Contact point heirarchy

[gryshnak]

I just discovered something (after a week of frustration) that I've never seen documented anywhere, so I'm going to post about it here.

These were the contact points from my 14bis:

[contact_points]
point.0= 1.0, -2.65, 0.0, -5.8, 1200.0, 0.0, 0.6, 0.0, 0.3, 2.5, 0.5, 0.0, 0.0, 7.0, 0.0, 0.0 //tailwheel
point.1= 1.0, 1.7, -1.6, -5.8, 1600.0, 0.0, 1.0, 0.0, 0.3, 2.5, 0.5, 0.0, 0.0, 2.0, 0.0, 0.0 //left wheel
point.2= 1.0, 1.7, 1.6, -5.8, 1600.0, 0.0, 1.0, 0.0, 0.3, 2.5, 0.5, 0.0, 0.0, 2.0, 0.0, 0.0 //right wheel
point.3= 2.0, 1.0, -12.0, -3.3, 800.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 5.0, 0.0, 0.0 //left skid
point.4= 2.0, 1.0, 12.0, -3.3, 800.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 5.0, 0.0, 0.0 //right skid
point.5= 2.0, 15, 0.0, -4.4, 800.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 5.0, 0.0, 0.0 //nose skid

When I tried it, the whole thing crashed and burned.  The wheels sank through the runway, the plane jumped up and down on the skids with showers of sparks, and the screen bounced rapidly enough to induce motion sickness.

I couldn't figure out why.  I tried for days and just couldn't understand what was going wrong.  I printed out the Aircraft Container System documentation, and read the whole thing.  It still wouldn't work.  It was getting late, I was tired, my brain was slowing down and the numbers were all starting to blur together.  I removed the skids from the .cfg file so I could concentrate on the wheels - and suddenly the plane behaved the way I wanted it to.  I could taxi, take off, and land safely with nothing more than a tweak to the CG.

As soon as I put the skids back in, it went back to the crash and burn scenario.  I got the impression the wheels were being ignored in favour of the skids, so I did a quick copy and paste to put the skids first, like this:

[contact_points]
point.0= 2.0, 1.0, -12.0, -3.3, 800.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 5.0, 0.0, 0.0 //left skid
point.1= 2.0, 1.0, 12.0, -3.3, 800.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 5.0, 0.0, 0.0 //right skid
point.2= 2.0, 15, 0.0, -4.4, 800.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 5.0, 0.0, 0.0 //nose skid
point.3= 1.0, -2.65, 0.0, -5.8, 1200.0, 0.0, 0.6, 0.0, 0.3, 2.5, 0.5, 0.0, 0.0, 7.0, 0.0, 0.0 //tailwheel
point.4= 1.0, 1.7, -1.6, -5.8, 1600.0, 0.0, 1.0, 0.0, 0.3, 2.5, 0.5, 0.0, 0.0, 2.0, 0.0, 0.0 //left wheel
point.5= 1.0, 1.7, 1.6, -5.8, 1600.0, 0.0, 1.0, 0.0, 0.3, 2.5, 0.5, 0.0, 0.0, 2.0, 0.0, 0.0 //right wheel

And now it works perfectly!  Same numbers, just a difference sequence.  The wheels work, and the skids work.

Is there some kind of heirarchy involved here?  Why oh why hasn't anyone mentioned this before?

Gryshnak

[Katahu]

I don't think so. Look at the following contact points below. These contacts are from the Nissan addon I created. Nothing bad happens here. Have you checked the reference datum and COG datum?

[contact_points]
//------0------1---------2------3-------4----5----6------7------8-----9------10---11----12-13
point.0=1,   4.152,  -2.547, -2.151, 9574.8, 0, 1.941, 30.66, 0.156, 1.73, 0.774, 0.0, 0.0, 0, 0.0, 0.0
point.1=1,   4.152,   2.547, -2.151, 9574.8, 0, 1.941, 30.66, 0.156, 1.73, 0.774, 0.0, 0.0, 2, 0.0, 0.0
point.2=1,  -3.888,  -2.547, -2.251, 9574.8, 1, 1.941,  0.00, 0.156, 1.73, 0.774, 0.0, 0.0, 3, 0.0, 0.0
point.3=1,  -3.888,   2.547, -2.251, 9574.8, 2, 1.941,  0.00, 0.156, 1.73, 0.774, 0.0, 0.0, 3, 0.0, 0.0
point.4=2,  -0.513,  -4.098,  0.003, 1574.8, 0, 0.000,  0.00, 0.000,  0.0, 0.000, 0.0, 0.0, 5, 0.0, 0.0
point.5=2,  -0.513,   4.098,  0.003, 1574.8, 0, 0.000,  0.00, 0.000,  0.0, 0.000, 0.0, 0.0, 6, 0.0, 0.0
point.6=2,   6.819,     0.0,  0.003, 1574.8, 0, 0.000,  0.00, 0.000,  0.0, 0.000, 0.0, 0.0, 9, 0.0, 0.0
point.7=2,  -6.873,     0.0,  0.003, 1574.8, 0, 0.000,  0.00, 0.000,  0.0, 0.000, 0.0, 0.0, 4, 0.0, 0.0

[gryshnak]

Have you checked the reference datum and COG datum?

Both are 0, 0, 0  ???

This is so screwy, I have no idea why it happens!

Gryshnak

[Milton]

Interestingly, what you are calling skids contact points are actually Crash points.  Skids = class 3 contact points; Crash points = 2.

Also, you show the "skids" (actually crash points) 2 feet above the tires and each has the sound of a Left Wing Scrape.

I think you need to go back to the beginning with each contact point, take one value at a time, get you measurements from the design program, and you will find all will work correctly.

Incidentally, skids should also have suspension like the tires.