In a message dated 5/20/2005 2:09:17 AM Eastern Daylight Time,  

But is is meant to be fixed at the aircraft and thus rotates due to the  
aircrafts velocity or is it fixed at the earth and relocated for every frame  
to display?

Or the other question is: what is that hud element meant  to display?
I cannot yet make sense of that.
Hi M,
The HUD element provides a visual indicator of energy within a  constant 
speed climb or descent thru the use of hud_ladr.cxx: 
 if(energy_marker) {
if (total_vel < 5.0)  {
t1 = 0;
t2 =  0;
} else {
t1 =  up_vel/total_vel;
t2 = asin((Vxx*Axx + Vyy*Ayy +  Vzz*Azz)/(9.81*total_vel));
pot_slope =  ((t2/3)*SGD_RADIANS_TO_DEGREES)*factor + vel_y;
The values of Vxx, Vyy, ... are called in for the JSBSim and YASim both,  but 
Axx, Ayy, ... are called for YAsim only, rendering the HUD element to  
indicate velocity only.
As for reference plane, I'm hazy as to the effect of the frame on the  
reference but would lean towards the geodedic or earth-referenced  
acceleration.  I 
have been through the flight.cxx and .hxx files along with  the 
src/FDM/JSBSim.cxx and YASim.cxx and have come away with the  "v_dot_local",  
"Get_" and 
"Set_Accels_Local" as the given  terminology in the NED sections, although 
may be in name only and not  function.
To refresh the initial query;  I would like to use the energy marker  within 
a training tool using a simple single trainer but I need 2 VOR's for  
intersection identification.  The PA-28 -YASim provides the energy  marker and 
c172 - JSBSim provides the avionics... hmmm...

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