FSXA Wing_Pos_Apex_Lon, Empty_weight_cg_position

Heretic

Resource contributor
#1
I know these things have been discussed 20000 times, but despite digging through the forums, the SDK and reading Yves' documentation, I'm drawing a bit of a blank here.

Wing_Pos_Apex_Lon decribes the point at which the leading edges of the left and right wing meet on the centerline and is expressed as longitudinal offset.
What I don't understand is the reference point used to which this offset is applied. Reference datum? VMO? 25% MAC? LEMAC?

Empty_weight_cg_position is an offset from the reference datum and defines the center of gravity at the weight specified as empty_weight in the aircraft.cfg.
Is this always supposed to be at 25% MAC when empty?
How do I find out if this also applies to the real aircraft and where would I find such data?
I've got a FCOM and the type certificate, but there's only references to CoG limits in either.
 
#2
Reference datum.

No... but it's important that the visual model's 3D center is at 25% MAC because for some reason that is what the sim believes it should be.
Real aircraft data usually is on the weight and balance sheet... for commercial tubes, I don't think you usually see them lying around.
 

Heretic

Resource contributor
#3
The model I'm working on fortunately has its VMO and RDP at 25% MAC, but I'm wondering what would happen if this wasn't the case.
Assuming that RDP is at +60 ft from the VMO and the result of my calculation for the wing apex* is +10 ft, I take that I'd have to set "Wing_Pos_Apex_Lon" to -50 feet since the apex is 50 feet aft of the RDP?

Anyway, thanks for the reply, Ed. Much obliged.


* Using Yves' equation on page 13 of Flight Dynamics for MSFS.
 
#4
FWIW, because I am the next-to-last person who should say anything about these things.

Here is some information from the FSX 747 aircraft.cfg that might answer your question in reply #3:

reference_datum_position = 83.5, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)
empty_weight_CG_position = -90.5, 0, 0 // (feet) longitudinal, lateral, vertical distance from specified datum
wing_pos_apex_lon = -58.2 //Feet, longitudinal distance from reference point, negative going aft

Here is a screenie using the Visualizer tool:
FSX_747.jpg
 

Heretic

Resource contributor
#5
Thanks for the image, Dave. I think this shows the relationship rather well.

According to AirWizEd, the default 747's empty CoG is at 6.6% MAC and 24.5% MAC with a full load (fuel+payload).
 

n4gix

Resource contributor
#6
I am a firm advocate of the Visualizer Utility. Too bad that the author hasn't updated it for 64bit so we could use it in P3Dv4.x... :(
 

Heretic

Resource contributor
#7
I am a firm advocate of the Visualizer Utility. Too bad that the author hasn't updated it for 64bit so we could use it in P3Dv4.x... :(
Visualizer is an external application, why would it need to be updated, unless it interfaces with something other than FSUIPC or SimConnect?

I've used ModelConverter to adjust the spatial locations for some parameters. While aircraft reloading takes considerably longer than in FSX, the wireframe view and animation slider is quite useful.
 

Heretic

Resource contributor
#9
Bjoern, I only know that it does not work in P3Dv3 or v4. I also have all "Legacy" versions of SimConnect installed.
Fair enough and as I've said, ModelConverter is a fairly valid substitute.
I've asked Arno for an improvement to the model refresh process to speed things up. Maybe it'll turn out a worthy alternative.
 
Last edited:

Roy Holmes

Resource contributor
#10
The importance of Wing_Pos_Apex_Lon is that it is the only indication of where the wing is placed.
From the wing span, area, root chord and sweep, the Mean Aerodynamic Chord is calculated and CG is always expressed in terms of where it is relative to MAC, typically at 25% MAC.
The CG is only at 1/4 root chord if the wing is unswept and rectangular in plan view.
The fore-aft location of MAC is a function of where the wing is placed and the CG is where it is specified in the Aircraft.cfg relative to the airfcraft reference point.
So the Wing_Pos_Apex_Lon is one parameter that adjusts the CG %MAC.
If you see an airplane CG, in the Fuel and Payload drop down, that is ahead of the wing graphic then the wing needs to go forward or the CG go back
The equations for Wing_Pos_Apex_Lon are quite complex with a swept and tapered wingform.
Obviously as sweep increases the Wing_Pos_Apex_Lon has to move forwards to keep the CG at the same percent MAC. Plus the MAC moves sideways away from the root as sweep increases.
Finally, if you have the Wing_Pos_Apex_Lon where is should be for the particular airplane and the CG is not around 25%, move the CG position.

Many military airplane manuals pay a lot of attention the CG shift with fuel and stores and their CG positions are given in %MAC, though weighing procedures use a datum on the fuselage when checking.
The weight of the airplane is measured at the nosewheel and each mainwheel and the CG is calculated from those figures and the known wheel positions.
There may be operational limitations imposed if the CG is at either extreme of the permitted brackets. For example the F-4 forward CG limit was 27%MAC and aft was 36%. With full fuel the F-4M CG was at 35.2% and it was decidedly twitchy until fuel burn brought the CG forward to 34% or so.
I guess there is little data about civil airplane CG because they are certified to fly without limitations of that kind, I think.
However I do remember being told to go forward in a passenger cabin because with a partial load my 140 lbs in a rear seat was a flight hazard.
Roy
 

Heretic

Resource contributor
#11
As usual, many thanks for the insight, Roy!

The equations for Wing_Pos_Apex_Lon are quite complex with a swept and tapered wingform.
Obviously as sweep increases the Wing_Pos_Apex_Lon has to move forwards to keep the CG at the same percent MAC. Plus the MAC moves sideways away from the root as sweep increases.
Finally, if you have the Wing_Pos_Apex_Lon where is should be for the particular airplane and the CG is not around 25%, move the CG position.
As posted before, I've used Yves' equation (ch. 3.1.4, p.13) to calculate the apex position. MAC length is the "official" value from the FAA type certificate.

I guess there is little data about civil airplane CG because they are certified to fly without limitations of that kind, I think.
There are CG limits for commercial jets and these are at least in the FAA type certificate. For, e.g., a DC-9-10, CG margin ranges between 38.6" and 12.8" depending on weight (MAC length 141.5").
If anything, CG is even more important than in military aircraft, because those likely kill not as many people onboard if CG is out of bounds.
 
#12
For what it's worth, the last pages of the FS2004 FSEdit SDK doc has some nice diagrams to illustrate many of the measurements including wing apex position and what it's referenced to.
 
#15
make sure to understand the reference datum is measured from the 3D model design 0,0,0 coordinate. It is NOT the center of the visual model you see and it can be ANYWHERE. The red + in the the top down view can help give a clue as to where it is positioned. Measuring the rendered wingtip light effects are also a good way to find out where the center is.

Some designers place the 0,0,0 point at the reference datum position, which does make setting up the cfg easier.
 
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