HI all!
To continue series of my stupid questions about aircraft design...
I still have very very big problem with placing my aircraft model on sim ground.
I read all your recommendations,learnt very closely
center of gravity tutorial and still one thing is completely unclear to me-what is that misterious REFERENCE DATUM POSITION?!
I know that it's not aircraft original(0,0,0),and I know that this is very critical parameter in aircraft cfg file...
According to SDK reference datum position-it's just "flightsim reference point"...

And one more simple question-how are contact points for wheels measured from RDP to center of wheel or to it's outer edge?

Minwhile my plane behaves in pretty strange way-one or two first seconds it stands on wheels-then suddenly falls on it's belly and crashes with wheels sunk deeply in ground...
Or it just  suddenly overturns it's nose and crashes with it's tail up...

Suddenly overturns and crashes with tail up

REFERENCE DATUM POSITION is the pivot point in FS used when you circle around the aircraft in spot view.

Usually many designers will also make that the CoG for simplicity.

To do this, in the design program, position the aircraft so the main crosshairs in the grid are at the approximate center of gravity.  This is usually 1/4 back from the leading edge of the wing longitudinally, at the water line (or at or near the engine thrust line) vertically, and at the centerline of the aircraft laterally.

With the aircraft set at this postition, and with gear extended, you can get the measurements for the contact points and cfg file for engines, lights, etc.  In Gmax, use the Tape Measure.  Always be in a standard View, like right/left/top/bottom when doing this.

The gear distance from the reference point to the ground is measured to the ground surface, not the hub of the wheel.

Also, get the precise measurement of suspension travel from the gear from frame 100-200.  You will need this to set static compression and max to static ratio.  Static compression can be figured at about 35% of total animated movement.

Hope this helps.

The Reference Datum Position can be anywhere you want it to be.  For example, manufacturers typically have it at the bulkhead toward the nose of the aircraft.

FS allows you to change the reference so that you can use manufacturers data as specified.

Setting it at 0, 0, 0 lessens the confusion of two sets of points.

The graphic you show looks good ... the aircraft appears to be positioned correctly longitudinally and laterally.

Vertically, I really don't know for that aircraft but the waterline is a good guideline, probably right at the nose tip level.