On 10/28/2013 02:00 PM, James Turner wrote:

On 28 Oct 2013, at 12:55, jean pellotier <jean.pellotier@wanadoo.fr> wrote:

Hi, following the discussion after a"fix" for the reported true air 
speed, for jsbsim planes, we've got yet another case where yasim and 
jsbsim diseagre when exporting a property to FG.

the property in cause is:

/velocities/mach

yasim report it as the mach value, along the u axis, if wind come from 
behind, the value is negative.
jsbsim reported it (before the patch) as the absolute value, the value 
was the same whatever the plane heading, this was changed to have the 
same as the yasim behaviour.

next come the question :)

what is supposed to be velocities/mach ?
should we keep the norm of the wind relative to the plane speed, in case 
pitot and other instrument using it should use alpha and side slip to 
give dynamique pressure, or should we use the yasim one, ie mach being 
the mach speed along the u axis?

I can't answer these questions, hopefully Jon / Eric / others can.

The jsbsim is what I would expect, thus /Mach being the total vector value. This value is used for any aerodynamic calculation (where aerodynamic coefficients are made non-dimensional with). Any projected values (which you might need also) would ideally need a reference in their names to the axis it is projected on or else you just dont know what you are dealing with. Like the jsbsim variant MachU, MachV, MachW for instance (Mach vector projected on body axis x,y and z respectfully).

Cheers,

Eric