Meaning of locations, htailarm and vtailarm in A320-211.xml

[gierschi@flightgear]

Dear FG community,

i have to find out the distance between the Center of Gravity and the Fin or Tail of the A320-211 but I don't know how. I think the following specifications in the A320-211.xml file are relevant:

Code: Select all

   <htailarm  unit="FT" >   57.88 </htailarm>
   <vtailarm  unit="FT" >   57.33 </vtailarm>
...

   <location name="CG" unit="IN">
   <x> 642.122 </x>
   <y> 0 </y>
   <z> -41.7398 </z>
   </location>



But i don't know the exact meaning of htailarm (Horizontal Tail Arm) and vtailarm (Vertical Tail Arm). Furthermore, the location of the C.G. is specified. But to which reference point do the x-,y- and z-values refer?

Best regards,
Sebastian

[erik]

The tail arm is used to calculate the momentums and is defined as:


htailarm is for the horizontal stabilizer
vtailarm is for the vertical stabilizer

Those distance are relative to the CG so you could easily measure them and use that number. As you can see in the image the horizontal tail arm is slightly shorter than the vertical tail arm for that particular model.

CG is relative to an imaginary location. Often used locations are front of the nose or at CG. All locations are relative to the same reference point and X is to the tail, Y is to the right and Z is up. See the Structural axis definition here:
http://jsbsim.sourceforge.net/JSBSimCoordinates.pdf

Erik

[gierschi@flightgear]

Thank you very much! Your answer help me a lot !

[Bomber]

Well that's not my understanding of these values..

Which are that they're between your datum point and the H-stab and V-stab...

jsbsim then works out your planes actual moment taking into account the CoG distance from your datum point..

It's a small difference but quite important... and i think some clarification is needed by jsbsim to nail the correct answer.

Reviewing the snipet of code, you have a distance between your datum and CoG of 53.5ft... and a distance between your datum and H-stab of 57.88.

Which would give a moment arm of 4.38ft...this being the distance between your CoG and H-stab.

I'll go out on a limb here and say the fdm isn't correct.

Simon.

P.S. don't be too quick to put the green tick on a topic, get a second opinion.

[Alant]

htailarm and vtailarm are not used internally by JSBsim. They may be usedful for FDM designers who wish to apply loads to the tail.

These lengths are also used by aerodynamacists when estimating some aerodynamic coefficients. (e.g. mwdot - which defines the pitch moment due to rate of change of incidence)

Bomber is correct when he say that they are measured from a fixed reference point and not from the CG, which moves with payload and fuel usesage.

In other words it is unlikely that you will have any need for them in your FDM.

Alan

[Bomber]

Ok so not 'used' internally by jsbsim directly... jsbsim does use it to produce other properties which are then used by a lot of flight modellers within their damping section for h-stabs and v-stabs.

End results still equals incorrect flight model