Asked

Aircraft was modeled with FSDS for FS2004. When viewed in FS, the main gear was buried in the ground about 1/3 of the way to the axle. The nose gear was also buried so the tire was up to the rim.

I also noticed that in slew mode the whole fuselage tilted so the nosewheel was about two feet off the ground. When leaving slew mode, the nose slammed back down to where it originally was.

I reset the contact points so the main gear tire bottoms and nose gear tire bottom are just barely below ground level to produce a very slight flattening effect. Now, when I go into slew mode everything seems OK, but when I exit slew mode the aircraft takes off like a scalded hog.

What other setting needs to be adjusted to correct this? Here are the contents of the contact points section of the aircraft.cfg file exactly as they appear.

Note that the parts commented out with "//" are the original settings. I did this so I would be able to get back to the original if needed.

[contact_points]

static_pitch=2.3
static_cg_height=9.2
gear_system_type=1
// 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
point.0= 1, 31.90, 0.000, -9.7, 1000, 0, 1.00, 52, 0.50, 1.00, 0.50, 10.5, 10.5, 0, 152, 152
//point.1= 1, -3.85, -14.000, -9.7, 1500, 1, 1.50, 0, 0.50, 1.00, 0.50, 10.2, 10.2, 2, 152, 152
//point.2= 1, -3.85, 14.000, -9.7, 1500, 2, 1.50, 0, 0.50, 1.00, 0.50, 10.0, 10.0, 3, 152, 152
point.1= 1, -3.85, -14.000, -9.7, 1500, 1, 1.50, 0, 0.50, 1.00, 0.50, 10.2, 10.2, 2, 152, 152
point.2= 1, -3.85, 14.000, -9.7, 1500, 2, 1.50, 0, 0.50, 1.00, 0.50, 10.0, 10.0, 3, 152, 152

point.3=2, 0 , -61.5, 6, 800, 0, 0, 0, 0, 0, 0, 0, 0, 5
point.4=2, 0 , 61.5, 6, 800, 0, 0, 0, 0, 0, 0, 0, 0, 6
point.5=2, -59 , 0, 0, 800, 0, 0, 0, 0, 0, 0, 0, 0, 9
point.6=2, 36.15, 0, 0, 800, 0, 0, 0, 0, 0, 0, 0, 0, 4

Expert Answered

Yep — that “launch” on exiting slew is almost always FS9 trying to resolve a ground-penetration / suspension mismatch in one frame. You fixed the visual sinking, but right now the contact geometry and the “resting” numbers (static_cg_height/static_pitch) still don’t agree, so when FS re-hooks the airplane to the ground after slew it winds up with a giant “spring unload” and tosses the model.

What I’d do next (in order)

1) Stop chasing the “slight tire flattening” for the moment
In FS2004 it’s very easy to create a situation where the airplane is already sitting with the gear compressed / points slightly below the surface. That may look nice while parked, but coming out of slew FS often reacts violently.

  1. As a test, raise the airplane very slightly by moving the gear Z values (the 4th number on point.0/1/2) so the tires are not below the surface at rest. In other words, aim for “just touching” or even a hair high.
  2. Reload the aircraft (don’t just hit OK in the menu—actually reload the flight/aircraft so FS reinitializes the ground solve).
  3. Test slew again.

If the “launch” reduces or disappears, you’ve confirmed it’s a penetration/solve issue (not a thrust/flight model issue).

2) Make sure static_cg_height matches your new gear height
Any time you change the gear Z positions, you’ve effectively changed the aircraft’s sit height. If static_cg_height=9.2 is now “wrong” for the updated gear geometry, FS will try to force the airframe to the attitude implied by those numbers when it re-contacts the ground.

  1. Once the aircraft is sitting without sinking, adjust static_cg_height in small steps (0.1 at a time) to get a stable rest attitude.
  2. Keep static_pitch in mind too — if the pitch is forcing the nose high/low relative to your gear points, you’ll get a big settle/pop when FS recomputes the stance.

3) That “52” in the nose gear line is a red flag
This bit jumps out:

point.0= 1, 31.90, 0.000, -9.7, 1000, 0, 1.00, 52, 0.50, 1.00, 0.50, 10.5, 10.5, 0, 152, 152
point.1= 1, -3.85, -14.000, -9.7, 1500, 1, 1.50, 0, 0.50, 1.00, 0.50, 10.2, 10.2, 2, 152, 152
point.2= 1, -3.85, 14.000, -9.7, 1500, 2, 1.50, 0, 0.50, 1.00, 0.50, 10.0, 10.0, 3, 152, 152

Even without debating which exact “field name” that is, having the nose gear behave wildly different from the mains in any spring/damping-related slot can absolutely cause the sim to “catapult” the airplane on ground re-contact (especially if it’s pitching at the same time).

  1. For testing, make that nose value the same ballpark as the mains (since both mains are “0” in that position, I’d try “0” on the nose as a diagnostic).
  2. Reload the aircraft and repeat the slew-exit test.

If that single change dramatically improves it, you’ve found the offender and can then fine-tune more gently from there.

4) Verify your scrape points aren’t acting like extra landing gear
After raising/lowering gear points, sometimes a type=2 point ends up contacting the runway at rest. Then on slew exit FS sees a different “support polygon” and tries to resolve it by rotating/launching the model.

Your ones to watch:

point.3=2, 0 , -61.5, 6, ...
point.4=2, 0 , 61.5, 6, ...
point.5=2, -59 , 0, 0, ...
point.6=2, 36.15, 0, 0, ...

  1. Temporarily move those type=2 points upward (increase their Z a bit) or temporarily comment them out for one test.
  2. Reload aircraft and test slew exit.

If the launch goes away with them disabled, one of those points is “biting” the runway.

A simple test routine (keeps you sane)

  1. Test on a default runway at sea level if possible.
  2. Load aircraft and do nothing for 20–30 seconds (make sure it’s stable without slewing).
  3. Enter slew, lift straight up 5–10 feet, stop movement, then exit slew.
  4. Only change one thing at a time (nose “52”, then scrape points, then static_cg_height).

Couple questions so I can point you to the exact knob instead of guessing

  1. Did you change only the three gear Z values (all currently -9.7) or did you touch any of the other numbers too?
  2. When it “takes off,” is it mostly a vertical jump, or does it pitch hard nose-up as well?
  3. What were the original values for static_pitch and static_cg_height before you started adjusting the contact points?

Still does not answer your question? Ask a new question!

If the question and answers provided above do not answer your specific question - why not ask a new question of your own? Our community and flight simulator experts will provided a dedicated and unique answer to your flight sim question. And, you don't even need to register to post your question!

Ask New Question...