# Contact point static compression coefficient

In the FSX SDK, this variable was named simply “Static Compression.” It’s
description stated, “This is the distance a landing gear is compressed when
the empty aircraft is at rest on the ground (feet). This term defines the
“strength” of the strut, where a smaller number will increase the “stiffness”
of the strut.” In the MSFS SDK, this variable is described as “The static
compression coefficient constant (which is used to compute spring reaction
when on the ground), in
ft. If the contact
point is rigid, then set this to 0.” This is confusing to me in that a
coefficient is normally dimensionless. Does the FSX description still apply,
that is, that this is the distance a landing gear is compressed when the empty
aircraft is at rest on the ground (in feet)? To follow on, the next variable
was named the “Ratio of maximum compression to static compression” in the FSX
SDK and was described as the ratio of the max dynamic compression
available. In the MSFS SDK, this same variable is called the “maximum static
compression ratio,” which implies that it is the compression ratio of a
stationary fully loaded airplane compared to an empty airplane, rather than,
say, the compression of an airplane landing at a high ground loading compared
to an empty static airplane. Can you clarify which is correct?

No need to spend too much time on this right now. I am considering deleting
the question. when I use IRL values for what I believe these variables
represent, the resulting external view I get is not what I expect. For
example, instead of the gear compressing, the bottom of the tire appears to go
through the pavement giving a flattened look to the bottom of the tire.

The static compression coefficient constant is the distance in feet the
contact point will compress when the aircraft is at a standstill and at max
weight. The maximum static compression ratio value times the static
compression coefficient will be the maximum amount of available compression in
feet. So figure out how much total compression you will need, then determine
what the resting compression distance should be, then use this formula to
determine the max compression: total compression / static compression
coefficient = maximum static compression When the suspension compresses, the
entire 3D model will move. So in order for the wheels to remain on the ground,
you need to animate the gear using frames 100-200. The wheels debugger in
aircraft editor will help by showing you the distances in cm. The last
parameter in the contact points list is an exponential factor for the
compression. You can try values between 1 and 2 to make the compression less
linear. The damping ratio constant controls how much oscillation the springs
have. Not too high and not too low will work well.

That seems to be quite a change from the FSX SDK, where the static compression
variable was the static compression at an empty weight. Now the static
compression coefficient variable is the compression at the max weight? That
doesn’t appear to match what I see when I look at the compression distance in
the wheel debugger, but I may not be doing this correctly. Since the force
applied to a given contact point depends not only on the airplane weight, but
also it’s cg position, what is used for determining the static compression
coefficient. Let’s take a nose gear for example since there is only one of
them to take all of the load. Let’s further use a static compression
coefficient of 1 ft (30.5 cm) and a maximum static compression ratio of 1.15.
This would give a total (static) compression of 1.15 ft (35 cm). When the
airplane is empty (airplane total weight = OEW), and the cg is set to the
forward limit, the wheel debug window gives a compression of 29/35 cm. This
happens to be very close to the static compression coefficient. When the
airplane is at the maximum takeoff weight, again at the forward cg limit, the
debug window gives a compression of 35/35 cm. This is not the static
compression coefficient amount; it is the total (static) compression. Even at
the aft cg limit, the compression in the debug window is 33/35.

I’ve only been able to determine these by testing many different values, so I
could have made some errors. I think with different values, you may see a more
obvious result. In your example, there is very little travel (only 5 cm) and
at rest, it already used up 85% of the travel. Since we cannot see the exact
formulas used, it’s difficult to determine the limits of the calculations. My
best results have been to have the static compression 30-40% of the total
range.

This is a great article that explains it for FSX and for all three of our
aircraft so far in MSFS holds true.
https://www.fsdeveloper.com/wiki/index.php?title=Contact_Points