Re: [Flightgear-devel] Abnormal surface friction factor

[Dany <da...@nu...>]

On 14/06/2023 à 13:26, Erik Hofman wrote :
> On 6/14/23 13:06, James Turner wrote:
>
>> Of course, I don’t know what units the friction is supposed to be in, 
>> to know if this understanding is correct, but it’s certainly very 
>> common to see such conversion factors being applied when values are 
>> read from files.
>
> The problem is that the code was never designed for different surface 
> frictions than dry asphalt. So What has been done is give rubber tires 
> on dry asphalt a friction factor of 1.0 and work from there.
>
> Just a to make sure: does any other JSBSim aircraft (say the Ercoupe 
> which is pretty simple) also have a friction value of 50 on the same 
> terrain type?
>
> If not it might be a aircraft problem. If it does then it's probably 
> something in the code.
>
> Erik
>

Hopefully, trying to help understanding of how these calculations are 
made... (I'm far from an expert, I try to think for wlbragg)

1- General physics (independently of FG)
Coefficients of...
static friction, dynamic friction, rolling friction (dimensionless)
To move an object of weight M on a surface, one need to exert a traction 
of M x [the coefficient]. Where [the coefficient] is static or dynamic 
or rolling friction.

(Obvious up to there...)

2- In FG, JSBSim
To move an object of weight M on a surface, one need to exert a traction 
of M x [the coefficient] x *[other factors]**
*
Where [the coefficient] is
/fdm/jsbsim/gear/unit[]/rolling_friction_coeff or static_friction_coeff 
or dynamic_friction_coeff. I guess, enough on hard asphalt, with [other 
factors] = 1.

Additionally, *[other factors]* are terrain defined, hard, soft, dry, 
wet, etc...
Giving (in FG Properties, /fdm/jsbsim/ground/ and /fdm/jsbsim/gear/unit/):
rolling_friction-factor, static_friction-factor, snow_friction-factor, 
etc... To add to confusion: static-friction-factor.
We also have (by geodinfo):
friction_factor, rolling_friction.
With (if I understand well)
rolling_friction_factor = rolling_friction / 0.02 = 50 x rolling_friction.

Erik, you seem having worked on this subject in JSBSim.
Can you explain what there is in [other factors]?
In order to, finally, have the 'A to Z' calculation for the traction to 
be exerted, starting from the basic wheel coefficients to the terrain 
coefficients.

I feel the confusion for wlbragg comes from here.

Sorry for writing here despite my lack of knowledge in the domain...

Best regards,

Dany