Mean Aerodynamic Chord

[Sergs]

Hello everybody,

I can not seem to find a definitive answer to how the Mean Aerodynamic Chord and it's position (relative to reference_datum_position) are calculated/defined for any given FSX aircraft. Has anybody managed to ascertain how these are precisely defined in FSX? Do they even exist, as such?

I noticed that the user 'mgh' has referred to MAC in this post (http://www.fsdeveloper.com/forum/showpost.php?p=268489&postcount=47). Is there any information on how the MAC was calculated in that case?

Any information would be appreciated.

Warm regards,

Sergs

 

[SkyMath]

Hello everybody,

I can not seem to find a definitive answer to how the Mean Aerodynamic Chord and it's position (relative to reference_datum_position) are calculated/defined for any given FSX aircraft. Has anybody managed to ascertain how these are precisely defined in FSX? Do they even exist, as such?

I noticed that the used 'mgh' has referred to MAC in this post (http://www.fsdeveloper.com/forum/showpost.php?p=268489&postcount=47). Is there any information on how the MAC was calculated in that case?

Any information would be appreciated.

Warm regards,

Sergs

Hi !

I recommend you read the Yves Guillaume document that precise exactly how to calculate the MAC length and its leading edge position.

http://library.avsim.net/download.php?DLID=170811&Cookie=1

 

[mgh]

The formulae I used were from the internet:

MAC length = A-(2*(A-B)*(0.5*A+B) / (3*(A+B)))

Distance of MAC LE aft of wing apex = (S*(A+2*B)) / (3*(A+B))

Lateral offset of MAC from wing centre-line = (2*Y*(0.5*A+B)) / (3*(A+B))

where

A = root chord (ft)
B = tip chord (ft)
Y = semi-span (ft)
S = leading edge sweep at tip (ft)


NOTE: Edited after original post

This was before Yves Guillaume document and I haven't compared the two.

 

[Sergs]

Thank you both very much, that's very useful.

 

[n4gix]

For general information, the reference_datum_position (RDP) corresponds to an arbitrary point in world space that the engineers have defined as THE reference point for all measurements on real world blueprints/drawings.

Typically this RDP is located several feet in front of the fuselage's nose, or the prop spinner of single engine aircraft, centerline, waterline.

The hierarcy of the various points are:

VMO - visual model origin (0,0,0)
|___ RDP - reference_datum_position (positive offset from VMO)
       |___EWCG - empty_weight_CG_position (negative offset from RDP)

The beauty of following this convention is that all other measurements in the aircraft.cfg are then defined as negative offsets from the RDP, so you can't get confused about whether they should be positive or negative offsets!

Now, that said the sad truth is that most modelers do not use this convention, but simply make the VMO, RDP and EWCG precisely the same point, which to my mind is just plain lazy...

Attached is a very handy utility that will allow you to SEE all of these points as well as all of the other critical points in the sim in real time. Simply unzip Visualizer into any convenient folder (I use ..\FS Tools), then double-click to run Visualizer while FSX is running.

 

[Sergs]

Hi Bill,

Sorry about the very late reply, unfortunately real life sometimes forces FS stuff on the back burner.

Thank you for the utility - very useful and time-saving!

Now, that said the sad truth is that most modelers do not use this convention, but simply make the VMO, RDP and EWCG precisely the same point, which to my mind is just plain lazy...

I completely agree about this. I much prefer to set the RDP to the manufacturer's datum, alowing me to use their data with very little, if any, conversion. Furthermore, of course, the EWCG seldom happens to be at either VMO, RDP or 25% MAC in aircraft.

Thank you again.

Sergs

 

[jx_]

as n4gix said, RPD is usually ahead of the nose. The pain is converting to FS model datum.

While mgh has posted an equation that would be fairly correct, it would be easier to simply download the aircraft type certificate data sheet.

Google: 'Aircraft ICAO ID' TCDS

or

Google: 'Aircraft ICAO ID' MAC


like: B747 TCDS


What Flight Sim needs is not the MAC length, but the leading edge position of the MAC.

Do you see the line that says 0% MAC?? That's what it needs. By setting:

wing_span=X
wing_root_chord=Y
wing_sweep=Z
wing_pos_apex_lon=-A


The sim has all the info it needs to find MAC, so long as those positions are referenced to the correct DATUM.

Other than that, make sure the center of lift is correct at empty weight in table 1534.


Once set correctly, the game simulates weight and balace characteristics very well.

 

[jx_]

better graphic with MSFS details...