Question on "contact points"

[JBaymore]

I am just beginning to explore some stuff on "contact points".

The contact points for a model I have are clearly located well ahead of the visual physical location of the nosewheel (15 -20 feet), and the main gear contacts are located a bit behind the main gear (3-4 feet).

I have tried moving them aftward and forward respectively, so that they line up with the visual model's actual wheels.  "Looks good on paper".

BUT...... when I do this, the model jumps up and down and batters itself onto the ground so hard that it looks like it will break apart  ;).  It activates the scrape points and sparks and smoke appear.  ;D

Is it "normal" to have contact points that don't line up with the visual model by a significant amount?  Visual stuff like the touchdown wheel smoke does line up correctly with the visual model.

Continuing on with this, the "scrape points" also do not match the physical model.  They are located out away from the wing tips and the rear of the craft by a good distance.  Is this typical too?

best,

......................john

[ozzy72]

John it sounds like you've adjusted the suspension settings rather than the "visual" contact points of the wheels!
The rules are as follows on the numbers in the aircraft.cfg;
[contact_points]
//0  Class
       //1  Longitudinal Position        (feet)
       //2  Lateral Position             (feet)
       //3  Vertical Position            (feet)
       //4  Impact Damage Threshold      (Feet Per Minute)
       //5  Brake Map                   (0=None, 1=Left, 2=Right)
       //6  Wheel Radius                (feet)
       //7  Steer Angle                         (degrees)
       //8  Static Compression           (feet)  (0 if rigid)
       //9  Max/Static Compression Ratio
       //10 Damping Ratio               (0=Undamped, 1=Critically Damped)
       //11 Extension Time              (seconds)
       //12 Retraction Time             (seconds)
       //13 Sound Type
       //14 Airspeed limit for retraction (KIAS)
       //15 Airspeed that gear gets damage at (KIAS)

[JBaymore]

Ozzy,

Nope, absolutely SURE that all I adjusted was the fore aft longitudinal distance....not even the lateral distance.    Did not change the CG static or tilt distance either.

best,

...................john

[microlight]

Hi John.

Would this be one of the Murchison 146's? I noticed on mine that the nose wheel tyre smoke was way ahead of the nose wheel itself - so much so that I could see the smoke from the cockpit! So I adjusted the longitudinal distance, and all is fine now - and with no jumping around. If it's doing this, then your moments of inertia could be set too low. I've fiddled with the FD in the 146 and here are my contact point and MoI settings from the aircraft.cfg file:

[contact_points]
point.0=1.000, 56.667, 0.000, -8.25, 1574.809, 0.000, 1.161, 80.000, 0.237, 2.500, 0.987, 5.000, 5.000, 0.000, 0.000, 0.000
point.1=1.000, -2.0, -6.500, -8.50, 1574.803, 1.000, 1.161, 0.000, 0.215, 2.500, 0.802, 5.000, 5.000, 2.000, 0.000, 0.000
point.2=1.000, -2.0, 6.500, -8.50, 1574.803, 2.000, 1.161, 0.000, 0.215, 2.500, 0.802, 5.000, 5.000, 3.000, 0.000, 0.000

point.3=2.000, -14.167, -47.333, 0.000, 787.402, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 5.000, 0.000, 0.000
point.4=2.000, -14.167, 47.333, 0.000, 787.402, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 6.000, 0.000, 0.000
point.5=2.000, -54.000, 0.000, 2.833, 787.402, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 9.000, 0.000, 0.000
point.6=2.000, 42.667, 0.000, -3.750, 787.402, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 0.000, 4.000, 0.000, 0.000
static_pitch=-1.099
static_cg_height=8.285
gear_system_type=0

MoI in the [weight_and_balance] section:

empty_weight_pitch_MOI=1200000.000
empty_weight_roll_MOI=680000.000
empty_weight_yaw_MOI=1900000.000
empty_weight_coupled_MOI=0.000

See if that helps any.

[JBaymore]

Microlite,

Yes, it is the the Murchison 146-200 with the updated FDE's from Matthias Liebericht that he supplies with his panels.

Thanks for the information.... I'll look at it later today and see how it works out.  Busy fixing the busted water heater in my studio right now.  Fun.

best,

....................john

[Falcon500]

what exactly causes aircraft to "jitter" when in the water or on the tarmac?

[microlight]

Falcon,

In the real world, planes (as do all objects) have a natural inertia and momentum. In FS, the FD designer has to tell the simulator what the inertia values for the object are. If these figures are set too low then you get something that is impossible to see in real life - a large object with a lesser amount of inertia than it should have. In FS this translates to the jittering seen when standing still.

You can fix it by increasing the moment of inertia figures in the aircraft.cfg, but not by so much that the plane starts acting much bigger (and more slowly) than it really is in the air. The MoI figures in the default planes can serve as a guide here.

[JBaymore]

microlite,

Well........ I did a bunch of editing of faulty locations and "adding" to the contact points for the Murchison BAe 146-200.

[microlight]

John,

I've been putting a taxi-light onto the nosewheel strut of the 146 (as it hasn't got one), and through that, pretty much agree with your nosewheel location. Using the same process, I've relocated the main gear forward 6 feet to where they should be (-2 to +4). Unfortunately, that takes all the points forward of the centre of gravity, and the whole plane flips over backwards! So I've cured that by adjusting the empty CofG point forward 10 feet, and it's normal again. Just a flight test left....

[5thranger]

I have a similar problem with a CASA 212-300 model I'm finishing up.

The datum for the plan is 0,0.

The plane is a high wing type frame, and thus the wheels touch the ground at -10.25 ft. from the datum.

But every time I select the aircraft and it gets positioned on the tarmac (or anywhere for that matter), the plane sinks for a second until the spinning props touch the ground....and it starts bouncing up and down....left and right like crazy until it suddenly gets thrust to like 500 ft AGL and at OVERSPEED IAS. So much so that I can fly and land it now....even though the engine is still idling.

No matter how many adjustments I've made...the thing sinks and bounces again.

This is not my first plane, I've modelled several, that's why im so stumped at this behavior.

Any ideas?

TIA

Hector

[Falcon500]

Well one thing that i learned from Morton i believe,

set the planes "wheels" well above ground, then use "nav" lights (the red and green steady burners) to place the lights, for/aft and port/starb..  Then put the wheels roughly back to where they were, but without the shocks (so you dont sink).

Start the sim, remove all the fuel and Passengers, then try to place the light at the very bottom of the wheel while (you can do this while flying)

As you adjust dont close the sim just hit Alt and "select" the same aircraft (make sure you save the .cfg after every edit)

When that is done, put your shock in.....

[Falcon500]

Another thing i noticed, the "wheel radius" entry in the .cfg, is actually the.......... it change the speed that the wheel revolves compared to your actual speed