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Help needed for moving aircraft reference datum

M

microlight

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7
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unitedkingdom
Hi all,

Can anyone help, please? I need to raise the reference datum by three feet in a cfg file to compensate for the reference datum being three feet low in the visual model. But by doing this, I'll need to lower all of the points in the cfg file that reference that point! Manually, it's a long task (it's not just one cfg file...) and I was told that a tool exists here that could speed the process up - does anyone know which one? After the website migration, there don't seem to be any resources on the tab above; would they be stored anywhere else?

Thanks for your input!
 
are you trying to set the CG height? What is the reason to move the reference point?
 
The modeller accidentally put the reference datum at the base of the fuselage rather than at the waterline point during the visual modelling process, which I'm trying to correct by re-locating the reference datum in the cfg file. Are you saying that I should actually be moving the CG point instead?
 
Ugh! I'm not aware of any method for moving the visual origin without having the source file(s) and recompiling. It would seem that you are stuck with modifying the aircraft.cfg point...

But! All is not lost. You can specify a new "reference datum" with a single entry in the aircraft.cfg file, since all other measurements already existing will be offset from that entry!

Most modelers either don't know about or have completely misunderstood this powerful configuration entry:
Code: 
reference_datum_position  =  40, 0, 0  // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)
Note in the example above that I've now forced the reference_datum_position to be 40' ahead of the model's visual origin (0,0,0) location. In your case, you can raise the waterline position by your desired 3' like this:
Code: 
reference_datum_position  =  0, 0, 3  // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)
All other measurement data will remain the same values, since they are based on the reference_datum_position, which is normally left by most modelers as 0,0,0 anyway! :teacher:
 
Many thanks for your feedback, Bill - that's what I originally thought, and exactly what I did before I wrote the original post. However, once I re-loaded the model into FSX after raising the reference datum, the plane was sunk into the tarmac until I corrected the contact points by lowering them by the exact same amount that I'd raised the reference datum. From that, I figured that I'd need to lower all of the other associated points too. I confirmed this visually by placing a beacon where one of these new points should be; in this case I placed a beacon at where the new level of the port wing tank would be, after lowering it by 3 feet (but ahead of it, so that I could see the beacon flashing), and it was in the right place. So it seems that I have to lower all of the relevant points: payload, engines, fuel tanks, etc etc, and I was wondering if there was a tool to simplify this task!
 
The reference datum point is an arbitrary point that is defined for all aircraft. The manufacturer usually has a value for that and it is almost always an imaginary point in front of the aircraft's nose. When you set this value in the aircraft's .cfg file, you are telling the simulator where the reference data point is located in relation to the visual model's 0,0,0 location. The visual model's 0,0,0 location is normally the 1/4 chord length from the front of the wing because it defaults to the center of lift. However, that doesn't mean a modeler defined their 3D wing position accordingly.

Once set, all other position tags in the aircraft.cfg file are directly related to the reference datum value.
 
yes. Move the CG if it concerns Flight Dynamics. Then move the center of lift in the air file to make sure they are placed correctly. As Warp D says above, a reference can be anywhere and is just a "starting place". It doesn't affect anything.
 
microlight said:
... it seems that I have to lower all of the relevant points: payload, engines, fuel tanks, etc etc, and I was wondering if there was a tool to simplify this task!
Click to expand...

There was, it was Aircraft Container Manager and it worked for up to FS9 models: it gave you a visual tool for setting contact points and many other things. Hard to find nowadays and not easy to install from scratch in modern Windows versions.

@jx_: the reference datum point ties together the flight model and the visual model. Without it, the sim doesn't know "where" your visual model is. A number of stock models in CFS3 were built with the model's 0,0,0 at the tip of the prop spinner: without a correct reference datum point in aircraft.cfg the models would have pitched, yawed and rolled around the spinner...:yikes:
 
Many thanks for all your replies. What I've done is to move the CG vertically up by three feet, which I think has done the trick. When I put the FDE together, I confirmed all the location points (e.g. engines) visually in the sim, so that they were in the right place - even if the reference datum location was a little vertically offset from where I thought it was. Consequently, the CG was low too, as it was 0,0,0 as was the reference datum. Raising the CG seems to have corrected the slightly odd roll, which previously happened around the fuselage base rather than where it should have been. The roll is better now.
 
@hairyspin You are making the assumption that reference datum and CG are tied. In real aircraft, the reference datum is normally some point in space AHEAD of the nose. All measurements are done from this point. Reference datum position can be anywhere so long as the coordinates of the geometry are correct.

"without a correct reference datum point in aircraft.cfg the models would have pitched, yawed and rolled around the spinner...:yikes:"

That statement is incorrect. If CG and Wing_COL are at the correct coordinate on the visual model, the flight axis will be correct regardless of reference datum position. If the control surface placements are correct (relative to CG/COL) the aircraft will fly as it should, assuming coefficients are correct.
 
If one does not put an entry into the aircraft's .air file that defines the center of lift, then FS defines the center of lift as defaulting to 1/4 of the wing chord. It also assumes (probably not the best idea) that the visual model will be created so that it's 0,0,0 position is also 1/4 of the wing chord. The reference datum in the aircraft. exists so that one can take an actual weight and balance sheet of an aircraft and define all of the correct arms in the aircraft.cfg. Otherwise it wouldn't be needed at all as there's no other reason to define it offset from 0,0,0 other than to be able to quickly set up the rest of your entries in the aircraft.cfg from an aircraft's actual weight and balance sheet.
 
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