Is there something going on with wind or runway friction?

Well, I finally had the time to test the new soft-contact ground handling model.

I’m pleased to say it has shown very nice results from my limited testing.

I used the CJ4 as a) there are no prop wash, torque or P-factor issues to complicate the evaluation and b) this aircraft has always suffered from excessive weathervane and c) I’ve done extensive crosswind flight testing in it before.

A 5knt crosswind had no appreciable effect now with the new model applied.

A 15knt crosswind had an effect from 40KIAS upwards but easily controlled with correct technique.

Not tried 25 of crosswind yet.

Landings are far smoother too - when the wheels touch they will track in the direction that intertial forces will take them, and not immediately pivot into the wind.

FINALLY!!!

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In my own testing of the new physics on the C172, I found that there’s a hard cutover in the model at 40 knots. Maybe that’s what you experienced? I found it really wonky in the C172 if you cross that speed with any control inputs in.

That will be because MSFS starts to apply air physics above Vso - so you can find sudden wing lifting and downwind drifting starting to occur, which would be around 40KIAS in the C172. I didn’t get a hard cutover in the CJ4 at that speed, just a gradual begining of the weathervane. There was a stronger effect at 85 (so Vso again in the CJ4), but I need to do more testing .

If a dev applies too much lateral friction, this doesn’t help, - yes it will eliminate all weathervaning at lower speeds, but pilots will then tend not position their controls correctly and be caught out once the wings start to lift. A problem, but a different problem.

Is this testing with or without modding the aircraft?

With. As far as I can tell from the SDK there have been no global changes to the ground physics. Improvement will only be felt if the new variables are added to the flight_model.cfg

Would you be willing to share the variables you entered and how they affected the aircraft?

Yes. I simply pasted the ones from the C172 !

As a start, I just wanted to see if they had any effect and if so, how much.

Without, and with a pure 5knt crosswind, the CJ4 will start to vane into the wind as soon as it starts to roll.

With the C172 values the CJ4 continues to track the centreline with no control inputs with only a slight deviation starting at higher speeds. An obvious improvement. No adverse impact on taxi. In fact now easy to straight line taxi without needing constant rudder input even in a light wind.

I dare say the values could be better tuned for the heavier CJ4 and as Noted there is still this transition from ground to air physics, but that was always there.

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We should almost do a series of tests and start a table

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Yes. I’ll do a YT with my findings so far.

It’s very important to do baselining with zero wind (the CJ4 goes straight down the centre line, but then it’s a jet) - gauging the amount of left turning in props in zero wind will be necessary.

This is what I used in the CJ4:


ground_crosswind_effect_zero_speed = -1000 ;
ground_crosswind_effect_max_speed = -1000;
ground_high_speed_steeringwheel_static_friction_scalar = 7; 10.0;
ground_high_speed_otherwheel_static_friction_scalar = 7; 10.0;
ground_new_contact_model_rolling_stickyness = 0.4; default = 1.0, ratio, further reduces sideways friction on wheels due to the effects of the rolling weel
ground_new_contact_model_up_to_speed_lateral = 1000.0 ; default = 0.1, feet per seconds, speed up to which the new contact model will be used, above we revert to the legacy contact model (lateral friction)
ground_new_contact_model_up_to_speed_longitudinal = 1000.0 ; default = 1.0, feet per seconds, speed up to which the new contact model will be used, above we revert to the legacy contact model (longitudinal friction)
ground_new_contact_model_up_to_speed_lateral_steering = 1000 ;  default = 1.0, feet per seconds, speed up to which the new contact model will be used, above we revert to the legacy contact model (longitudinal friction)
ground_new_contact_model_ gear_flex = 0.007 ; default = 0.0, compliance in feet per pound of force, defines the added compliance (inverse of the stiffness) of the gears with the new soft contact simulation physics
ground_new_contact_model_gear_flex_damping = 5 ; default = 0.0, damping force pounds per feet per seconds, defines the added damping (energy dispersion in heat) of the gears with the new soft contact simulation physics
enable_high_accuracy_integration = 1; default = 0, enables more accuracy of the physics simulation (1 = accurate to the nanometer, 0 = accurate to the millimeter), required to simulate micro vibrations

The SDK doesn’t make it clear how Gear flex and Gear flex damping should be calculated. But these have less relevance for lateral friction considerations. The key value IMO is the ‘rolling stickyness’. Note that the old crosswind effect and high-speed static friction scalars are disabled by the setting the new ‘up to speed’ values to 1000 (that’s 592knots…)

I found that whether or not the CFD model was applied matters a lot on how the new ground contact model works. If the CFD model is disabled in the aircraft flight model config, you get a worse transition at 40 knots in the C172 than if CFD is on. But the CFD model feels much more toy-like than the default C172 classic flight model (compared to the real C172 that I fly), so you’re ■■■■■■ if you do and ■■■■■■ if you don’t.

That’s an important point - CFD will definitely make a difference. And another reason why its quite hard to use the C172 G1000 as a baseline - its got CFD, NPS and other new physics enabled by Asobo.