Simviation Forum - Question on "contact points"

Apr 29^th, 2006 at 9:51am

JBaymore Offline
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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 Wink

. It activates the scrape points and sparks and smoke appear. Grin

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

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Reply #1 - Apr 29^th, 2006 at 11:39am

ozzy72 Offline
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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)

There are two types of aeroplane, Spitfires and everything else that wishes it was a Spitfire!

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Reply #2 - Apr 29^th, 2006 at 12:14pm

JBaymore Offline
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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

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Reply #3 - Apr 29^th, 2006 at 12:41pm

microlight Offline
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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.

Wink

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Reply #4 - Apr 29^th, 2006 at 1:35pm

JBaymore Offline
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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

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Reply #5 - Apr 29^th, 2006 at 4:27pm

Falcon500 Offline
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what exactly causes aircraft to "jitter" when in the water or on the tarmac?

What do I do you ask? I struggle! Then destroy! Then try to put back together what I just broke on accident.....&&&&&&&&&& The GMax effect The GMax effect

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Reply #6 - Apr 30^th, 2006 at 4:27am

microlight Offline
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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.

Wink

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Reply #7 - Apr 30^th, 2006 at 9:48pm

JBaymore Offline
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microlite,

Well........ I did a bunch of editing of faulty locations and "adding" to the contact points for the Murchison BAe 146-200. I have added a bunch of more realistic "scrape points" for things like the under side pitot tubes, the engine cowlings, the tail center top and stabilizers ends, and so on.

The nose wheel (point 0) here is lined up exactly on the visual model now. I still haven't fixed the main gear locations.... they are still a good bit behind the visual model in this one.

[contact_points]
static_pitch = -1.099
static_cg_height = 8.285
tailwheel_lock = 0
gear_system_type = 0
max_number_of_points = 17

point.0 = 1, 40.667, 0, -8.65, 1574.809, 0, 1.161, 80, 0.237, 2.5, 0.987, 5, 5, 0, 0, 0
point.1 = 1.000, -2.0, -6.500, -8.65, 1574.803, 1.000, 1.161, 0.000, 0.215, 2.500, 0.642, 5.000, 5.000, 2.000, 0.000, 0.000
point.2 = 1.000, -2.0, 6.500, -8.65, 1574.803, 2.000, 1.161, 0.000, 0.215, 2.500, 0.642, 5.000, 5.000, 3.000, 0.000, 0.000
point.3 = 2, -2.25, -42.333, 2, 787.402, 0, 0, 0, 0, 0, 0, 0, 0, 5, 0, 0
point.4 = 2, -2.25, 42.333, 2, 787.402, 0, 0, 0, 0, 0, 0, 0, 0, 6, 0, 0
point.5 = 2, -20, 0, -4.1, 787.402, 0, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0
point.6 = 2, -48.2, -18, 19, 0, 0, 0, 0, 0, 0, 0, 0, 0, 5, 0, 0
point.7 = 2, -48.2, 18, 19, 0, 0, 0, 0, 0, 0, 0, 0, 0, 6, 0, 0
point.8 = 2, 42.667, 0, -5.25, 787.402, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0
point.9 = 2, 15, -21.9, -3.4, 787.402, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0
point.10 = 2, 18, -13.3, -2.7, 787.402, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0
point.11 = 2, 18, 13.3, -2.7, 787.402, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0
point.12 = 2, 15, 21.9, -3.4, 787.402, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0
point.13 = 2, -41.5, 0, 20.5, 787.402, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0
point.14 = 2, 34, 0, -6.2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0
point.15 = 2, 23.5, 0, -6.2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0
point.16 = 2, -6, 0, -5.8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 9, 0, 0

Hope this is useful to you (or someone else too).

best,

......john

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Reply #8 - May 1^st, 2006 at 12:58pm

microlight Offline
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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....

Wink

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Reply #9 - May 1^st, 2006 at 8:23pm

5thranger Offline
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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

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Reply #10 - May 1^st, 2006 at 9:10pm

Falcon500 Offline
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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.....

Wink

What do I do you ask? I struggle! Then destroy! Then try to put back together what I just broke on accident.....&&&&&&&&&& The GMax effect The GMax effect

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Reply #11 - May 1^st, 2006 at 9:14pm

Falcon500 Offline
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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

What do I do you ask? I struggle! Then destroy! Then try to put back together what I just broke on accident.....&&&&&&&&&& The GMax effect The GMax effect

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