regards, >> Dave P. > > Hi Dave, > > The issue is, we used to use a constant pressure vs. altitude rate. > With this model sea-level-pressure and QNH are the same. However, we > now use a more realistic model whose pressure vs. altitude rate varies > based temperature. This had the impact of making sea-level-pressure and > QNH different. > > QNH is based on field elevation, so it may/will vary based on what > airport you're trying to set it for. Perhaps a nasal script would be > best for determining QNH on a per airport basis?
In any event, I realize that the label "Alt (in HG)" is totally wrong. If it is useful to set the QNH directly in this dialog, then the interface will need to be recoded a bit. Tim ------------------------------------------------------------------------------ Come build with us! The BlackBerry(R) Developer Conference in SF, CA is the only developer event you need to attend this year. Jumpstart your developing skills, take BlackBerry mobile applications to market and stay ahead of the curve. Join us from November 9 - 12, 2009. Register now! http://p.sf.net/sfu/devconference _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
