• Which the release of FS2020 we see an explosition of activity on the forun and of course we are very happy to see this. But having all questions about FS2020 in one forum becomes a bit messy. So therefore we would like to ask you all to use the following guidelines when posting your questions:

    • Tag FS2020 specific questions with the MSFS2020 tag.
    • Questions about making 3D assets can be posted in the 3D asset design forum. Either post them in the subforum of the modelling tool you use or in the general forum if they are general.
    • Questions about aircraft design can be posted in the Aircraft design forum
    • Questions about airport design can be posted in the FS2020 airport design forum. Once airport development tools have been updated for FS2020 you can post tool speciifc questions in the subforums of those tools as well of course.
    • Questions about terrain design can be posted in the FS2020 terrain design forum.
    • Questions about SimConnect can be posted in the SimConnect forum.

    Any other question that is not specific to an aspect of development or tool can be posted in the General chat forum.

    By following these guidelines we make sure that the forums remain easy to read for everybody and also that the right people can find your post to answer it.

P3D v4 Aircraft Datum Point in P3D v4

S

Sundog

Messages
11
Country
us-indiana
I've developed flight models for FSX before and, IIRC, the datum point for aircraft models used to be the most forward point on the model center line, correct? So I started reading the P3D SDK for the aircraft datum point and it reads this;
reference_datum_position Offset (in feet) of the aircraft's reference datum from the standard center point, which is on the center line chord aft of the leading edge. By adjusting this position, actual aircraft loading data can be used directly according to the aircraft's manufacturer. If not specified, the default is 0,0,0.
Click to expand...

Which basically reads, "On yonder, back that-a-way, is a point upon which is the center." i.e.- I haven't any idea what they are talking about. Is there anyone here that can translate that for me? Are they going from some magic middle point in the model? Also, the empty weight C.G. would be the best reference, since aircraft rotate around their C.G. when in the air. I assume they are talking about that location from some datum point that was placed on the model when it was compiled, but I haven't found a reference in the SDK to tell me where that point is located. Any help would be appreciated.

Regards,

Ken
 
hairyspin

hairyspin

Resource contributor
Messages
3,253
Country
unitedkingdom
The model centre 0,0,0 is wherever the modeller makes it in Gmax/Blender/FSDS/whatever and any clot could make that the starboard wingtip or the pitot root if he were daft enough. Sensible modellers make the model centre on the aircraft centre line, at 25% of wing chord back from the wing leading edge and at the same height as the prop centre or jet thrust line. This should coincide fairly well with the CoG and is where the simulator expects the model centre to be.

However, not everybody does, sometimes with good reasons, so the reference_datum_position in the cfg tells the flight model this is not the case and to look for model centre at the offset given.

This has been the case since at least FS8. A number of stock models from back then have model centre at the prop spinner tip.
 
Last edited:
n4gix

n4gix

Resource contributor
Messages
11,674
Country
unitedstates
Reference Datum Point (RDP) is easily the most misunderstood part of modeling. It truly is quite simple in principle.

If not specified, it is ignored (essentially considered to be co-located with the model's origin (0,0,0) point.

You can set the RDP to be any darn thing you wish it to be, unless you have manufacturer's data that provides the RDP they used. In all cases though the RDP uses the centerline and waterline of the aircraft, which is of course x, 0, 0 where x is the distance forward of the model's origin. For example, in the B737-200 aircraft.cfg I've set the following:

Code: 
reference_datum_position     =  40, 0, 0         // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)
empty_weight_CG_position     = -39.5, -0.00, -2.0         // (feet) longitudinal, lateral, vertical distance from specified datum

Note that the RDP is 40' forward, and the empty weight CG position is set as -39.5' aft of the RDP, and -2' below the waterline. This system allows one to micro-manage the empty weight CG in case the model's origin isn't precisely at the 25% MAC point, and the waterline is too high for proper balance.

The other major advantage of using this system is that all points of the aircraft will be negative values since they are all located behind the RDP by definition! This totally eliminates the need to remember if a defined position is fore or aft of the model's origin point... :teacher:

Making the job easier is that you can create a 'ruler' in Max/GMax/Blender marking the precise RDP you want, then use that point in the modeling program to create another 'ruler' to establish the precise points for engines, flaps, landing gear, et cetera.
 
S

Sundog

Messages
11
Country
us-indiana
Thank guys. So, if I want to update an existing flight model, I can just add a light at the 0,0,0 point to determine the models reference point. Eventually, I'll get back to actually modeling, but given that 3DS max can only be leased now, I think I'll use Rhino to make models and use my old 3DS max (If I can get it to work on this new PC) to export the model to FS/P3D. The problem is I have so many projects, not FS related, going on now. But nobody is ever going to model the planes I want, so I had better get at it soon. :)
 
You must log in or register to reply here.
Top