Contact Points Demystified

Getting your model into the sim can often be frustrating. Here is the method I use.

The gMax origin is where the camera is pointed when you are in Spot View so I start with making sure
I am "visually" centered, front to rear, top to bottom with the gear down, and naturally, side to side. Next I
look at the wing, project the leading edges in to the point they cross as this will be the longitudinal apex. The
Reference Datum Position is placed one fourth of the wings cord back from this apex. This should also generally
be somewhere between of the main gear and nose gear. Vertically I place it where I figure the thrust vector of
the engine(s) is going to be. Of course, it is centered side to side. You can place it where ever you like, just
remember all of your measurements in the aircraft.cfg and *.air file are based on this point.


I am going to assume you have your gear animation done so move your animation slider to frame 100.
Measure either by using a Point and Tape helper tools and or just doing the math and calculate your contact
points. This will be the lowest point of your tires and is measured from the reference datum. These points tell
the sim when your aircraft is "on the ground" when landing and to start the gear suspension animations between
frame 101 and 200.

Next move your animation slider to frame 150. The distance your gear animation moves between frame
101 and 150 determines the static compression. The distance the lowest point of your main gear to the reference
datum is the static_cg_height and tells the sim how to position your aircraft when it is first loaded on the ground.
The static_pitch angle tells the sim to either drop the nose or the tail so that all tires are on the ground.
Positive angle values = nose up/tail down and negative angle values nose down/tail up. As I recall,
the default B737 has its pitch set wrong or its contact point to low and its nose wheel looks about 6"
off the ground.
Now for the Max/Static Compression Ratio. This is the most difficult part to get your head around I think.
Here is how I deal with it. My animation moves just as far after frame 150 as it did up to that point so I
make my ratio 2.000. If I wanted to make my ratio 2.500 I would divide my total animation distance by 5
and then make the frames from 101 to 150 move three-fifths of the total distance and the other two-fifths
between frames 151 and 200 or vice versa. This may or may not look good, so you kinda have to play with
it a bit till it looks good for your model. You want your model to look right when it sits on the ground and not
have the gear works crash into the closed gear doors and other sheet metal in the area when the gear is fully
compressed or during extension/retraction.
Here are some shots of this model in the sim.