Hey Ian, I am not really an expert in this field but I disagree. Provided there are enough satellites in view I would prefer GPS Altitude for final glide calculations over QNH corrected Pressure Alt. If I recall some of my Flight Mechanics and Aerodynamics classes correctly, the glide ratio L/D is completely independent of Air density, temerature, etc. Its a function of the Wing-Profile (in our case the whole Glider) dependent of the AOA. Therefore, lets say for example at best L/D, with a given Altitude the glider can travel a given distance. If you know the Altitude precisely, you can calculate the distance. You can also think of it in terms of pontential energy... The GPS-Altitude is pretty precise in my opinion. The pressure altitude (corrected by QNH) is only precise at the ground of the airport, where the QNH was measured. If you fly cross country you get an error, but I think more important you get a temperature error if you fly and the atmosphere is not at ISA condition (which it never is). Lets say for example you do a wave flight in the spring. the temperature is 15° less than ISA and your flying at 2500m MSL true (!) altitude. Your QNH corrected Pressure Altimeter will show approx. 2650 m ( 4% per 10K ISA Difference) In Summer with Temperature at 15° above ISA, you're still at 2500m but your Altimeter will show 2350m. Remember the old saying: "In Winter the mountains are higher" ;-) With GPS-Altitude, you don't get this error.
However for vertical airspace distances, I think QNH-Alt should be the Master, since Airspaces are defined that way. The "true" vertical boundarys just "fluctuate" a little bit with temperature. Anybody know how the usual competion scoring software calculates if you vertically violated an Airspace? The same algorythm in XCS would be the right thing to have ;-) Since I don't know the XCS internal computations it would be nice to get Max or Tobias comment on these issues. Thanks :-) My personal preference for the use of Altitude sources in XCS would be: Final glide calculations: GPS-Alt primary, fallback to QNH-Alt. Airspace vertical distance warnings: primary QNH-Alt, secondary GPS Just my 2 cents... Henrik Am 28.09.2010 10:02, schrieb Ian: > > I would think for final glide calcs, the best source of altitude data is > (in order of accuracy): > > - Barometric altitude with QNH correction. (Can this be done > automatically before take off?). > > - GPS altitude. > > - Barometric altitude without QNH correction. (Often out by 100's of feet). > > - Barometric "flight levels" rounded off to 100' as per the B500. > > I know the B500 uses barometric pressure and outside air temperature to > provide corrections to TAS and vario readings. The "flight levels" data > would be good enough for this purpose. I am not sure which altitude > source it uses for its internal final glide calcs. > > I will have to find something else to display in that box with the "---" > indication ;-) > > Thanks > > Ian > > ------------------------------------------------------------------------------ > Start uncovering the many advantages of virtual appliances > and start using them to simplify application deployment and > accelerate your shift to cloud computing. > http://p.sf.net/sfu/novell-sfdev2dev > _______________________________________________ > Xcsoar-user mailing list > Xcsoar-user@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/xcsoar-user > ------------------------------------------------------------------------------ Start uncovering the many advantages of virtual appliances and start using them to simplify application deployment and accelerate your shift to cloud computing. http://p.sf.net/sfu/novell-sfdev2dev _______________________________________________ Xcsoar-user mailing list Xcsoar-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/xcsoar-user
